Survey of Recent Theoretical Results

The SCF-A a-SW calculation also confirms the importance of the back donation component of this model, and a contour plot of the relevant molecular orbital is shown in Fig. 3. Johnson and his co-workers (193) have estimated that this component may contribute up to 25% of the total metal-olefin bond energy. These calculations have given a more satisfactory account of the electronic absorption characteristics of this 

Chemical considerations suggest that metal-olefin back donation will be less important for silver(I) than for platinum(II), and Basch s ab initio calculations on [Ag(C2H4)]+ (75) have confirmed this view. These calculations suggest that most of the electronic rearrangement of the ethylene unit in this complex ion can be accounted for by the polarization effects induced by the positive charge on the silver atom. Indeed, the bonding metal-olefin molecular orbital has only 6.5% Ag 5s orbital character. This result agrees nicely with recent ESR studies on y-irradiated silver-olefin complexes which estimate a 5s spin density of 4.6% for this molecular orbital 92, 93). 

Accurate structural analyses on metal-olefin complexes during the last 10 years (14, 106, 147, 162, 195) have suggested the following generalizations. 

Hybridization is a concept normally associated with the valence bond approach but may also be derived from molecular orbital theory by using second-order perturbation theory in the following way (78, 117, 190). 

The preceding perturbation theory analysis is supported by extended Hiickel calculations by Cusachs and his co-workers (166, 167, 237) on model platinum(II)- and platinum(0)-olefin and -acetylene complexes and Hoffmann and Rossi s extensive analysis of five-coordinate transition metal complexes (194). By using similar arguments, Hoffmann and Rosch (190) predicted that the planar conformation would be energetically preferred for d10 M(C2H4)3 complexes. This geometry has now been established by Stone (214) and his co-workers for the platinum-olefin complex shown in Fig. 12. 

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