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Studies in the coordination chemistry of selenium and tellurium


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A thesis presented for the

degree of Doctor of Philosophy in Chemistry

5.n JGhe University of Canter-bury, Christchurch9 New Zealand.



I acknowledge the award of a research grant from the Selenium-Tellurium Development Association, New Yorka

I am grateful to many members of the staff of the

Canterbury Chemistr•y Department~ particularly Dr G. A. Rodley, for helpful discussions.

A special mention is due to Professor Harry B. Gray9

who9 during his visit hePe in 1968, suggested and largely

contributed to the work described in chapter 1.

I am most grateful to Professor C.J. Wilkins for his guidance and encouragement throughout the course of this work.


2-The electronic spectra of the octahedral Tec1 6

and TeBP~- complexes are reported and assigned. Bands

due to transitions to components of the 3T1u (sp) excited state are found at 24,240; 25, 71+0 and 30,670 cm-1 in


and 21,400; 22,400 and 26,200 cm-1 in



The spin-allowed s-+ p transition to the T1u excited state gives rise to three bands in



34,530 and 36,510 cm-1) and


(28,130; 29,730 and 31,200 cm-1), which is taken as


that the excited states


distorted to lower symmetry. The lowest

allowed halide _,. Te charge transfer transi tiona are at 44,100 cm-1 and somewhat higher than 52,000 cm-1 in

2-TeC16 ; analogous transitions appear at 37,300 and


2-44,500 em in TeBr6 • The observed spectral data indicate that the "inert" 5s2 Te(IV) electrons, which reside in the a


ga~ level, are partially delocalised to

2-the halide ligands. The spectra of th~ complexes SeBr


and Tei6 are also presented.

Complexes of, selenium and tellurium tetrahalides with nitrogen and oxygen donor ligands were prepared, and their constitutions studied by spectroscopic and conductivity measurements. The compositions MX








ligand are also found. Complexes are formulated


(Tec1 3.L

9 tel-, [ MX

3.2L' J+x-, [MX3.L" ]+x-, [SeX2.2L" j2+2X-,

t -

f ]2+

2-[MC13.terpyridyl Cl and LTeC1 2.terpyridyl Tec1


(M = Se,Te; X= Cl,Br; L', L"= monodentate, bidentate ligand) in solution. Some are differently constituted

in the solid state. A strong tendency to five-coordination in fluoro-complexes is'found and discussed. New

complexes are formulated SeF


.pyridine, [SeF2.bipyridyl]2+ 2SeF5 and [ TeF3 .. terpyridyl


TeF5 •

An attempt to prepare





reaction of

· cs 2TeBr


and hydrogen fluoride was unsuccessful.

Despite earlier reports, tellurium tetrachloride generally undergoes condensation with primary and

secondary amines, yielding amine salts and other products, rather than complexes. The condensation products


' \

bis(diethylamino)tellurium dichloride/and dipiperidino-)1,

tellurium dichloride;(,were isolated and characterised. The reactions of tellurium tetrachloride with aromatic amines are complex; earlier work is shown to be in error. Tellurium tetrachloride is extensively solvolysed in


Products from the reactions of sel.enium and tellurium tetrachlorides with compounds containing activated hydrogen atoms, including phenol- anisole, acetone and acetophenone,


were isolated and characterisedf Tellurium tetra-chloride and ·acetophenone yield several compounds,

including dypnone derivatives. The -Te012- group causes

proto~ magnetic resonance chemical shifts of

0.5 - 2.3



/Infrared absorptions due to vibrations of selenium

/' . r..' .t~

and tellurium atoms are assigned in the following ranges ( cm-1)

.M - 0 (alkyl) M-phenyl






540 - 630

302 - 355

271 - 283

580 - 610(?)




498 - 550


- 311





290 - 340



General Introduction

Review: The constitution of the selenium and tellurium tetrahalides

Chapter 1: Electronic spectra of the hexahalo-complexes

Chapter 2: Complexes of the tetrahalides




with nitrogen and oxygen donor ligands 50 Chapter


Reactions of tellurium tetrachloride

with primary and secondary amines



Reactions of' the tetrachlorides

with organic oxygen compounds. Chapter 5: IMrared spectra

Experimental References



Table 1





6 7




Constitution and transition temperatures of the tetrahalides.

Published vibrational spectra of the tetrachlorides and trichloro-cations. Electronic spectra of the hexahalo-complexes.

Complexes of the tetrahalides with nitrogen and oxygen donor ligands.

Conductivities and proposed constitution of complexes.

Results from X-ray powder patterns. I

Analyses of TeC1


-n-butylamine reaction products.

Infrared spectra of organo-selenium and -tellurium compounds.

Published assignments of infrared absorptions to Se-alkyl and Te-alkyl vibrations.

Infrared absorptions assigned to Se-Cl and Te-Cl vibrations.





56-57 94





Figure After page 1





Simplified molecular orbital diagram

2-for Tex

6 .

Assignments for hexahalo complexes.

2-Relative orbital energies for Tex




2-Electronic spectra of Tec1


and TeBr

6 ·

Electronic spectra of Tl+ in KI and

2-TeC16 .


Infra~ed spectra of tellurium tetrachloride

complexes. 7


Proton magnetic resonance spectra. · Infrared spectra of acetophenone and derivatives.






58 117


The work described in this thesis is concerned with reactions of the tetrahalides of selenlum and tellurium, and the nature of a variety of derivatives which can be obtained directly from them.

The tetrahalides may be regarded as the principal halides of the two elements; except for the fluorides,

they are the most stable and accessible halides formed. Selenium and tellurium both have very stable and

unre-active hexafluorides. Selenium also forms a monochloride,

a dichloride, TeC12, and a dibromide, TeBr2 • No

iodides of selenium;are known; tellurium forms only the tetra iodide. Seven of the eight possible tetrahalides are known, and give ample scope for comparisons, of selenium with tellurium, and among the halogens.


from Grignard reagents to give stable organic derivatives; they are mild oxidising agents, halogenating agents,

halide ion donors and, as will be shown, strong electron acceptors. (In this last respect they differ from the sulphur tetrahalides.)

