Coordination studies of the halides of groups (IV), (V) and (VI)

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COORDINATION STUDIES OF THE HALIDES OF GROUPS (IV), (V) AND (VI)

By

STEVEN RONALD WADE B.Sc

Submitted to the U niversity of Warwick in p a rtia l fu lfilm en t of the

degree of Doctor of Philosophy.

Parts of the work contained in t h is th e sis have been published

in the s c ie n t if ic lit e ra t u r e with the following references

S. R.

S. R.

S. R.

S. R.

S. R.

S. R.

Wade and G. R. W ille y ,

Wade and G. R. W ille y ,

Wade and G. R. W ille y ,

Wade and G. R. W ille y ,

Wade and G. R. W ille y ,

Wade and G. R. W ille y ,

Inorg. Nucl. Chem. L e t t ., 1978, 1£, 363.

Inorg. Chim. Acta, 1979, 35, 61.

J . Less-Common. M et., 1979, 68, 105.

J . Inorg. Nucl. Chem., 1980, 42, 113.

Inorg. Chim. Acta, 1980, in press.

J . Inorg. Nucl. Chem, 1980, in p ress.

The donor-acceptor chemistry of some Groups ( IV ) , (V) and (V I) halides was investigated with a number of lig ands.

Trimethylamine provided donor complexes with Hf (IV ), Mo (V ), Mo ( I I ) , As (TIT) and Sb (TIT) h alid es. With Cp2T iC l2, M0OCI4 and AuCli trim eth yl- amine was found to a c t as a reducing agent. The use o f MCl4.2NMe7

CM = Z r, Hf) adducts as intermediates in complex synthesis was b r ie f ly studied, and the Megtren adducts (MCl2Megtren)z+2Cl~ (M = T i , Z r, Hf) iso la te d .

Hexamethylphosphoramide (HMPA) gave adducts with both M (IV) (M = T i , Z r, Hf, Sn) and M C H I) (M = T i , V, Cr) ch lo rides. From the TiCVHMPA system, adducts of 2:1, 1 : 1 and 1 ; 2 stoichiom etries were iso late d .

O C I3.3HMPA was iso la te d as both fac and mer isomers. The (Me2N)xC l3_xPS Cx = 1 ,2 ,3 ) se rie s are much weaker donors than HMPA* although they

provided a number of adducts with hard Lewis acid s. Changes in the e le ctro n ic structure of the phosphoryl and thiophosphoryl ligands that occur on complexation were examined by *H, 13c, 31p an(j i r spectroscopy.

(Me2N)2CH2 and bis(dipheny1phosphino)methane gave 1 : 1 adducts with MCIV) h a lid e s. The dominant reaction in the CpTi(NMe2)3 and Cp2Zr(NMe?)o/ MX4 systems, however, was group exchange to give the respective CpM halides. Me2Si(NMe2)2 gave both 1:1 adducts (M (IV) = T i , Z r, Hf, Sn) and group exchange (M = S t , Ge, T i j . The donor-acceptor chemistry of Me2SiCl(NMe2) and C l3Tt(NMe2) was B r ie f ly studied, and was found to be dominated by disproportionation.

(CH20)3 and CCHoS)3 act as weak monodentate donors towards hard M (IV) a c id s. Dynamic behaviour in the SnCl4.2(CH2S ) i system was investigated by 'H NMR and interpreted in terms of both ring inversion and donor-site interchange. (MeBNMe)3 was found to a ct as a bridging bidentate o-donor in (MeBNMe)3.2T iC l4.

