Bonding in transition metal organometallic complexes

The present-day qualitative approach to the bonding in transition metal organometallic complexes is based on a combination of two models. The first, the 18-electron rule, has already been met in Chapter 2 (see Table 2.8)—iigands that bond to a transition metal in a low valence state normally do so in such a way that the metal atom is, formally, surrounded by 18-electrons . Of course with a few exceptions, the classical complexes of the Werner type and which are the subject of most of this book do not obey this rule, so that it is clear that much hinges on the low valence state requirement. Equally, the fact that most transition elements form neutral bis-fy -cyclopentadiene complexes, M(C5H5)2, shows that even for organometallic complexes of transition metals it is only an approximation. Many examples of molecules with more, and many examples with fewer, than 18 valence-shell electrons are known. Those with more tend to be readily oxidized, those with fewer tend either to be sterically hindered or readily add further ligands. 

The second model, which is widely applied to organometallic complexes of transition metal ions, is the requirement that the bonding interactions are symmetry-determined. A very common and important case is provided by the bonding of cyclic conjugated C R systems—of which C5H5 is perhaps the best known example. These are discussed in more detail in Appendix 13 

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