The chemistry of selenium and tellurium has been very much neglected, by comparison with that of the similar elements of groups IIIb, IVb, Vb and VIIb (apparently because of the largely undeserved ill-repute brought upon the two elements by a small group of their organic compounds.) This is particularly true of their halides, whose chemistry has been much less studied than that of the halides of the other elements mentioned above, so much so that even the constitution of the tetrahalides

themselves is not fully known. Because knowledge of the nature of the tetrahalides is important to the

understand-ing of their chemistry, the evidence for their constitution, which has not previously been fully reviewed, is presented in the second part of this introduction. Their widely varying physical properties and thermal stabilities are also summarised in that section.

There are two features of the electronic structure of the tetrahalides which have an important bearing on this work. First, they are 11inert pair" compounds;


than the group valency, so that they contain an unshared pair of electrons, which has a marked effect on the

stereochemistry and electronic properties of the com-pounds. The significance of the unshared pair is greatest in the six-coordinate compounds formed by Se and Te(IV). The simplest of these, the hexahalo anions,

2- ( )

MX6 M = Se, Te; X = halogen are theoretically important because their structures are the subject of a conflict between various current bonding theories (which was largely overlooked before the discovery of the iso-electronic compound, xenon hexafluoride.) Chapter 1 of this thesis describes a study of the electronic spectra of the hexahalo complexes, which gives useful information on the role of the "lone pair" of electrons in their


The second important feature is the readiness with which selenium and tellurium vary the size of their valence shells. Unlike the elements of the first and second.'rows of the periodic table, those of the third and fourth rows, including selenium and tellurium,readily expand their valence shells from eight or ten to twelve or fourteen electrons. This readiness, combined with the strong electron-withdrawing effect of the four halogen atoms, make_a the central atom of the tetrahalides a


are of two closely related types: {1) formation of donor-acceptor complexes, the tetrahalide acting as a strong Lewis acid (2) substitution {condensation) reactions where the selenium or tellurium atom acts as an electrophile.

Complex formation may be d~vided into two types: (1) the formation of the penta- or hexahalo complexes, mentioned above, from the tetrahalides or dioxides in aqueous halogen acids (2) reaction of the tetrahalides in anhydrous conditions with neutral Lewis bases such as pyridine, giving complexes that are neutral, or cationic through loss of one or more halide ions.

(Anionic complexes of selenium and tellurium containing ligands other than halides are so far unknown.)

Similar complexes are formed by the tetrahalides of group IVb and the trihalides of groups IIIb and Vb.


than between elements of any other group. This diversity can generally be related to the large variation in

11metal11-halogen bond strengths on passing from fluorides

to iodides, and the lower strength of selenium-halogen bonds than the corresponding tellurium-halogen bonds.

It was clear that these simple complexes should be more closely examined before more complicated reactions, which may proceed via complex formation, were studied. Chapter 2 describes the complexes which have been prepared from the tetrahalides and a number of ligands, particularly tertiary amines, which were selected as being unlikely to undergo any reaction other than coordination.

(Because of some overlapping- it is necessary, in order to describe the present work fully, to include much of what was done by two previous students in this department Miss M.L. Ve1tch(1) and Miss P.S. Markham. ( 2) Due credit for their work is given.)


the reactions of the selenium and tellurium tetra-halides, which is fragmentary and largely unreliable, contains claims of both types of reaction, sometimes even from the same reactants. Chapter 3 describes the author's study of the reactions of tellurium tetra-chloride with a number of primary and secondary amines, both aliphatic and aromatic.

This thesis is based on a specific research proposal, supported by the Selenium and Tellurium Development

Association. The title of the proposal is "Selenium and Tellurium Complexes and their Rearrangement." The first part of it proposes the work on complexes, leading to the reactions of primary and secondary amines, outlined above. The second part is concerned with the reactions of the tetrahalides with certain other types of organic compounds containing activated hydrogen atoms. Two examples are: (1) the initial reaction of selenium tetrachloride with


(2) the reaction of tellurium tetrachloride with anisole: OH3oc6H

5 + Tec14 -+ p-CH3oc6H4TeC13 + HCl Both of these reactions may be regarded as electrophilic


are relatively slow; and it has been shown that the same or similar compounds can also, more rapidly, form

coordination compounds with the tetrahalidea, coordination taking place through oxygen or nitrogen atoms. The

implicit assumption of the proposal was that one could isolate one of these complexes, and then allow its components to undergo the slower condensation reaction, the "rearrangement" being observed kinetically. While it still appears very likely that complex formation is an essential part of the mechanism of at least some of

the condensation reactions, it was found that observation of solution reactions of well-defined complexes is not practicable, because of side reactions and because no suitable solvent could be found for the stable complexes. The situation is discussed further in chapter L~, which also describes the author's attempted kinetic studies of a condensation.

Moat of the previous work on condensation reactions was carried out in the period 1920- 1940, and there is naturally very little discussion of the theoretical

significance of the results. In connection with the aims described above, it was decided to re-examine the

reactions of selenium and tellurium tetrachlorides with a representative range of certain types of organic


use of modern techniques, particularly spectroscopy. Information on the relationships between reactions, structure and spectra in such compounds has previously been conspicuously lacking. The author has made a

number of useful correlations of this type, particularly in the fields of proton magnetic resonance spectra,

infrared spectra and electronic and steric effects in reactions, which are described in chapters




Differences between selenium and tellurium are moat evident in this group of reactions. Past experience had shown that the occurrence of any reaction of a selenium compound does not necessarily mean that the corresponding tellurium compound will undergo the same reaction, and viae versa. For example, the reaction of tellurium tetra-chloride with acetylaoetone had been shown to yield only compounds of tetravalent tellurium, while selenium

tetrachloride and acetylacetone always gave compounds of divalent selenium\3) Further examples of such differences were found in the course of the present work; they can generally be related to the greater ease of reduction of selenium(IV) than of tellurium (IV).