C O N T E N T S

Page

ABSTRACT

CHAPTER 1 - INTRODUCTION

1.1 The Early Tr r s it io n Metals 1

1.2 Structure and R e a ctiv ity of the Early t-M etals 4

1.3 Coordination Chemistry of the E a rly t-Metals 20

1.4 Halides and Complexes of Group IVB and Group VB 28

CHAPTER 2 - COMPLEXES AND REACTIONS OF TRIMETHYLAMINE

2.1 Introduction 31

2.2 Reactions of HfX^ (X = B r, I ) and ZrCl4 with NMe3 34

2.3 Reactions of NMe3 with CpgMClg (M = T i, Z r, Hf) 36

2.4 Reactions of NMe3 with some Group VB Halides 37

2.5 NMe3 Complexes of some Halides and Oxyhalides of Mo 40

2.5.1 Reactions of NMe3 with MoC13 and MoCl^ 40

2 .5 .2 Reactions of MoOC13 , MoOC14 and NMe3 42

2.6 Reactions of NMe3 with Zirconium Trih alid e s 45

2.7 Reaction of NMe3 with AuC13 46

2.8 Ligand Exchange Reactions of NMe^ Complexes 47

2.8.1 Substitution reactions of MC1^.2NMe3 (M = Z r, Hf) 47

2 .8 .2 Complexes of MC14 (M - T i , Z r , Hf) with tren and Megtren 48

2.9 Experimental 51

CHAPTER 3 - COMPLEX CHEMISTRY OF DIMETHYLAMINO SUBSTITUTED PHOSPHINE CHALCONIDES

3.1 Introduction 62

3.2.1 Adducts o f (NMe2)3P0 with MC14 (M = T1, Sn, Zr and Hf)

and Snl4 68

3 .2 .2 MC13.3HMPA complexes (M = T i , V, Cr) 76

3.3.1 Complexes and reactions of (NMe2)xC1 3-xPS ( x = 1»2»3 )

with metal chlorides 80

3 .3 .2 Complexes with SnCl4 , Z rC l4 , HfCl4 and SbClg 81

3 .3 .3 Reaction of (NMe2)2ClPS with BC13 93

3.4 Experimental 95

CHAPTER 4 - C0MPLEXATI0N AND EXCHANGE REACTIONS OF SOME DIMETHYLAMINO SUBSTITUTED GROUP (IV ) COMPOUNDS

4.1 Introduction 102

4.2 Complexes o f (Me2N)2CH2 with MC14 (M = T i , Sn), VCI3 and

C rC l3.2NMe3 110

4.3 Reaction o f bis(diphenylphosphino)methane (dpm) with T i C l4 112

4.4.1 Exchange reactions between Me2Si(NMe2)2 and MC14 (M = S i , Ge) 115

4 .4 .2 Exchange reaction between Me2Si(NMe2 )2 and T iC l4 118

4 .4 .3 Mechanism of Si-N bond cleavage 119

4 .5 Complexes o f Me2Si(NMe2 ) 2 with MX4 (M = T i , Zr, Hf, X = C l,

M = Sn, X = C l, Br) 121

4 .6 Reactions o f Me2SiCl(NMe2 ) with MC14 (M = Sn, T i) 125

4.7 Reaction of C l3TiNMe2 with MC14 (M * S i , Ge, Sn, T i) and

Donor Solvents 127

4 .8 Exchange Reactions between CpTi(NMe2 )3 and Cp2Zr(NMe2 )2 with some

Covalent Metal Tetrahalides 128

4.8.1 Reaction o f Cp2T iC l2 with Me2NLi 128

4 .8 .2 Reaction of CpTi(NMe2 )3 and Cp2Zr(NMe2) 2 with MC14 (M = S i , Ge,

Sn, T i , Z r , Hf) 129

Page

CHAPTER 5 - SOME COMPLEX CHEMISTRY OF INORGANIC HETEROCYCLES

5.1 Introduction 146

5.1.1 Bonding In inorganic heterocycles 146

5.1 .2 Donor chemistry of alternation heterocycles 150

5.2.1 Reactions of (CH20)3 with MC14 (M = T1, Sn) and ZrCl4.2NMe3 153

5.2 .2 Reaction of (CH20)3 with MCl3.2NMe3 (M = V, Cr) 154

5.3 Complexes of (CH2S) 3 with Covalent Metal Chlorides 155

5.4 Reactions of (Me2S10)3 4 with some Covalent Metal Halides 163

5.5 Complexes of Hexamethylborazine w ith Covalent Metal Halides 164

5.6 Comments on the Synthesis of (RgTiPR )x 167

5.7 Experimental 170

CHAPTER 6 - COMPLEXES OF SUBSTITUTED OXAMIDES, THIOOXAMIDES AND MALONAMIDES WITH METAL TETRAHALIDES

6.1 Introduction 176

6.2 Complexes of Substituted Oxamides, Thiooxamides and Malonamides

with MC14 (M = T t , Sn) 179

6.3 Experimental 186

APPENDIX A 189

APPENDIX B 197

I would l ik e to express my thanks to Dr G* R. W illey fo r his

continued help and encouragement in the three years 0976-1979)

during which the work described in t h is th esis was undertaken.

t would l i k e to thank the techn ical s t a f f of the U n iv ersity of

Warwick for t h e ir a ssista n ce , in p a rtic u la r the glass-blowing

department under the directio n of Mr E. Burgess fo r an inexhaustible

supply of * p ig s‘ . The contributions o f Mr D. Rylance and

M ile. C. M abillard who a ssisted in the work described in Chapter 6 as

part of a t h ird year p ro ject, are acknowledged.

A very sp e cia l thankyou goes to Ms. Vicki Narbett fo r the typing

of th is manuscript.

A postgraduate award from the Science Research Council i s also

T A B L E S

<T* *,v

CHAPTER 1

Table 1.3.1 P-21

Table 1 .3 .2 P - 21

Table 1 .3 .3 p.22

Table 1.4.1 p.29

CHAPTER 2

Table 2.4.1 p. 38

CHAPTER 3

Table 3.1.1 p.64

Table 3 .1 .2 p.65

Table 3 .1 .3 p.66

Table 3 .1 .4 p.66

Table 3.2.1 p.70

Table 3 .2 .2 p.73

Table 3 .2 .3 p.74

Table 3 .2 .4 p.78

Table 3.3.1 p.83/4

Table 3 .3 .2 p.86/7

Table 3 .3 .3 p.89

CHAPTER 4

Table 4.1.1 p.104

C la s s if ic a t io n of Bases

C la s s if ic a t io n of Lewis Acids

Heats of Formation of MX^ + 2L -*■ MX^.2L in

Standard S tates

Donor Numbers of some Selected Ligands

p .38 NMR Data fo r MX3.M!1e3 and MX3.2NMe3

V e rtic a l Io n isatio n Energies fo r (NMe2) xC l3_xP0(S)

Thermodynamic Data fo r (Me2N)3PY + I2

Selected o-Phosphoryl Substituent Constants

Thermodynamic Data fo r the Reaction

(NMe2)xCl 3-xP0(S) + l 2 = ( NMe2) xC l3. xP 0 ( S ) .I2

IR Spectra o f M(IV) HMPA Adducts

Low IR Spectra of some Sn (IV ) Species

NMR Data fo r M (IV) HMPA Complexes

IR Spectra o f M (III).HMPA Adducts

IR Spectral Data of (NMe2)3PS Complexes

IR Spectral Data of (NMe2) 2ClPS Complexes

^H and 31P Spectra of the (NMe2 ) 3PS and (NMe2) 2ClPS

Complexes. (MeN02 s o ln ., T = 298 K)

Table 4 .1 .2 p. 104

Table 4 .1 .3 p.105

Table 4.4.1 p.116

Table 4.5 .1 p.124

CHAPTER 5

Table 5.8.1 p.157

CHAPTER 6

Table 6.2.1 p.181

Table 6 .2 .2 p.184

Table 6 .2 .3 p.185

Table 6.3.1 p.188 13

C-H Coupling Constants fo r Selected Amines

220 MHz NMR Chemical S h if t Data fo r Various

S ila n e s, Aminosilanes and Aminogermanes (2%, CgDg)

NMR Spectra of MX4 .L (220 MHz in CgDg) (R elative

In te n s itie s given in brackets)

p.157 IR Spectra of (CHgS^ Adducts

P rin cip le IR Bands of MCl^.B Complexes

] H NMR Spectra of MC14.B, C0^02/TMS 90 MHz)

13C NMR S h ift Data of the Ligands (a) CD^OD and

(b) CDCI3

'A nalytical Data of MC14 .B Complexes

F I G U R E S

I

r

CHAPTER 1

F ig . 1.2.1 p.7 MgXg(a) and MgX1 2(b) Metal C lu s te r Centres

F ig . 1 .2 .2 _P-9 C ry sta l Structure of NbClg

F ig . 1 .2 .3 p. 10 C ry sta l Structure of ZrC^

F ig . 1 .2 .4 p . l l S tru ctu re of NbCl^ (39)

Fig . 1 .2 .5 P-12 S tru c tu ra l Units of (a) s-TiC l^ and (b) a , y and

« T i C l3

F ig . 1 .2 .6 p.14 C ry sta l Structure of Nb-jClg

F ig . 1 .2 .7 p . 15 Schematic Representation of a Sheet of (MoCl2)C lg^2

Octahedra Showing Short (— ) and long (---- ) Mo-Mo

Bonds

Fig . 1 .2 .8 p . 15 C ry sta l Structure of MoBr3

F ig . 1 .2 .9 p.16 S tru ctu re of WCI3 (50)

F ig . 1.2.10 P-17 The Cadmium Diiodide Structure

F ig . 1.2.11 p.18 Unit C e ll of C rC l2 Showing Short (2.39 8) and Long

(2.93

X)

F ig . 1.3.1 p.25 C ry sta l Structures of TiCl^.ZPOCl^ (83) and (TiCl^ .

P0C13 ) 2 (87)

F ig . 1 .3 .2 p.27 Schematic Structure of T lC l^ .Z d ia rs

F ig . 1.4 p.30 Usual Geometries of Group VB H alide Adducts

CHAPTER 2

F ig . 2.1.1 p.32 X-ray Structu re of C rC l3.2NMe3 (69, 78)

F ig . 2.4.1 p.39 Proposed Geometries of AsBr3.NMe3 and SbX3.2NMe

(X * C l , Br)

F ig . 2 .4 .2 p.40 Proposed Structure of SbX3«NMe3 (X « C l, Br)

F ig . 2.7.1 p.47

F ig . 2.8.1 p.50

CHAPTER 3

F ig . 3.1.1 p.63

F ig . 3.2.1 p.72

F ig . 3 .2 .2 p.73

F ig . 3 .2 .3 P - 7 9

F ig . 3.3.1 p.91

F ig . 3 .3 .2 p.92

F ig . 3 .3 .3 p.93

CHAPTER 4

F ig . 4.1.1 p.103

F ig . 4 .1 .2 p.106

F ig . 4.2 .1 p. 1 1 2

F ig . 4.3.1 p.113

F ig . 4 .3 .2 p.114

F1g. 4.4 .1a p.119

F ig . 4.4.1b p.119

F ig . 4 .4 .2 p.119

F ig . 4 .4 .3 p.119

F ig . 4.5.1 p.122

F1g. 4 .5 .2 p.124

CHAPTER 5

F ig . 5.1.1 p.147

Reaction Scheme for AuC13 + NMe3 Au

Proposed Structure of MCl4.Megtren

X-ray Crystal Structures of ^NJ-jPO and (Me2N)3PSe

Proposed Structures of T iC l4.2HMPA, TiCl^.HMPA and

2TiCl4 .HMPA

Possible Structures fo r SnCl^.HMPA

Proposed Structures of MC133HMPA

] H NMR Spectra (.220 MHz, CD3N02 s o in ., r e l . TMS)

for SbCl5.(NMe2)3PS + (NMe2)3PS 1:1

NMR Spectra (220 MHz of SbCl5.(NMe2)3PS,

CD3N02 soin.

