Those, such as Pt(0) and Rh(I), which show Increases in stability as electronegativity increases.

This effect is related to an alteration in the enthalpy term w hich decreases for group (1) and increases for group (2). The difference in behaviour is related to which of the components is the

m ost im po rtant in the synergic bond. In group (1) the o component is the more im portant. Therefore electron w ithdrawing substituents, which reduce the ease w ith which electrons can take

part in this a bond, decrease bond stability. In the second group the m ajor component is found to be the % back bonding. Thus the increase in the n acceptor ability of the alkene caused by electron withdrawing substituents strengthens the m etal alkene bond.

1.4.3 Properties of the M etal

M etal-alkene complexes are produced only when the

d-orbitals of the m etal of % symmetry are full, thus transition metals w ith d-electron configurations d^®, d®, d® or d"^ fit this criteria. The differing k bond stabilities exhibited w ithin this group are due to the abilify of the m etal to donate electron density to the alkene via the tc m etal alkene bond, increasing this ability increases the tc

bond stability(52). The ionization potential of the m etal gives a rough guide to the ab ility to donate electrons. An increase in ionisation potential is found to be m irrored in the stability of the complexes. It is essential when studying this effect th at there is no steric or electronic differences thus the complexes compared m ust have id en tical ligand system s, for exam ple the complexes

[L2M (C2H4)1 where M = Ni, Pd and Pt have been studied.

1.4.4 Effects of Non-Olefinic Ligands

The non olefinic ligands present w ill also effect the stability of the TC bond. A stable complex is more likely to be formed if the

non olefinic ligands are soft llgands(S3.54), This situation results in an increase in the energy of highest occupied m olecular orbital on

the m etal, reducing the AE value between it and tc* orbital of the

alkene. As a resu lt the interaction between these orbitals is increased and so the strength of the m etal-alkene bond is in c re a s e d (s e e example of orbital diagram. Figure 1.4).

np = f ' '^ s * ... IT*

" % —

M etal Complex Alkene

Fig. 1.4

Molecular orbital scheme for platinum (II)-paHadium (II) alkene complexes. Only the alkene ligand orbitals have been shown.

1.4.5 Effects of Cyclic Ligands

The effect of introducing cyclic ligands is m ainly steric bu t as ring form ation also effects the electronic environm ent of the alkene, electronic effects w ill also be of some significance. Five membered rings provide the most stable complexes, the fu ll order shown by ring alkene complexes being C5 > C7 > Ce > Cg(53). This difference is linked to enthalpy effects due to firstly the weakening of the m ultiple bond th at occurs in an attem pt to relieve

ring strain by lengthening of the double bond, secondly the ring strain producing deformations of the % orbital which enhances complex stability and thirdly the transannular hydrogen atoms are also found to promote complex stability although the method by which this occurs is not fully understood(^G).

1.5 The Development of Chelate Mono-Alkenes

As already stated the development of an adequate bonding theory and reliable methods of identification in the 1950's led to the rapid expansion of the chemistry of jc bound alkenes. Such has been the increase in the volume of mono olefinic complexes isolated and th e ir related organic applications th a t one review could not adequately cover the whole topic. Thus this section w ill deal almost entirely w ith the discovery and early developm ent of complexes containing alkenes for w hich the coordination is stabilised by chelation.

Despite the attention given to olefinic ligands and the study of th eir coordination to transition m etals, interest in the coordination and use of alkenes containing functional groups has been comparatively recent. Although the first complex isolated and determined to have a functional group close to the alkene group was "acechlorplatin" isolated by Zeise during the last centuryfS), it was not u n til the years following 1960 th at interest in this area really exploded. Furtherm ore it was observed th a t if the olefinic substituents contained a second moiety which possessed a lone pair of electrons then coordination was possible, in m any cases, via both the double bond and the lone pair donor to produce a chelate mono

C helate olefin co ordination was firs t noted w ith unsaturated amine ligands. Both Rubinshtein and Derbisher(^7) and Gelm an and Essen(58) showed th a t reaction of diallylam ine and am m onium te tra c h lo ro p la tin a te (1 1) produced two isolable complexes, PtLClg and (PtLCl2)2 where L = diallylam ine. The dim er

was determ ined to be [(P t-(diallylam in e)2)P tC 14] whereas the monomer was discovered to be [Pt-(diallylam ine)Cl2l. Furtherm ore the diallylam ine ligand was observed to be bound in a chelate fashion via the two olefin species as seen in Figure 1.5.

N H

Fig. 1.5

Such chelate diolefin complexes were found to produce examples of chelate mono olefinic ligands by subsequent nucleophilic reactions. The reaction (A) shown in Figure 1.6 shows nucleophilic attack of an alkoxide ion a t one of the olefinic carbon atoms

resulting in a attachm ent. However the reaction process did not end a t this point. The nucleophilic attack, in this example, was found to be accompanied by the loss of a chlorine atom leading to the dim erization (B), and ultim ately the isolation of a chlorine bridged

RO OR