Some severe experimental difficulties are encountered in the chemistry of the selenium and tellurium tetrahalides. The greatest of these arises from their reaction with


Most of their derivatives, especially complexes, are similarly attacked by moist air; some are extremely sensitive to even minute traces of water. Consequently rigorously anhydrous conditions are necessary, not only for the reactions, but for all subsequent manipulations of the products. All manipulations were therefore

made in a glove box, except that some reactions involving volatile components were carried out in a vacuum system. All apparatus, solvents and reagents were carefully dried. Procedures for the preparation and handling of samples for X-ray powder photographs, spectra, etc., were likewise designed to prevent access of moisture.

A second difficulty also arises from the high reactivity of the tetrahalides. Because they react with a wide variety of organic compounds, the choice of solvents which can safely be used is very limited. No suitable solvent was found in which selenium tetrachloride or the tetrabromides showed useful solubility; this is one reason why tellurium tetrachloride, which is readily soluble in ether and acetonitrile, was studied rather more than the other tetrahalides. . (Apart from these two

the only other solvent which was used at all extensively in this work was dichloromethane.) Some examples of the use of unsuitable solvents by previous workers, with


A third limitation, already mentioned, is the low solubility iii any solvent of' many of the products

studied. This is particularly true of the complexes described in chapter 2, most of which are barely soluble enough, even in such solvents as nitrobenzene or dimethyl-formamide, for conductivity measurements (10-3 molar). Many of the compounds described are too insoluble for recrystallisation, so that close attention to the

conditions of the initial reactions is necessary to obtain pure products. Low solubility also limited the range of molecular weight determinations, P.M.R. spectra, etc., which could be obtained, the difficulty being increased by the reactivity and instability of the compounds.

In summary, underlying features of the chemistry of the tetrahalides which influenced the course of the work are: ,,

(1) the presence of an unshared pair of electrons on the central atom;

(2) the variation in the strength of 11'meta111-halogen

bonds on passing from fluorides to iodides;

(3) differences in behaviour between selenium and tellurium halides;


the ability of the tetrahalides to form stable donor-acceptor complexes;



the occurrence of condensation reactions leading to the formation of covalent bonds to carbon, nitrogen and oxygen;

(7) experimental limitations on the media in which reactions can be carried out, the products which can be isolated, and the measurements which can be made.

For purposes of description, the work falls conveniently into the following divisions:

(1) the electronic spectra of the hexahalo complexes, with particular attention to the role of the 11lone pair"

of electrons in bonding (chapter 1);

(2) the formation of donor-acceptor complexes of the tetrahalides with aprotic amines and oxygen donors (chapter 2);

(3) condensation reactions between the tetrahalides and various organic compounds containing activated hydrogen atoms (chapters 3.and





Simple non-metal or semi-metal halides of formula MXn commonly have one of four tyPes of structure:

(i) molecular MXn, (11) associated or polymeric (MXn)m,' (iii) the simple ionic form MX~_


x- or (iv) a complex ionic form









etc. The

trihalides of groups IIIb and Vb and the tetrahalides of group IVb gener~lly have structures (1) or (ii); i.e.,

they are non-ionic. In the crystalline state, phosphorus pentabromide, which is not a halide-ion acceptor, has a structure of type (iii), viz. PBr4 Br-, while phosphorus pentachloride, which is a halide-ion acceptor, has a

stvuature of type (iv), viz$ POl4POl6 in the solid state.

One might expect that the tetrahalides of selenium and tellurium, being strong acceptors, would have type

( iv structures, i.e., 2MX) +




or, less likely,







In fact~ there is no experimental support

for such a composition in any state. Instead, the

considerable evidence has been variously interpreted in terms of the alternative constitutions MX





)m or


acceptor to ionise to free halide ions.

Only two of the tetrahalides, selenium tetrafluoride and tellurium tetrachloride, are stable in the gaseous state. Both consist of neutral, monomeric species, as shown by vapour density measurements.(4,5) Electron diffraction studies( 6


show that the stereochemistry of both is a slightly distorted trigonal bipyramid with the non-bonding pair of electrons in an equatorial

position, as predicted by Gillespie and Nyholm.(B) There is little doubt that the fluorides are non-ionic in all states. Selenium tetrafluoride is a low-melting liquid (m.p. -9.5°) with a low electrical

conductivity (which has been suggested(9) to show a slight ionisation to SeF3 and SeF5). The infrared spectra of the solid and vapour( 10) and the Raman spectrum of the liquid( 11 ) are all very similar1 and are all interpreted in terms of the non-ionic structure. Infrared spectra of the solid using the matrix isolation technique,< 1o) 19FN.M.R. spectra< 12) and the high


square-pyramidal TeF


units sharing cis-qorners.

There is still much debate over the c,onsti tution of the chlorides, bromides and iodides in the solid and liquid states and in solution. None of the usual

techniques - X-ray diffraction, infrared and Raman spectra, and conductivity and molecular weight measure-ments - can distinguish unequivocally between non-ionic and simple ionic



structures. The evidence which has been obtained by each of these means will be considered in turn.

Tellurium tetrachloride, in the molten state, has a salt-like electrical conductivity,(1


which has been interpreted as showing extensive ionisation, by contrast to the non-ionic structure of the vapour. But this interpretation is open to question, because the range of

cond~ctivities observed (specific conductivity 0.1 - 0.2 -1 -1)


A number of attempts to determine the crystal structures of selenium tetrachloride and tellurium

tetrachloride, tetrabromide and tetraiodide by X-ray diffraction methods(iG-i9) have encountered unusually great difficulties, mainly because of persistent twinning, whose exact nature could not be determined. Only one study,(i9) on tellurium tetrabromide, proceeded beyond the determination of space group and cell parameters, and then the data obtained were not of sufficient quality

to distinguish between covalent and ionic structures. In view of the possibility that the tetrahalides contain MX3 cations, a number of workers have compared the vibrational spectra of the tetrahalides with those of compounds known to contain these cations. Gerding and Houtgraaf( 20) compared the Raman spectra of solid selenium tetrachloride and solid and liquid tellurium tetrachloride with those of the aluminium chloride adducts, formulated

+ - .




4 ,

and those of the isoelectronic arsenic and antimony trichlorides. They concluded that the ionic formulation is most likely for the solid and liquid phases.