^H NMR Spectrum of (BClgNMeg^. (CDC13, 220 MHz)

Bonding Schemes for M-NR3 Species

Crystal Structure of T iC l3(NEt2) (268)

Proposed Structure of MCl4.(Me2N)2CH2 (M = T i , Sn)

Proposed Structure of TiCl^.dpm

*H NMR Spectra (220 MHz, CD3N02) o f dpm and T iC l4 .d

^4-Centre* Mechanism

'Sommer-Type Mechanism*

6 Coordinate Intermediate

5 Coordinate Intermediate

Proposed Structure of MX4.Me2Si(NMe2 )2

Possible Structures for MX4.Me2Si(NMe3 )2

Schematic Diagram of Borazine Showing B -*• N

o-Donat1on, N + B ir-Donatlon and Overall Charges on

F ig . 5.1 .2 p.148 Formation of Skeletal dir-pw Bonds

F ig . 5.1.3 p.149 "Island" Model fo r Cyclophosphazines (R2PN) 3

F ig . 5.1.4 p.151 Crystal Structure of Cr(CO)3.(EtN BEt)3

F ig . 5.1 .5 p.152 Proposed Structure of (Me2SiNH)3MCl3 , (M = T i , V)

F ig . 5.3.1 p.156 C rystal Structure of (CH2S)3

F ig . 5.3.1 p.158 C rystal Structure of SbCl3.(CH2S) 3 (projection on

xy plane)

F ig . 5.3.3 p.159 Proposed Structures of MC14.2L (M = Sn, T i ) ,

AuCl3 .L and SbClg.L

F ig . 5.3.4 p.160 s-T rith ia n e Showing Axial and E q u ito ria l Protons

F ig . 5.3 .5 p.160 V ariable Temperature NMR Spectra of (CH2S)3

(.400 MHz, CDC13)

F ig . 5.3.6 p.160 V ariable Temperature ^H NMR Spectra of SnCl4.3(CH,

(220 MHz, CDC13)

Fig . 5.3.7 p.161 SnC1^.2(CH2S)3 Showing Magnetically Inequivalent

.Protons

Fig . 5.4.1 p.166 Proposed Structure of 2TiCl4 .HMB

CHAPTER 6

F ig . 6.1.1 p.176 X-ray Crystal Structures of (C0NH2 )2 and (CSNH2 )2

Fig . 6.1.2 p.177 C rystal Structure o f N,N-dimethyldithiooxamide

F ig . 6.2.1 p.182 Proposed Structures of MCl^.B

F ig . 6.2 .2 p. 183 P ossible Isomers o f MC14 (Y2C2N2(NR)2)

APPENDIX A

Fig . A1 p.192 Apparatus fo r the Preparation of MX4 (M ■ H f,

X = C l , B r, M - Z r, X = Cl)

Fig . A2 p.192 Furnace Tube

APPENDIX B

Fig . B1 p.197 Vacuum Line

Fig. B4 p.198

Fig. B5a p.198

Fig. B5b p.198

Fig. B6 p.201

Fig . B7 p. 201

Single (a) and Double (b) Bomb Glass Reaction V e sse ls

Loading on the Vacuum Line

Extraction on the Vacuum Line

UV Cell

ABBREVIATIONS

L a monodentate Ligand

B a bidentate Ligand

DEDTO N,N -diethyldithiooxam ide

DEM N,N -diethylmalonamide1

DEO N,N -diethyloxamide

d iars o-phenylenebisdimethylarsine

DMDTO N,N -dimethyldithiooxamide•

DMO N,N -dimethyloxamide•

dpm Bis(diphenylphosphi no)methane

Megtren T ri s(.2-di methyl ami noethyl) ami ne

HMB Hexamethylborazi ne

HMPA T r i s (d i methy 1 ami no) pho s ph i ne oxi de

py Pyridine

THF Tetrahydrofuran

THT Tetrahydrothrophene

tren T r i s (2-ami noethyl)amine

Abbreviations used in the description of IR Spectra

s strong

m medium

w weak

br broad

1. INTRODUCTION

The follow ing introductory sectio n s are concerned with the

binary h alid es of the T i , V and Cr subgroups, th e ir stru ctu ral

chemistry and donor-acceptor properties in non-aqueous media. The

halides of Groups IVB and VB plqy a le ss important role in terms

of the su b ject matter of th is th e s is , and are therefore examined

only in a general sense (Section 1.4) with the relevant chemistry

being introduced to the text as and where appropriate.

1.1 The E a rly Transition Metals

The elements of groups IVA, VA and VIA exh ib it many common

p ro p erties, and are c o lle c tiv e ly known as 'the ea rly tra n sitio n

m etals'. A ll have r e la t iv e ly few electrons outside an in e rt gas

configuration, and low e ffe ctiv e nuclear charges. The resu lta n t

expansion in the size of the d -o rb itals gives r is e to low ionisatio n

p o te n tia ls, and the p o s s ib ilit y of high oxidation states with no or

few e le ctro n s. One cla ss of compounds which tends to take fu ll

advantage o f the possible range of oxidation states i s the h a lid e s,

this being p a rtic u a lly noticeable for the second and th ird row

elements. The halides play an important part in early t-metal

chemistry fo r a number of reasons. In high oxidation sta te s they

are often e it h e r monomeric, or possess weakly bonded la t t ic e s , and

can thus provide a wide range of donor chemistry. This is in contrast

to the e a rly t-metal chalconides, fo r example, the chemistry of

which i s dominated by polymeric la t t ic e s and polyanions, both

being unreactive in a donor-acceptor sense.

Another feature of early t-metal chemistry concerns metal-metal

elements has important repercussions for the coordination chemistry

of the lower valence s t a t e s , and i s further discussed in Section 1 .1 .3 .

In those h alid es where metal-metal bonding i s not present,

cleavage of the metal-halogen bond i s unusually f a c i l e , and although

th is presents some technical problems in m aterial handling, the

inconvenience is more than o ffset by the extensive f ie ld of halo-

substituted d e riva tiv es made possible by such reactio ns. The

halides can, therefore,be u t ilis e d as syn th etic precursors fo r a

whole range of 0 , N, S , and P bonded species as well as in the

rapidly expanding region of organometallic chemistry. Since the

problems of hydrolysis and a eria l oxidation have larg ely been

overcome by the widespread introduction of high vacuum techniques,

the e a rly t-metal halides have become the focus of considerable

attention, and several texts have appeared, the works of Canterford

and Colton (1 ,2 ) , Kepert (3) and Clark (4) being of note.

