Trends in Cyclopentadienyl−Main-Group-Metal Bonding

In contrast to transition metals (TMs), main-group metals (MMs) show a bewildering variety of bonding arrangements to cyclopentadienyl (Cp) groups, including electron-precise, electron-excess, and electron-deficient structures. From a theoretical examination of a large number of representative species, and a comparison with experimental structures, we have deduced three factors which combine to produce this variety. (1) The drive toward electron-precise structures is caused by the preference for electronic saturation (8e rule, similar to the TM 18e rule). (2) A high degree of ionicity leads (if space permits) to increased hapticity and hence to excess-electron structures; this is seen in complexes of Li, Na, Mg, and Al and, to a lesser extent, Pb and Bi. (3) The metal n s orbital has a preference for forming σ-bound structures. Thus, heavy involvement of this orbital in the bonding leads to a preference for σ-bound structures, even if these are electron-deficient. This last factor, in particular, leads to the paradoxical situation that adding a donor can increase the hapticity of the system. An examination of the slippage curves for a large number of systems shows that Cp−MM interactions are generally much “softer” than the corresponding Cp−TM interactions, leading to easy accessibility of a wide range of geometrical arrangements for many compounds. For intermediate-hapticity structures, valence-bond calculations indicate that the general preference for η 2 over η 3 structures is primarily due to ionic and σ bond interactions, the latter having a better orbital size match in the η 2 situation.