Robinson and Ciruna( 22 ) compared the Raman spectra of the solid tetrachlorides with those of solutions in chloro- and fluorosulphuric acids, which were shown to contain the Mel; cations. They pointed out that certain Raman lines in the solids are at significantly lower

frequencies than the corresponding lines in the solutions, and consequently drew, from a similar comparison, the opposite conclusion to the previous workers, i.e., that the solids are non-ionic.

There have been at least six other studies of the vibrational spectra of the tetrachlorides and tetra-bromides,(23-29) of which three favour the simple ionic formulation and three the covalent. All agree that

2-structures containing


or MX


anions are ruled out~ It is generally agreed that tellurium tetrachloride is non-ionic in benzene solution, and that its structure is concentration-dependent. The observed dipole moment was viewed(30,3i) as resulting from monomeric molecules with the polar structure described above. But infrared spectra and molecular weight measurements, over the same concentration ranges, have been variously interpreted to indicate monomeric( 21 , 26 , 29) and approximately trimeric ( 32 , 33) species.


solvents was studied in this department by Miss Veitch



and Miss Markham,(2 ,


using conductivity and

molecular weight measurements. Their results indicate that the tellurium halides are fully ionised as Tex; x-, while the extent of ionisation of the selenium halides depends mainly on the electron-donor properties of the solvent.

Greenwood, Straughan and Wilson(


found similar results for tellurium tetrachloride and tetrabromide. Molecular weight values measured for acetonitrile and acetone solutions are consistent with the ionic


-tion Mx

3 X , as are conductivity values for the same solvents, which show the "expected" linear relationship,




Aa -

k c2, between molar conductivity and concentra-tion. (Conductivities in other solvents are anomalous, as are results obtained for tellurium tetraiodide.)

By contrast, the ultraviolet absorption at a single wavelength (360 m~~) of tellurium tetrachloride in aceto-nitrile solutions of various concentrations obey Beer's law very closely, indicating that there is no change in constitution with varying concentration.<21)

Moreover, a recent paper(


presents conductivity and molecular weight data which suggest very strongly


tetra-bromide is stated to be completely dissociated to a lower bromide and bromine in solution.) The conductiv-ity values again increase with decreasing concentration, but values are lower by factors of 10- 100 than those found by the previous workers.

Vibrational spectra of tellurium tetrachloride in (probably fairly concentrated) solutions in acetonitrile and propionitrile were interpreted in terms of the

molecular monomer.( 2i)

Thus direct interpretations of the results of

conventional measurements have led to highly conflicting conclusions about the constitution of the tetrahalides. It is therefore necessary to examine the evidence more critically and to consider other lines of argument.

Interpretation of low-frequency vibrational spectra is difficult at best. Intensities of absorptions vary greatly, bands are often broad or asymmetric, and spectra are complicated by noise, spurious bands and absorptions by solvents and optical materials. The difficulty is well illustrated here by consideration of the number of bands expected for each of the structures proposed for the tetrahalides. The trigonal pyramidal


cation should give four bands in both infrared and Raman, while the MX


molecule (c2v symmetry) should give eight bands in the infrared and nine in the Raman. Yet such


of the spectra: bands missing from the expected spectrum are written off as 11 too wealt11 or 11 obscured",

while extra bands are equally readily explained as being due to resolution of degeneracy by solid-state effects, or merely 11spuriousu. So it is not surprising that

studies of the spectra of the tetrahalides alone have been indecisive.

As noted above, some authors have detected a

similarity between the spectra of the tetrachlorides and those of the MC13 cations, and therefore assumed these cations to be present in the halides. But more recent and discriminating studies have shown that there is a significant difference between the two sets of spectra. A further study(35) of the Raman spectra of compounds formulated as MC13 AsF6 has established the vibrational frequencies of these cations more firmly. Some of the

+ various published spectra of the tetrachlorides and MC1

3 cations are listed in Table 2. It appears that the differences in the spectra are too large to be accounted for by solid-state effects, etc., and therefore favour the non-ionic structure. This is supported by a nuclear quadrupole resonance study(3G) of solid tellurium tetra-chloride, in which the six observed lines were interpreted as indicating two pairs of equivalent chlorine atoms, i.e., a non-ionic (c


Similarly, cond~ctivity and molecular weight measure-menta must be interpreted with caution. With such

reactive compounds as the selenium and tellurium tetra-halides, the purity of solvents is of prime importance, because even the minute traces of impurities remaining after the moat rigorous purification procedures can have a significant effect, especially in dilute solutio.ns. Katsaros and George( 29) mention, for example, that

solutions made from one sample of acetonitrile, carefully purified by standard methods, gave considerably higher conductivities than solutions made from a sample obtained from another source. Since high conductivities and low molecular weight values are readily explained in this way,

one must reject such values when presented with a reliable observation of low conductivities and high molecular

weight values for similar solutions, which are otherwise difficult to account for. So, in view of the results of Katsaros and George,( 29) one can only conclude that the

tetrahalides are also non-ionic even in solution in

moderately polar solvents such as nitrobenzene and aceto-nitrile. Further evidence·for the non-ionic composition in solution is provided by the observation( 2i) that


This conclusion does not invalidate the finding that strong electron donors, including solvents such as

dimethylformamide, which form very stable complexes with the tetrahalides, promote ionisation (see chapter 2). Further, in defence of Miss Veitch's work, it must be pointed out that she used a wide variety of solvents, and observed 1:1 electrolyte conductivities for the tellurium tetrahalides in all of them (except alcohols, which react with the halides: see chapter


results which were corroborated by Greenwood and co-workers. It seems most unlikely that several different solvents would all contain about the same concentration of impurities. Moreover, Miss Markham, using similarly purified solvents, obtained much lower conductivities for. the selenium halides in acetonitrile and nitromethane. It therefore appears unlikely that the conductivity of the tellurium halides could have resulted entirely from reaction with impurities in the solvents, since the same impurities would almost certainly react with the selenium halides.