1.1.1 Occurrence o f oxidation states of e a rly t-metal halides

Despite the a v a ila b ilit y of much thermodynamic data (5 ), no

simple pattern has y e t emerged for the prediction of stable

oxidation states e ith e r along a row (e .g . T i , V, Cr) or down a

group (e .g . T i , Z r, H f). One point th at i s f a i r l y c le a r , however,

i s the a b ilit y of highly electronegative elements such as flu o rin e

and oxygen to s t a b ilis e high oxidation s t a t e s , whereas bromine and

iodine favour low oxidation sta te s. For example, the known stab le

binary halides of molybdenum consist o f

:-MoFg

m°f5 MoC15

MoF4 MoC14 MoBr4

are as yet unknown.

S im ila r ly , although VC14 is thermally unstable, with slow

decomposition to VC13 and chlorine being found even at room

temperature, examples of sta b le V(IV) and V(V) species e x is t in

the oxohalides (V(IV)0C12 , V(V)0C13 and V(V)02C1).

1.1 .2 Metal-metal bonding in early t-metal halides

Metal-metal bonding i s an important feature of e a rly t-metal

chem istry, dominating the stru ctu ra l (1.2) and complex (1.3)

chemistry of the second and third row elements in low oxidation

s ta te s. Kepert (3 , 6, 7) and Vrieze (6) have reviewed the

su b je c t, and have summarised on an em pirical b asis the factors

which tend to produce metal-metal bonding rather than mononuclear

or halogen bridged sp ecies:

(a) High energies of atomisation and d-orbital overlap, both

of which are favourable towards metal-metal bonding, are more

pronounced for the early t-metals than fo r elements la t e r in the

d block. Thus metal-metal in teractio n s are found in B -T iC l3 but

not VC13 or C rCl3, in HfCl3 , TaCl3 and WC13 but not OsCl3 or

I r C l 3, in NbCl^ and a-MoCl4 (m arginally), but not in b-MoC14 or

TcC14. As these properties are enhanced in the second and third

row elements, a s im ila r trend i s found on going down a p a rticu la r

group. For example, metal-metal bonding e x ists in TaCl3 and

NbCl3, but not VC13 , in TaCl4 and NbCl4 but not VC14 and in WC14,

with o-MoC14 (m arginal), but not b-MoC14.

s ta te s , where contracting d -o rb itals and a reduction in the number

o f electrons availab le fo r m-m bonding are unfavourable in flu en ces.

In addition, a high metal:ha1ogen ra tio would produce weakening

o f a projected m-m in teractio n on s t e r i c grounds. Examples of

t h is phenomenon are numerous and e x is t in T i , and a ll the

second and third row elements of the f i r s t three periods.

(c) A decreasing la t t ic e energy in the order F>Cl>Br>I

s t a b ilis e s metal-metal bonding in the reverse order. I t i s , therefore

found that while m-m bonded bromo and iodo species are comparatively

numerous, NbgF i5 appears to be the so le example involving flu o rin e (6)

An estimation of the re la tiv e cap acities of F vs. I for m-m bonded

sp ecies, purely on the b asis of the number of known compounds,

should be approached with caution, however, as F favours high

oxidation states which are usually F bridged polymers for reasons

examined in (b). However, the se rie s MoX3 (X = F , C l, B r, I ) does

provide a v a lid example, with MoI3 and MoBr3 consisting of

metal-metal bonded chains, MoC13 of chains of metal-metal bonded

metal atom p a irs , and MoF3 showing no signs of any interaction

at a l l .

1.2 Structure and R eactivity of Ea rly T-Metal Halides

I t i s a generally held p rin cip le that the structure of the

e a rly t-metal halides pleys an important ro le in th e ir r e a c t iv it y ,

with decreasing r e a c tiv ity accompanying an increase in the

polymeric nature of the metal halide. At a f i r s t approximation th is

that have to be broken in order to achieve a mononuclear metal

centre. A metal halide with a highly polymerised la t t ic e would

then be expected to be less vigorous in i t s reactio ns than that

of a sim ila r mononuclear or weakly associated sp ecies. With

the Ti ch lo rid es, fo r example, monomeric TiCl^ i s a strong

acceptor, forming donor adducts with a large number of both

strong and weak donors. The chioro-bridged stru ctu re of a -TiC l^ ,

however, requires the use of stronger donors and more forcing

conditions, while the donor chemistry of the even more highly

polymerised T iC l2 i s minimal and poorly understood. The introduction

into the la t t ic e of a strong metal-metal in te ra ctio n has a

large influence on r e a c t iv it y . Thus the metal-metal bonded

6 modification of T iC l3 is not only chemically in e r t , but i s

capable of surviving an e le c t r o ly t ic oxidation-reduction c y c le ,

apparently without change (8).

Despite th is general correlatio n between stru ctu re and

r e a c t iv it y , i t is impossible to predict the reactio ns of a p a rticu la r

metal halide purely on stru ctu ral grounds. The factors involved

are many and complex, and knowledge of some a re a s , p a rtic u la rly

the nature of the reacting metal halide su rfa ce , i s v irtu a lly

non-existent.

1.2.1 Structural aspects

Structural data have now been obtained fo r the majority of

the e a rly t-metal h a lid e s , and from th is work several points have

emerged.

(a) Within a sin g le oxidation s t a t e , sp ecies with eith e r

stru ctu ra l type has been studied by sin g le crystal X-ray methods

with the remainder of the group being confirmed as isomorphous

by comparison of powder X-ray photographs.

(b) Many of the early t-metal haltdes exh ib it polymorphism.

Th is phenomenon can e ith e r take the form of metal-metal bonded vs.

non-metal-metal bonded m odifications as in B-and a - T iC l3 re sp e c tiv e ly ,

or i t can be a difference in ha lid e ion packing (e .g . a , y» and 6

forms of T iC l3) . M odifications of the second type, present no

change in the immediate environment of the metal ion, and there

i s no evidence for any d ifferen ce in re a c t iv it y between polymorphs.

Consequently, modifications o f th is type w ill be omitted from the

te x t.

(c) The small s iz e and high electroneg ativity o f fluorine

gives r is e to metal fluo rides which are atypical in both a chemical

and a stru ctu ra l sense. As the fluorides have l i t t l e d ire c t

relevance in terms of the su b je ct matter of th is d isse rta tio n ,

they have been excluded from the following sectio n s. The fluorides

have been the subject of se ve ra l reviews (9-13).

(d) Recent studies have found that a number of metal halides

previously thought to be of a simple composition are in fact non-

sto ich io m etrie, e .g . Z rC l3, Z rC l2 and H fl3 (14-16). The extent to

which th is phenomenon applies to the other metal halides has yet

(e) The lower halides o f Nb, Ta, Mo and W are 'c lu s te r'

compounds, based on an octahedral arrangement of s ix strongly

metal-metal bonded metal atoms. A ll the c lu s te r compounds have

h a lid e ions eith e r positioned above each face of-the core

octahedron, or bridging each edge, to give MgXg and M6X12 units

resp ectively (F ig . 1 .2 .1 ).

