GEOMETRIC DISTORTIONS IN 4-LEGGED PIANO-STOOL CYCLOPENTADIENYL TRANSITION-METAL COMPLEXES

Ab initio calculations with effective core potentials are used to obtain structural parameters of ''four-legged piano-stool'' (also viewed as pseudo-square-pyramidal) cyclopentadienyl transition-metal complexes, CpML4-nLn' (n = 0-3), where M = Ru, Os, Rh, and Ir, and L, L' = PH3, CO, H, SiH3, Cl, Li, and Be. These four-legged piano-stool complexes distort from an ideal pseudo-square-pyramidal geometry in such a way that the two larger L-M-Ct (Ct = centroid of the Cp ligand) angles correspond to trans ligands, as do the two smaller ones. These distortions are explained in terms of a sigma-interaction model that differs considerably from a previous one, which emphasized the M-L pi-interactions. A rehybridization between d(z2) (one of the two nonbonding d orbitals in a pseudo-C4v CpML4 complex) and d(x2-y2) (sigma antiboding with L's) occurs in the process of distortion from pseudo-C4v to C2v symmetry. The result of the rehybridization is a preference for strong covalently sigma-bonding ligands to be coordinated to the transition metal with a small L-M-Ct angle.