Thus, while one must accept the low conductivity figures of Katsaros and George,( 29) there are features of the earlier, higher values which cannot be simply explained in terms of impurities, particularly water, in the solvents.


been made, which may help to resolve some of the anomalies in spectra and solution measurements. It has been

proposed( 24•26) that the structure of the solids may lie somewhere between the ionic and covalent extremes, i.e., that there is one abnormally long bond in the MX


molecule, Examples are known( 37,3B) of selenium-halogen and tellurium-halogen distances in crystals which are

longer than the sum of covalent radii but shorter than the sum of van der Waals radii. It has also been proposed(33) that the tellurium tetrahalides have a polymeric structure, analogous to that described for tellurium tetrafluoride, which is partially broken down in solution, to an extent depending on the electron donor properties of the

solvent. This wo.uld explain the observed trimerisation of these halides in benzene solution.

In summary, while the question is still far from settled, a small preponderance of the evidence, including the two most recent papers,< 22 • 29) favours covalent rather than ionic structures for the tetrahalides. It may be noted that there is no single piece of unequivocal

evidence for an ionic structure in any state.

A final question which must be considered is that of solvation of the tetrahalides in solution, particularly in concentrated solutions of tellurium tetrachloride.




1 .60


22L~0 comp.







tube tuoe

Liquid Assoc sociated ?











Point de cornu.



detoYI'\ TcF4-\?. · SeC12 + Tee


Solution As soc d As soc d SeBr2 +


Comnarison of snectra of Sec1


and SeGl; +

SeCl~, SeC12

'"-e-Ref. 20 25 29 29 27 27 26 21 22 35

R(s) i.r.(s) Lr .. (a) i.r.(b) R(s) Lr.(s) R( s) i .. r.(s) R(f) R(s)

(560) 590


474 430 437

441 415 390

388 384

371 375 371

362 366 361 367 354

348 341 348 348 351 346 348


275 281 268

258 249

206 205 206 200

190 172 168

164 165 163


127 132


80 69

-=.~=~--r.,_-,.~. - . ._..__..~_._,..



+ Comparison of snectra of Tec1


and TeC13


TeCl4 TeC1














R(s) (s) i.r.(a) i.r. (s) R(s) Lr.(s) i.r.(s) i.r.{b) R(s) i.r.(s) R(c) .(a) R(f) R(s)














































143 150.



1 1











(a) in acetonitrile solut (c) in opionitri solution (s) sol




The work described in this chapter was carried out in conjunction with Professor H.B. Gray of the California Institute of Technology, U.S.A.

Structure and Bonding

Molecules containing four-, five- and six-coordina~

selenium and tellurium show many features of structural interest. The hexahalo anions are particularly

important in view of their deviation from the structural trends set by the compounds of lower coordination numbers.

In all known examples of four-coordinate selenium and tellurium containing a lone pair of electrons, the lone pair is sterically active, giving a pseudo-trigonal-bipyramid structure (e.g. di-p-tolylselenium dichloride and dibromide, (40) tellurium tetrachloride, (G) dimethyl-tellurium




and diphenyltellurium

dibromide.(42)) Similarly, in the only known structures containing five-coordinate selenium and tellurium, those of SeOC12.2pyt•idine(37) and polymeric tellurium tetra-. fluoride,(13) the ligands are


a square-pyramidal arrangement with the central atom approximately at the centre of the base. This suggests pseudo-hexacoordination with a sterically active lone pair (although such a


pyramid is also a possible shape.(43))

The valence shells of the hexahaloselenate and -tellurate(IV) anions contain six bonding pairs and one non-bonding pair of electrons. On this basis, Gillespie and Nyholm predicted(B) that


and similar molecules (e.g.


IF6 and XeF


would· have a structure

based on seven-fold coordination, giving a shape markedly different from a regular octahedron.(44) However, some crystal structures of compounds containing hexahalo

complexes of selenium and tellurium(IV) and antimony(III) have been determined,(45-4B) and none shows any significant deviation from the regular octahedral arrangement. This is supported by infrared,< 21 ,49,50) Raman,< 21 ,49,51)

nuclear- quadrupole resonance (5 2 ' 53) and M"ossbauer (


studies ..

2-The electronic configuration of the MX


anions has been discussed by several authors. Pauling and Beach(55) give a simple description, proposing that the

2-lone pair in SeBr6 is in the 4s orbital, while an

octahedral 4P


4d25s hybrid is used for the six pairs of


2-bonding electrons. Similarly in Tex


the orbitals


compared with those in the corresponding tin compounds, which do not have the lone pair (5 6)







2- 0






This description, with full use of d orbitals, as well as outer s orbitals, represents one extreme in

bonding theories. The other extreme is the three-centre-four-electron bond approach in which ~either d orbitals nor outer orbitals are used at all. This approach also predicts both the regular octahedral structure and the· long bonds, and has been used to describe the bonding in XeF6,(57) as well as being generally applied to compounds

in which an atom has more than four pairs of valence electrons.(5B)

Other authors have taken intermediate views,

proposing partial participation of d orbitals and some-times of outer s orbitals. (e.g. 59,60)

It will be noted that in all of these theories the lone pair of electrons is assumed to reside in what is essentially a pure 4s (or 5s) orbital, which is reasonable because in a regular octahedral molecule the electrons could be expected to have a totally symmetrical

distribution about the central atom.( 61 ). Such an


theory, (B) of which an implicit assumption is that all of the electron pairs in the valence shell of any atom occupy positions on the 11surface" of the atom, their

separation being angular, rather than radial as is assumed in the alternative descriptions above.

2-Gillespie has more recently classified the Tex6 anions as being among the few examples where stereochemistry is governed by interactions between ligands rather than

interactions between valence-shell electron pairs. ( 61 ) It has been pointed out(47) that the interactions between the bromine atoms in the proposed seven-coordinate

2-structure of TeBr6 would be very large.

2-Since XeF6 , which would be isoelectronic with TeF6 , is known to be distorted,( 62 ) it is unfortunate that

2-TeF6 cannot be prepared for comparison. Gillespie's

2-explanation of the regular structure of the other Tex6 anions is based on inter-ligand interactions, and because TeF7, IF7 , XeF7 and XeF8 are all claimed to exist, he predicts that


should be distorted.( 61 ) The

2-question of the existence of TeF6 is discussed further in chapter 2.