Fig . 1.2.1 MgXg (a) and MgX^2 (b) Metal C lu ste r Centres

Additional halide ions can be bonded to the vertice s o f the Mg core

e it h e r in a terminal cap acity, or be engaged in bridging to adjacent

c lu s t e r s . To sim plify the descrip tio n of such c lu s t e r s , the

formula 1s often w ritten in the form:- (c lu s te r core)(number of bridging

halides/number of clu sters bridged by each halide)(number of terminal

would be described as (MOgClg)Cl4/2C l2. Sim ilarly» WC13 (F ig . 1 .2 .9 )

would be (WgCli2)C lg . This nomenclature has also been applied to

no n-cluster metal-metal bonded compounds (e .g . MoCl3 (F ig . 1 .2 .7 ) is

(MoC12)C18/2).

The following sections describe the stru ctu ra l chemistry by

oxidation s ta te :

Oxidation State VI

The v o la t ile liq u id s MFg (M = Mo and W) and c r y s t a llin e

so lid s MoClg, WXg (X = C l , Br) are the only known examples of the

+6 s ta te . In the s o lid s t a t e , the structures show a high degree

of polymorphism, but a ll contain a metal atom in an octahedral

monomeric environment (17). The hexahalides being coordinatively

saturated have no w ell established coordination chem istry, but often

give complexes as part of th e ir reduction products, e . g .,

MeCN

W C l g

T H F

— > W C l 4 . 2 M e C N ( 1 8 )

( 1 9 ) w d g / W U L I j t l H r

Oxidation State V

Of the known pentahalides, NbXg (X = C l , B r, I) (2 0 ), TaXg (X = C l,

B r, I ) , MoClg (21) and WXg (X = C l , Br) (22) are a ll iso stru ctu ra l

9.

Fig . 1.2.2 Crystal Structure o f NbClg (20)

D istortion of the MXg octahedra occurs hy mutual repulsion of the

two Nb atoms from th e ir octahedral centres. Nblg (23) and VFg (24)

are more highly associated forming in f in it e halide bridged chains.

A ll of the pentahalides are v o la t ile , the gas phase stru ctu re being

based on a monomeric trigonal hipyramidal unit (25).

Oxidation State IV

A wide range of stru ctu ra l chemistry is presented by the +4

s ta te . I t is also the highest oxidation state in which metal-metal

bonding is of any sig n ifica n c e . Titanium i s the only f i r s t row element

that forms thermally sta b le tetra h a lid es. TiCl^ i s a colourless fuming

liq u id , with a monomeric tetrahedral stru ctu re , both in the gas phase

(26) and in solution (27) although Raman studies of the neat liq u id

in d icate some association via chloride bridges (28). A monomeric

structure was also found for the low melting point so lid s TiBr^ (29, 30)

and T i I4 (3 0 ); the X-ray crystal structures show the la t t ic e to

slow ly disproportionates to VCl^ and C l2 a t room temperature and

rap id ly on heating in vacuo (3 1 ). As with T iC l4 a monomeric

tetrahedral configuration i s adopted in both solution and the gas

phase, but here d isto rtio n occurs due to Jahn -T eller e ffe cts (32).

VBr4 is unstable, decomposing above -45°C w hile CrCl4 has only been

observed as a constituent of high temperature gas phase e q u ilib r ia ,

and i s unknown in the condensed phase.

The second and th ird row halides c r y s t a llis e in only two b asic

stru ctu ra l types. ZrC14 , ZrBr4 , HfCl4 and HfBr4 are isomorphous,

the stru ctu re being ty p ified by ZrCl4 (F ig . 1 .2.3) (33, 34).

Here, octahedrally coordinated Zr atoms each share two edges with

adjacent octahedra via chloride bridges to form an in f in it e one-dimensional

'zig -zag ' polymer. Variations in the bond lengths of the Zr-Cl bridges

O C l

between alternate octahedra are caused by d isto rtio n s from ideal

octahedral geometry rather than metal-metal bonding, which was

eliminated on the b asis of a value for the shortest Zr-Zr

interatomic distance of 3.962 8.

C ry sta llin e ZrF^ and HfF^ have structures based on eight

coordinate Zr or Hf. Each square an tip rism a tica lly coordinated

Zr (Hf) i s linked to neighbouring anti prisms through a ll eight

vertice s to form a three-dimensional s o lid (35).

In the gas phase, electron d iffra ctio n studies reveal the MX^

(M = Z r, Hf; X = F , C l , B r, I ) polymers d isso ciate to give regular

tetrahedral species as with the TiX4 se rie s (36-38).

The second stru ctu ra l type (F ig . 1.2.4) accounts f o r the so lid

phase chemistry of a-MoCl^, MoBr^, MX^ (M = Nb, Ta, W; X = C l , Br)

a ll of which are iso stru c tu ra l (7 , 39).

11.

•N b

O C l

NbCl^ consists of edge sharing octahedra forming an in f in it e chain,

the Nb being displaced towards each other in altern ate octahedra

forming d is t in c t Nb-Nb p a ir s , i . e . metal-metal bonding. Nbl^ (40)

i s stru c tu ra lly re la te d , but a ltern a te Nb-Nb distances of 3.31 and

MoCl^ has also been obtained in a second 8 form (41). This was

found to be related to a - T i C l j , having the same b asic l a t t i c e , but

with only three-quarters of the metal s ite s occupied. The Mo atoms

are located in s t a t i s t i c a l l y disordered 'domains'. In each domain,

h a lf the metal s it e s are vacant in alternate layers to give

2+ 2

-alternating sheets composed of (Mo2Clg) and MoClg u n its. There

is no evidence for any metal-metal bonding in the 8 m odification.

Oxidation State I I I

In the +3 sta te a l l the early t-metals except vanadium and

chromium exh ib it metal-metal bonding in one or more m odifications.

T iC l3 has four m odifications, a ll of which have been studied by

Natta and coworkers (F ig . 1 .2 .5 ) (42).

4.36 8 were considered long, and the metal-metal in te ra ctio n , th erefo re,

weak.

(b)

o Tt

Cl

The mauve a , y and <5 forms are c lo se ly related forming layer

la t t ic e s in which each Ti atom i s octahedrally coordinated to s ix Cl

atoms. In the a form the chloride ions are hexagonally close

packed, and in the y form cu b ica lly close packed. Prolonged

grinding of eith e r the a or r forms re su lts in fi-TiCl^ which has

a disordered la t t ic e co n sistin g s t a t i s t i c a l l y o f 63% hexagonal close

packing and 37% cubic close packing. The stru ctu re of the brown e

metal-metal bonded form is quite d iffe re n t. As with a - T iC l3 , the Cl

atoms are hexagonally clo se packed, but the Ti atoms are arranged

in such a way that the stru ctu re can be considered as an in f in it e

one-dimensional polymer formed by T iC lg octahedra sharing opposite

fa ce s. Although the T i-C l bond lengths in a ll four modifications

are id en tica l (.2.45 - 2.46 J?), the T i-T i interatom ic distance

in the 6 form is much shorter (2.91 X) than that found in the a , y

and 5 forms (3.54 8) , equalling the T i-T i distance in m eta llic

t i taniurn.

