Jr-backbonding

The water molecule is a good cr-donor ligand, while jr-backbonding is negligible. For this reason, higher-valent transition metals form the more stable metal complexes, but the nature of the metal by itself is important, too (Table 1). 

From these properties it is concluded that low-valent metals do not favor water in the ligand sphere. Not that Cr(CO)6 is a stable compound because of the outstanding jr-backbonding of carbon monoxide, while Cr(FI20)6 does not exist -quite contrary to the common [Cr(H20)6]3+. On the other hand, trivalent chromium does not form the (hypothetical) cationic carbonylchromium complex Cr(CO)6]3+. This is, in short, one major reason why so little is known about typical organometallic water complexes. 

The nature of the bonding in silylene-metal complexes, as compared with the better known metal-carbene complexes, is a question of considerable interest. MO calculations on H2Si=Mo(CO)s indicate that the Si—Mo bond consists of a cr-donor and jr-backbond component, like the carbon-metal complexes. The r-component is, however, weaker than for metal carbenes. Infrared C=0 frequencies for the base-free silylene metal complexes snpport this model. Theoretical considerations of the bonding in silylene-metal complexes are treated more fnlly in Section IV.E. 

The final step puts the two bridging carbonyls into the Fe3(CO)io unit. As expected, in the Fe2(p-C)2 pseudo-plane, the asymmetric combination of 2% orbitals of the bridging carbonyls significantly lowers the energy of the Jt af) Fel-Fe2 MO and transfers the two electrons from the Fel-Fe2 jr bonding MO to the Fel-Fe2 jr MO (Fig. 17d). Such backbonding stabilization prevents the formation of the... 

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