For the purposes of the present·discussion, the bonding orbitals of the central atom will be considered


molecular orbital of' a1g symmetry. While this will have little bonding effect, because both bonding and

antibonding components of the resulting molecular orbital are occupied, the extent of the interaction, and of' the delocalisation of' the lone pair1 is of' greatest interest.

The bonding scheme is shown in a simplified form in Figure 1.


2-§uitability of Se~


and Tex


for S~ectral Stud~

Spectroscopic studies of hexahalo complexes of post-transition elements have concentrated mainly on d10

systems, particularly Sn(IV) and Sb(V). ( 63, 64) Of the relatively few studies on d10s2 systems, a recent

investigation(64) has shown that the interpretation of absorption spectra of halo complexes of Sb(III) and

Bi(III) is complicated by uncertainties in the structure of the species under investigation. The most extensive spectral studies( 65) on halo complexes of Se(IV) and Te(IV) have been in aqueous media, where hydrolysis is a serious problem, and there is evidence that the predominant species is not




or hydroxy complexes. ( 21


However, two independent Raman spectral studies are in agreement that the anions TeCl~- and TeBr~- , at

least, retain their octahedral structure in acetonitrile solution .. ( 21



measured of compounds in acetonitrile and dichloromethane solutions and in the solid state as the thin

non-crystalline films obtained by evaporating dichloromethane solutions of the tetra-n-butylammonium salts on a silica plate. ( 67)

Theory for d10s 2 Systems in Oh_§xmmetrl

From a spectroscopic point of view, the central

atom may be regarded as M4+, whose ground state configura-tion will be 4d105s25p0 for tellurium, i.e. 1A1g in Oh symmetry. Atomic spectra reveal( 6B) that the order of orbital energies for uncomplexed d10s2 atoms and ions

is nd << (n+1)s < (n+1)p. While nd ~ (n+1)s transitions have been observed in the d10s 0 systems cu+ and Ag+, (69)

the inner d orbitals rapidly become more tightly bound toward the right of the periodic table, and even in

spectra of group IIIb metals (e.g. Tl+) transitions from them are no longer observed. Furthermore, the spectra of the complexes Snc1 6 and SbC16 below 50,000 om-1 have been assigned entirely as p~ ~ a1g (X ~ s) charge




Te orbitals Oh molecular




Halogen orbitals

Figure 1. Simplified molecular orbital diagram



u (n+1~p ( n+ 1 ) p -A\ ~~-





g (n+1)s ( n+1) s •·

P~u P~g

(b) poo· 0

g nd nd


orbitals orbitals



Highly simplified diagrams showing previous assignments for d10s 0 (as in SnXg- (63)) and d10 s 2 (as

in SbXt- (64)) hexahalo complexes. In each case, the lowest-energy observed band has been assigned to


From a theoretical point of view, then, two types of electronic excited states are expected: those that are derived from the free ion s 2 ~ sp transitions (the form of the s and p orbitals being essentially preserved in the complex) and should therefore be more localised on the central atom; and those that involve strong charge tre.nsfer between the halides and the central atom.

The s2


sp transition in atoms and ions leads to excited


3P0, 3p1, 3P2 and 1P1 • The transition to


P0 is severely forbidden in octahedral symmetry; that to 1P

1 is fully allowed; and the 3p1 transition is spin-forbidden but gains fairly high intensity

through intermediate coupling with 1P1 in elements with large spin-orbit coupling constants. The


1 transition should therefore have higher intensity in tellurium

than in selenium. The 3p2 transition also gains

observable intensity through vibrational coupling. The excited states in octahedral complexes are analogous. The sp states are designated 1T1u and 3T1u in octahedral symmetry.

Since charge transfer bands of d10s 0 complexes have beeri assigned to transi tiona o;f; :the P?Cu ~ a1 g (X ~ s)


63, 6


it is reasonable to assume the order of coulomb energies pn(X) < (n+1)s. Furthermore, previous


10 2 interpretations of the electronic spectra of d s

complexes have assumed(G4,G5) that the lowest observed




it t i -+ t a ( s -+ T'l) , not p7C -+ t a exc a one are a1


1 u v g 1 u ( p (X) -+ p ( M) ) ( see Figure 2) •

Thus the overall order of coulomb energies is nd <

p (X) < ( n + 1 ) s < ( n + 1 ) p. The full sequence of molecular orbital energy levels is shown in Figure 3. The halogen p orbitals are split into a and 7C components, the

difference in energy ( p a< p7C), which is observed in

charge transfer spectra of hexahalo complexes of transition metals, being interpreted as the "crystal field" splitting of the halogen orbitals by the positively charged

"metal".( 69, 70) The order of p7Cu(X) levels is taken from magnetic circular dichroism evidence. (63 ) Further details of the splitting of the ligand p7C orbitals are assigned partly on the basis of the present work. The non-bonding ego orbital is assumed to be at higher energy than the bonding a orbitals.

It has been pointed out(71) that there is a formal analogy between the expected overall pattern and that of


1 .

n-a d ow-sp1n transition metal complex MX



where the



11 internal" a

1 go -+ t1 u o the t 2 g -+ e g, excitation;


however, is that a

1 go -+

transition takes the place of an important difference,


t 1 u a is Laporte The predicted spectrum for SeX~- and



is as follows, in order of increasing energy:

(1) 1A1g



T1u unobserved


of low intensity in


2-and of moderate intensity in Tex

6 ,

with two or three components (transition (a) in Figure 2B).

(2) 1A


1T of higher intensity (also transition (a)).

1g 1 u

( 3) There are two allowed charge transfer transitions

.. ,

of the type ?Cg(X) ~ t1 u o~ (transition (b) in Figure 2B) ,



namely t1g?C ~ t1uo and t 2g?C ~ t1uo • These excited. states are likely to appear close together and may be difficult to resolve. Intensities will be very high.


At considerably higher energy, the two allowed


ag(X) ~ t

1uo' transitions (transition (c) in Figure 2B),



namely ega~ t1ua and a1go ~ t1uo. Their intensities will also be very high.