T iB r3 e x ists in both a and & forms, but T i l 3 , ZrCl3 (4 3 ),

ZrBr3 (4 4 ), Z r l3 (45) and H fl3 (45) are known only in the metal-metal

bonded B m odification.

VC13 is a mauve c r y s t a llin e so lid with an « - T iC l3 type stru ctu re.

VBr3 forms continuous s o lid solutions with VC13 over the whole

composition range, and i s , therefore, considered as isomorphous with

VC13 (3 , 4 ). The isomorphous C rC l3 and CrBr3 stru ctu res (46) are

made up of close packed halide sh eets, with Cr atoms occupying two-thirds

of the octahedral vacancies in every second layer to form repeating

14.

Niobium and tantalum form no simple t r ih a lid e s , MX3 g being

only one constituent in a continuous phase between upper and lower

lim its (e.g. for ‘NbClj*, NbCl2 g7 - NbClg 1 3). The lower lim it

correspond to M3Xg, the stru ctu re of which is known in the case of

Nb3Clg (Fig. 1.2.6) (47, 48).

Fig . 1.2.6 Crystal Structure of NbgClg

The main feature of this structure is a strongly metal-metal bonded

Nb trian gu lar c lu ste r which occupies three out of four site s in a

sheet composed of Cl octahedra. A niobium-niobium distance of

2.81 8 was found which is shorter than the metal-metal distance in

NbCl^ (3.06 8) or in the metal i t s e l f (2.94 8) . Increasing the Nb:Cl

ra tio resu lts in the disruption of the Nb3 clusters to form diamagnetic

Nb2 units u n til an upper lim it of NbCl3 13 is reached. Perhaps not

su rp risin g ly , there is no evidence for any coordination compounds with

a-MoClj is related to both Cr C l3 and CrBr3, the structure (^39)

consisting of Mo atoms occupying two-thirds o f the octahedral vacancies

in a sheet of cubic close packed chloride ions. Metal-metal bonding

between p a irs of Mo atoms d isto rts the M0C I3 octahedra, i . e . Mo2C12C1q^2

Instead of forming sheets of octahedra by edge bridging (as in MoC13) ,

MoBr3 (49) and MoI3 (49) co n sist of in f in it e chains of MoXg octahedra

sharing oppositve faces with the two adjacent u n its, sim ila r to B -TiCl3.

Metal-metal bonding between pairs of Mo atoms create Mo2Brg u n its,

i . e . (Mo2Br3)Brg^2 . as can be seen from the altern ate short and long Mo-Mo

bonds (F ig . 1 .2 .8 ).

(F ig . 1 .2 .7 ) . Consequently there i s one short (2.77 8) and two long

(3.71 8) Mo-Mo intern u clear d istan ces, as opposed to the three equal

C r- C r i n t e r n u c l e a r d is t a n c e s in C r C l , or C rB r-,.

F i g . 1 . 2 . 7 Schematic R e p re s e n ta tio n o f a Sheet o f (MoCl? )C l

c

Octahedra Showing Short (— ) and Long (---- ) Mo-Mo Bonds.

®Mo

O B r

¿92

consisting of Mo atoms occupying two-thirds of the octahedral vacancies

in a sheet of cubic close packed chloride ions. Metal-metal bonding

between p airs of Mo atoms d isto rts the MoC13 octahedra, i . e . Mo2C1 2C1 8^2

(F ig . 1 .2 .7 ) . Consequently there i s one short (2.77 8) and two long

(3.71 8) Mo-Mo intern u clear d istan ces, as opposed to the three equal

C r- C r i n t e r n u c l e a r d is t a n c e s in CrC l-, or C r B r , .

*->

J

F i g . 1 . 2 . 7 Schem atic R e p re se n ta tio n o f a j h e e t o f ( M o C ^ j C l ^ , Octahedra Showing Short (— •) and Long (---- ) Mo-Mo Bonds.

Instead of forming sheets of octahedra by edge bridging (as in MoC13) ,

MoBr3 (49) and MoI3 (49) co n sist of in f in it e chains of MoXg octahedra

sharing oppositve faces with the two adjacent u n its, s im ila r to e -T iC l3.

Metal-metal bonding between pairs of Mo atoms create Mo2Brg u n its,

i . e . (Mo2Br3)Brg^2» as can be seen from the alternate short and long Mo-Mo

bonds (F ig . 1 .2 .8 ).

<I< )1

WCIg (50) and WBrg (51) are both c lu ste r compounds. The tric h lo rid e

(F ig . 1.2.9) contains a Wg metal bonded octahedral core with tw elve-

edge bridging Cl atoms and s ix terminal Cl atoms, i . e . (WgC 1 -|2)C16*

Fig. 1.2.9 Structure of WClg (50)

'WBr3 l (51) is based on a (WgBrg)6+ octahedral core with two bridging

O C , n

polybromide (Br4 ) groups per c lu s t e r , i . e . (WgBrg) ( Br4 )4^,2Br2 *

WIg i s isomorphous with MoBrg and MoIg. The stable c lu ste r structures

of WXg (X = C l, Br) prevents the d ire c t formation of coordination

complexes, although reduction of complexes of the higher halides does

provide a synthetic route to W (III) adducts (52).

Oxidation State I I

The structures of the dihalides o f titanium and vanadium a l l belong

F ig . 1.2.10 The Cadmium Diiodide Structure

Sheets of close packed halide ions form planes with the metal atoms

f i l l i n g the octahedral s it e s between alternate pairs of la y e rs.

The dihalid es of chromium (F , C l , Br, I) are related to a R utile

structure (3 ). At a f i r s t approximation the structure can be considered

as four chromium atoms occupying octahedral vacancies between close

packed halide sh e e ts. Considerable d isto rtio n of the CrXg octahedra

occurs, however, as a consequence o f the Jahn-Teller e f f e c t , producing

four short eq u ito ria l and two long a x ia l Cr-X bonds (F ig . 1 .2 .1 1 ).

L i t t l e was known of the stru ctu ra l chemistry of the Zr and Hf

dihalides u n til very rece n tly , when Corbett and coworkers (16)

succeeded in is o la tin g pure samples o f Z rC l2 and the clo sely related

non-stoichiometric Zr-j QgC l2 . Both these compounds are based on a

18.

O

O cr

F ig . 1.2.11 Unit C e ll of C rC l2 Showing Short (2.39 8) and

Long (2.93 8) Cr-Cl Bonds (53)

Z r l2 provides the sole example of a neutral Group IVA c lu s t e r , the

structure being based on an I bridged trigonal antiprism (ZrgI 12) (54).

Niobium and tantalum form no true d ih a lid e s, instead giving a

se rie s of clu ste rs based on (M6X3) 3+, (MgX12) 2+, (MgX12) 3+ and (MgX12) 4+

with average metal oxidation states of 1.8 3, 2.33, 2.50 and 2.67

resp ectively. The chemistry of these compounds is extensive, and has

been summarised (3 , 6).

MX2 (M = Mo , W; X = C l , B r, I) are isomorphous. S tru ctu ra lly they

are related to WC13 (F ig . 1 .2 .8 ) , but with four meridinal centrifugal

halide atoms bridging to four adjacent clu sters to form sheets, i . e .