2-The electronic absorption spectra of Teo1


measured at 300°K and 18°K from a thin film of (n- Bu

4N)2Teo16 are shown in Figure


As set out in Table 3, spectra

2-of Te01


in acetonitrile and dichloromethane solutions are very similar to the thin film spectrum, and it appears that the octahedral structure of the anion is preserved in these solvents.

It is reasonable to assign the bands at 24,240, -1



t1 u1C



t 1 1C __.,;;,

g / . t2g1C


ega t1 ua----? a 1ga _.711'

Te orbitals

--.,.. , ;

Oh molecular orbitals




Halogen orbitals


Relative orbital energies estimated for


~ CD 0


!)) ::s Q (!)




... c:+ I";S !):) I";S ~


... c+ CD























































\ 30,000 - -1 v, em



. •. .







• •



. .


· . .






Fi~. Elec onic absorption spectra of thin films of (n-Bu


N)+ salts.


300bK;. - - -


18°K; •••••









I l









I ' 3p I ~







. P,





Top: spectrum of Tl+ in KI(G9)

77°K, - - - 4°K.

Bottom: spectrum

of thin film of (n-Bu


3p excited state in the Te4+ ion has components 3P0 , 3p

1 and 3p2 with energies of 75,109, 78,023, and 85,997 cm-1 respectively above the ground state. ( 6B) Comparison

+ 2- ( )

of the spectra of Tl and Tec16 Figure 5 shows

clearly that the 25,740 and 30,670 cm-1 bands correspond to transitions to the components of 3T1u related to 3P1

3 1 . 3

and P

2 respectively. But since the transition


0 ~ P0 has not been observed( 6B, 69) in Te4+ ions or octahedral Tl(I), and the analogous transition was not observed in Sb (III) complexes, even in low symmetry, ( 6


1 t appears that the band at 24,240 cm-1 cannot be assigned as the third component of 3T1u.


The principal maximum at 34,530 em is assigned to the transition to the 1T

1u excited state. The observed splitting of the 1T1u system is too large to be a

vibrational progression and suggests that the excited state is significantly distorted from the octahedral

structure. One would expect promotion of an electron to


the strongly anti bonding t1 u a level to result in a sharp structural change. It is difficult to predict the shape of a molecule in an electronically excited state, but a c 2v structure in which one Cl-Te-01 grouping becomes non-linear has been suggested.


1 ) In c2v, allowed transitions to 1A1 , 1B1 and 1B2 excited states are expected. This


in the splitting as the temperature is decreased to 18°1<. At the higher temperatures, because excited vibrational

states in low frequency 01-Te-01 bending modes are substantially populated, vertical excitations to lower vibrational levels of the distorted excited state are possible. As the "hot" transi tiona are removed, the two side maxima would be expected to move closer to the

principal maximum, as is observed.

A similar mechanism may account for the presence of more than one band below 28,000 cm-1 ; i.e., the shoulder at 24,240 cm-1 seems more likely to be due to splitting

of the lower 3T

1u component by lowering of symmetry than to a separate component of 3T1u. It may be noted that the low temperature spectrum also shows these two bands moving closer together.

The low-temperature spectrum also helps to co.n;firm the assignment of the 30,670 cm-1 band in TeOl~- to the



transition to the T1u component related to P

2; since its position is not altered, but its intensity is much lower than at 300°K, consistent with its intensity being due mainly to vibrational coupling. By comparison, the band related to 3p1 has about.the same intensity at 18°K as at 300°K, since it does not depend on vibrational coupling.


40,000 cm- 1 , which has no maximum within the instrumental limit at 52,000 cm-1 Both acetonitrile and film spectra show a distinct shoulder on this intense system, at about 44,200 em -1 • Also, a weak band at 3 ,2 0 em 8 1 -1 . 1s

revealed in the film spectrum when the 1T1u system narrows at 18°K.. Although several assignments are possible, it is most probable that the weak band at 38,210 cm-1


represents the forbidden t 1 u 1c -l-t 1 u o: transition, while

the moderately intense 44,200 cm-1 shoulder and the very intense maximum (above 52,000 cm-1) should be designated Support for this interpretation is obtained from comparison with the

2-TeBr6 spectrum.

The spectra of


in a film at 300°K and 77°K and in solution at 300°K are given in Table 3. The spectrum of the film at 300~ is also shown in Figure 4. Again, the spectral data do not suggest any change in the

octahedral structure of the anion in the solutions studied. As in the case of


the lower energy portion of the

2- 3 1

TeBr6 spectrum shows the T1u and T1u transitions (Figure 4) . Components of' the 3T

1 u system in acetonitrile solution appear at 21,400, 22,L~OO, and 26,200 cm-1 • In the film spectrum, the splitting of the 1T

1u band into three components is again evident, but it is much less . pronounced than in Teo1


this suggests that TeBr~- is more nearly octahedral in the 1T1u excited state.

2-The charge transfer spectrum of TeBr


in aceto-nitrile consists of a show.der at




of moderate

-1 intensity (s



and an intense peak at


em . These two features correspond to the shoulder and strongly

2-rising absorption observed for Tec1


with the band

positions substantially red shifted. The large shift confirms that these bands are of the halide-+ Te charge transfer type. The enormous intensity of the



2-band (e


of TeBr

6 .

leads one to assign it as a o->o* transition, namely ego-+ t



(72a) This means that the allowed t


g~ ~ t

1uo* transition must give moderately intense shoulder at



se to the

As mentioned above, the most interesting point to explore is the extent to which these electronic spectra shed light on the question of the bonding involvement of the "inert" pair of s electrons in these d


s 2 complexes.

At the very least it may be concluded that the a 1g . O* electrons are not spectroscopically 11inert", because relatively low energy transitions are observed involving them. (cf. ref. 59). The much lower energy of 1A

1g -+ 3T1u


changing the nature of the s ~ p transition.