(M6X8^X4/2X2 ( 39)- 4s W1th the niob'*un' and tantalum c lu s te rs, oxidation

y ield s a variety of c lu ste r based compounds, depending on the conditions,

e.g. (WgBrg)Br^2Br2 + Br2 -*• °~WgBr-|g, 6- WgBr-jg, WgBr.jg, WgBr-j^ and

Rather su rp risin g ly , the molybdenum and tungsten d ihalid es also have

a coordination chem istry, which is discussed in Chapter 2.

Oxidation State I

ZrCl and HfCl appear to be the only monohalides known. The

cry sta l stru ctu re (55) o f ZrCl (F ig . 1.2.12) and that of the isomorphous

HfCl show some rather remarkable fea tu res, including a Zr-Zr interatomic

distance (3.09 ft) which i s considerably le ss than that in the twelve

coordinate metal (3.19 ft).

The unique four layer structure (here presented as a projection on the

001 plane) consists of Cl-Zr-Zr-Cl stacked sh eets, with each Zr atom

The monohalides have no known coordination chem istry, as might be

expected from such a highly metal-metal bonded stru ctu re.

O Zr below olane Zr above plane

Fig. 1.2.12 C rystal Structure of ZrCl

having s ix in phase Zr neighbours (a t 3.42 ft), three Zr neighbours in

the sheet below (a t 3.09 ft) and three Cl neighbours in the sheet above

20.

1.3 Coordination Chemistry of the Early T-Metals

The complex chemistry of the tra n sitio n metal halides has long

been a su b ject of in te re s t. In p a r t ic u la r, the non-aqueous chemistry

of the anhydrous metal halides has recently received much attentio n.

There are two inherent problems associated with th is area of chem istry,

(a) the production of pure samples of the anhydrous metal h a lid e s , and

(b) handling such m aterials once prepared, as most are e a sily hydrolysed

by moist a ir to oxo and hydroxo sp ecies.

Coordination compounds are u su ally prepared by one of two b a s ic

methods. Complexes of monomeric or weakly associated sp ecies, e .g . NbClg,

are read ily obtained by simply mixing the reactants in a su itab le

non-coordinating so lven t, or by d isso lvin g the metal halide in excess

lig an d, the ligand here acting as solvent. With more highly polymerised

metal h a lid e s , ligand sub stitutio n reactions provide an altern a tiv e

syn th etic route, e.g .

MeCN, THF, py 3L

MX3 (M = T i , V, C r ) --- >MX3.3 S ---> MX3.3L

\ NMe3 l

\ --- > MX3.2NMe3---»MX-J.3L

1.3.1 Hard and so ft acids and bases

In the terminology of G. N. Lew is, complexation can be thought of

as an acid-base reactio n. Metal ions are examples of Lewis a cid s, and

considering the vast amount of potential donor-acceptor complexes, i t

would be advantageous to have a q u a lita tiv e idea of the s t a b ilit y or

v ia b ili t y o f a projected adduct. An attempt at such a prediction can

be made using the p rin cip le of hard and so ft acids and bases (HSAB).

Sidgwick (56) and la t e r Ahrland, Chatt and Davies (57) d ifferen tiated

21

p block bases (N, 0 , F) which they ca lle d c la s s A, and those species

preferring to complex to second and th ird row donors (P , S , As, S e ),

called C la ss B. Pearson (58, 59) extended t h is id ea, c la s s ify in g bases

in which the donor atom is at high e le c tro n e g a tiv ity , low p o la r iz a b ilit y ,

and hard to oxidise as 'hard' (C lass A) bases, and those of low

e le ctro n e g a tiv e ly , high p o la riz a b ilit y and e a s i ly oxidised as 's o f t '

(Class B) (Table 1 .3 .1 ).

HARD SOFT BORDERLINE

h2o, oh" , ro" , r2o r2s, rsh, rs" CgHgNHg, CgHgN

ROH, P043" , S042" SCN", R3P, R3As N3' , B r ', N02" , N2

C l ' , F" I ' , CO, (R0)3p

nh3 rnh2 , n2h4 c2h4, h"

Table 1.3.1 C la s s if ic a t io n of Bases

Lewis acids were likew ise categorised by t h e ir a f f in it y fo r e ith e r

hard or s o f t bases (Table 1 .3 .2 ).

HARD SOFT BORDERLINE

H+, L i +, Na+, K+ Cu+, Ag+, Au+ Fe2+, C o " , N i " , Cu2+

C r3+, Co3+, Fe3+, As3* Pd2+, Cd2+, Pt2+ Pb2+, Sn2+ Sb3+, B i3+

S i 4+, Sn4+, T i 4+, Zr4+, Hf4* T l3+, BH3 , GaCl3 Rh3+, BMe3 , S02

W04+, V02+, Mo03+ In C l3, Brg GaH3

BeMe2 , BF3 , B(0R)3

AlMe3, A1C13 , A1H3

Table 1.3.2 C la s s ific a tio n o f Lewis Acids

The character of an acid or base can be modified by changing the oxidation

sta te . Thus Ni(0) is a so ft a cid , N i( I I ) b o rd e rlin e , and N i(IV ) hard.

Addition of so ft ligands has the e ffe c t of softening the acceptor atom,

22.

Having estab lish ed the nature of both Lewis acids and Lewis bases,

the s t a b ilit y of an acid-base complex can be described by the rule

'Hard acids p refer to bind to hard bases, and so ft acids p refer to bind to

so ft b ases'. This i s a t o t a lly em pirical co rrela tio n .

Inspection of Table 1.3.2 reveals that the early t-m eta ls, where

present, are a ll hard, and the preference fo r donor atoms should thus

be in the o rd e

r:-N » P > As > Sb

0 » S > Se > Te

F > Cl > Br > I

L i t t l e qu antitative evidence e x ists to substantiate the expected trend,

but i t is a general observation that the number of known complexes with

0 and N donor atoms f a r outweighs those with so ft donors. Chung and

Westland (60) have, however, obtained thermochemical data fo r the

reaction of THF and THT with some Group IV te tra h a lid e s, which does

illu s t r a t e the HSAB p rin c ip le as in Table 1 .3 .3 .

COMPLEX -AH (KJ mol-1 )

Z rC l4.2THF 140.5 ± 2.0

Z rC l4.2THT 124.6 ± 3.8

ZrBr4.2THF 151.3 ± 1.7

ZrBr4.2THT 36.4 ± 1.7

HfC14. 2THF 144.6 ± 0.8

HfCl4.2THT 138.3 ± 0.8

Table 1 .3 .3 Heats of Formation of MX4 + 2L -*■ MX^.2L in Standard States

For HfCl4 , the small d ifference in enthalpy between the reaction between

THF and THT was seen in terms of Hf possessing more B (s o ft) character

23.

Several workers have attempted to explain the HSAB phenomenon,

the ir-bonding theory of Chatt (61) being p a rtic u la rly appropriate

fo r metal ions. Here, the important feature of so ft acids was the

presence of loosely held outer d -electro n s, which can form ir-bonds

w ith a ligand by donation into empty d -o rb itals on so ft bases such as

P , S e tc . Hard acids have t ig h tly held outer electrons but empty d

o r b it a ls , which again, can form ir-bonds by accepting electron density

from a hard donor atom. A hard base/soft acid interaction would be

disfavoured by the repulsion between the f ille d o rb ita ls on both

metal and ligand.