Although a detailed analysis is not possible, >:'

delocalisation of the a1ga electrons to the ligands may be inferred from two distinct features of the spectra. (1) The variation in the energy of the 1T1u and 3T1u


transitions (free ion >> Te016 > TeBr6 must be attributed largely to a nephelauxetic effect, i.e.,

delocalisation of the tellurium orbitals over the halides. A larger series of complexes of the d10s2 ion Pb 2+

follows the nephelauxetic series very closely.(72b) Gray(7i) has suggested that the rather substantial


2-energy difference between the Te016 and TeBr6 bands (approximately 5000 cm-1) evidence of a particularly


large delocalisation of the a1ga electrons, while Jprgensen(72b) has suggested that decreases in the E(sp) - E(s 2) separation in the series In(I)-Sn(II)-Sb(III)-Te(IV) and Tl(I)-Pb(II)-Bi(III) may also be


attributed to covalent bonding through the a

1go orbital.

Detailed discussions of these effects are not available. (2) The separation between the 1T

1u and 3T

1u bands is also significant. The magnitude of this term separation depends on the effective spin-orbit interaction parameter,



the electrons spend on the central atom, and the a 1 go* orbital will be of the form





"'o*= blJrs + -6- I)vpo(X) •

Thus a decrease in the term separation is evidence of

a decrease in


and therefore in b, which in turn implies, from the above equation, a greater delocalisation to the ligands. The separation (1P

1 - 3P1) in the Te4+ ion is 33,684 cm-1; (68 ) the separation of the analogous terms is approximately


cm-1 in


and 7500 cm-1 in


A similar trend in Tl(I) as the free ion, and in. crystal lattices where 1 t is surrounded octahedrally by halide ions, has been accounted for in the same

way. (69)

Another feature of interest is the approximately 7000 cm-1 separation of the t 1g7t-+ t 1uo'* and ego-+ t 1uo}:: charge transfer transitions. This has been compared(71) with the 10,000 - 15,000 cm-1 separations of


-+ego* and a (X) -+ e o* found in d6 low-spin(70) and d10

u g

complexes. (63) However, a different result for d 1 0s 2 complexes is not unexpected, because different lower levels are involved in the comparison. It is generally accepted( 63 ) that the lowest charge transfer transition




on a level of t


uo"~ symmetry. However, it may be noted

2- 1

that t1u?C -~o t1uo* in Te016 has been placed at 38,000cm-, which separates it from the a -~o a* type transitions by at least 10,000 cm-1


2-SeX6 and Tei 6

2-Salts of Ser6 are difficult to prepare in a pure state, and too unstable to give reliable electronic spectra. Nor could reliable spectra of the very


2-scopic Se016 salts be obtained. Ammonium and


2-butylammonium salts of SeBr

6,. and Tei 6 were prepared, and their spectra recorded in acetonitrile and dichloro-methane solutions. These are listed in Table 3. While the spectra show intense charge transfer bands, the s ~ p transitions are represented only by very weak shoulders on the low energy side of these bands. Assuming that the anions retain their structure in solution, this might

2-Tei6 since extrapolation from the have been expected in

2-spectra of Te016 and TeBr6 shows that the charge transfer 2-and s -> p systems should be almost coincident. Similarly

2-in SeBr6 the two systems may be co2-incident, because comparison of atomic spectra of


and Te4+ shows ( 68 ) that the s -~o p transitions have considerably higher energy in se4+ than in Tel.t.+.

However, it is rather doubtful whether the ·species

2-which gave these spectra were in fact SeBr


Hendra and Jovic(49) report that, while



2-TeBr6 give Raman spectra in solution which are similar


2-to those of the respective solids, SeBr6 and Tei 6 do not. Moreover, .the spectrum of the supposed Tei~- is disturbingly similar to that of


Unfortunately, film spectra of the tetra-n-butylammonium salts could not be obtained because (n-Bu

4N) 2Ter6 crystallised, and (n-Bu




decomposed rapidly in air, presumably




Electronic Absorption Spectra of TeC12- 2- 2- and 6 , TeBr6 , Ter6


Complex Exnt~ A. ,m!J.




4) 2TeC16 OH30N Lj.1 0 24,L~OO sh 700a soln., 300°K 385 26,000 1 '900

323 31,000 3,000

304 32,900 sh 8,ooo8 287 !34,900 9,000

277 36,100 8,600

227 44,100 sh 5,000a (n-Bu

4N)2TeC16 OH2012 . L~1 0 24,400 sh 1 , 300a soln., 300°K 385 26,000 3, 700

321 31,200 6,000

301 33,200 15,0008 288 34,700 21,000 276 36,200 19,000 (n-Bu


Table 3 Continued

( n-Bu4N) 2:TeCl6 Thin film~ 405.7 24,650

18°K 391.2 25,560

325 .. 2 30,750 304.1 32,880 290.0 34,480 280.3 35,680 261 .. 7 38,210 226 .. 4 44,170 (NH

4) 2TeBr6 CH3CN 467 21,400 sh 1,800


a oln. , 300


447 22,400 3,900 381 26,200 sh 1, 200a 330 30,300 19,000 268 37,300 sh 10,000a 225 44.soo 77,000


4N) 2TeBr6 CH2Cl2 467 21,400 sh 1,oooa soln., 300°K 446 22,400 2,600


Table 3 Continued (n-Bu

4N)2TeBr6 Thin film, 473 21 , 100 300°K 449.2 22,260 384 26,000 355.4 28,130 336.3 29,730 320.5 31,200 272 36,760 237 42,190


4N) 2TeBr6 Thin film, LJ.62 21,700

77°K 451 22,200

381 26,300 351.2 28,470 336.4 29,730 324.2 30,850 270 37,030 235 42,550


4) 2Ter6 CH3CN 575 17,400 sh 100a




Table 3 OH

2012 soln., 300°K

CH 30N soln., 300°K ·

OH2o12 soln. , 300 °K

Continued 575 360 294 251

395 267

495 390 271

17,400 sh 27,800


39,800 sh

25,300 sh 37,500

20,200 sh 25,600 sh 36,800

400a L,.3,000 84,000 8,oooa

500a 80,000



Table 1 Constitution and transition temperatures of the tetrahalides.
Figure 1 Simplified molecular orbital diagram 6 . 2-
Table 2 Comnarison of snectra of Sec14 and SeGl; SeCl~,
Figure 1. Simplified molecular orbital diagram 2-


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