Care should be taken in applying the HSAB p rin cip le too s t r i c t l y ,

as the predicted and experimental behaviour of a complex do not always

co incid e. For example, the complex T iC l4. 2 thioxan (thioxan = S(CH2)40)

i s S bonded rather than the 0 bonded isomer expected from HSAB (62).

1 .3 .2 Geometry of coordination complexes

The following section is intended as a b rie f introduction to the

types of geometry encountered in th is area of chemistry, with (where

p o ssib le) v e rifie d examples of each c la s s .

Low coordination numbers (<5)

There are no true three coordinate halide sp ecies, although a

number of compounds of f i r s t row elements of the type MR3 (R = NR2 ' , I

SiMe3 , CH(SiMe3) 2, SR , e t c .) have been isolated v ia halide substitution

rea ctio n s.

Four coordination i s very ra re , the VX4" (X = C l , Br) ions observed

complexes prepared by Scaife (64) being amongst the few known examples.

5 coordination

Penta-coordinate complexes appear to be r e s t r ic t e d to f i r s t row

elements, possibly due to the increased s iz e (and thus a b ilit y to attain

higher coordination numbers) of the second and th ird row metals. A

common facto r in a ll of these complexes is the presence of a bulky ligand,

such as NR3, PRg, SR£, e tc . A ntler and Laubengayer (65) iso late d the

f i r s t 5 coordinate complexes in 1955 by the reaction o f T iC l4 with

NMe3 , giving TiCl^.NMe^ or T i C l3.2NMe3 depending on the reaction

conditions. Fowles and coworkers (66-69) extended the trimethyl amine

se rie s in both the +4 (T iB r4.NMe3 (6 7 ), VX4.NMe3 (66) ) and +3 (T iB r3.2NMe3

(6 7 ), HCl3.2NMe3 , M = V (66) , Cr (68) ) sta te s. These complexes are

discussed in greater d e ta il in Chapter 2. Further work by a number of

groups has sin ce produced 5 coordinate adducts with a varie ty of

nitrogen (66, 7 0 ), phosphorus (7 1 ), a rsen ic (6 2 ), oxygen (72, 73) and

sulphur (68, 70) donors.

Spectroscopic (74, 75) and cry sta llo g ra p h ic (6 9 , 76-78) studies

have confirmed a trigonal bipyramidal (D3h or C3V) stru ctu re in a

number of in sta n ce s, the lig an d (s) occupying an axial position in

each case. S t e r ic compression prevents the occupation of adjacent

s i t e s , i . e . 1 axial + 1 equatorial or 2 e q u a to ria l, elim inating the

p o s s ib ilit y of geometric isomerism. The tendency to dimerise in the

so lid sta te forming 6 coordinate species has also been noted (68, 70).

5 coordinate species have been reviewed (79-81). 6

6 coordination

One of the dominant facto rs 1n e a rly tra n sitio n metal chemistry

pseudo-octahedral 6 coordinate sp e c ie s. Consequently this i s the most common

geometry, adopted by the larg e majority of early t-metal complexes.

Examples can be found fo r a l l the early t-metals in a v a rie ty of

oxidation states and sto ichio m etries.

For an adduct at general formula MXy L2 , when x + y = 6 an octahedral

environment is obtained simply by the ligands occupying vacant s it e s on

the metal io n , e.g . MXg.L (8 2 ), MX4.I_2 ( 83 , 84), MX3L3 (85) and

MX2L4 (86). Complexes which are coordinatively unsaturated as monomers,

i . e . x + y < 6, can achieve 6 coordination by halogen b ridging, e .g .

(MC14. L ) 2, M = T i , L = P0C13 (8 7 ), MeN02 (88) , EtOAc (8 9 ), M = W,

L = C I3CCN (9 0 ), (MX4) 2L (9 1 ), (MX3L2) 2 (68, 70), (MX2L2 ) x (92).

Oct

#n

O

p

00

Fig . 1.3.1 C rystal Structures of T iC l4.2POCI3 (83) and ( T iC l4.P0Cl3 )2 (87)

When the number of ligands in a simple complex is two or more,

geometrical Isomerism can o ccu r, e.g. c is T iC l4.2P0Cl3 (83) and trans

WCl4.2py (84). In certain favourable in stan ces, e.g . with the In e rt

OOl

26.

L = NH3 (9 3 ), THT (9 4 ), dien (95).

7 coordination

Theoretical aspects of 7 coordination have been examined by

Kepert (3 ). A number of 7 coordinate adducts have been iso late d with

the polydentate arsine ligands o -p h en ylen ebis(d im ethylarsin e)(d iars),

methylbis(o-dimethyarsinophenyl)arsine(o-TAS) and t r i s - 1 ,1 ,l-(d im eth y l-

arsino methyl)ethane(v-TAS),

r ^ ^ Ix A s M e2

diars

AsMe

2

AsMe2/

AsMe

AsMe

2

o-TAS

^ C H

2

AsMe

2

Me—C— CH

2

As M e

2

v-TAS

^ C H

₂

As M e

2

Clark and coworkers have prepared the 1:1 adducts MXg.diars (M = Nb, Ta,

X = C l, Br (96); M = W, X = Cl (9 7 ), MC14(o-TAS) and MC14(v-TAS) (M = T i , V)

(98). These are diamagnetic and non-conducting. Equivalent AsMe sig nals

in the NMR spectra were taken as evidence of 7 coordination. The

p o s s ib ilit y of a more common 6 coordinate structure with rapid exchange

occurring in solution between free and coordinated donor s it e s (which

would give an id en tical re su lt to above) was, however, not discussed.

Confirmatory X-ray, or variable temperature NMR studies on these

species have yet to be carried out.

A number of potential 7 coordinate trimethyl amine adducts have been

isolated (WClg.NMe3 (99), MCl5.2NMe3 (M = Nb (100), Ta (101), Mo (102) )

L = NH3 (9 3 ), THT (9 4 ), dien (95).

7 coordination

Theoretical aspects of 7 coordination have been examined by

Kepert (3 ). A number of 7 coordinate adducts have been iso late d with

the polydentate a rsin e ligands o -p h en ylen eb is(d im ethylarsin e)(d iars),

methylbis(o-dimethyarsinophenyl)arsine(o-TAS) and t r i s - 1 ,1 ,l-(d im e th y l-

arsino methyl)ethane(v-TAS).

Clark and coworkers have prepared the 1:1 adducts MXg.diars (M = Nb, Ta,

X = C l, Br (96); M = W, X = Cl (9 7 ), MC14(o-TAS) and MC14(v-TAS) (M = T i , V)

(98). These are diamagnetic and non-conducting. Equivalent AsMe sig n a ls

p o s s ib ility of a more common 6 coordinate structure with rapid exchange

occurring in solutio n between free and coordinated donor s it e s (which

would give an id e n tica l re su lt to above) was, however, not discussed.

AsMe

2

AsMe

2

AsMe

2

o-TAS

2

diars

^ C H

2

AsMe

2

Me—C—CH

₂

As M e

2

v-TAS

^ C H

₂

As M

g

2

in the H NMR spectra were taken as evidence of 7 coordination. The

Confirmatory X-ray, or variable temperature H NMR studies on these

species have y et to be carried out.

A number of p otential 7 coordinate t r i methyl amine adducts have been

isolated (WClg.NMe3 (9 9 ), MCl5.2NMe3 (M = Nb (.100), Ta (101), Mo (102) )