TRANSITION-METAL POLYHYDRIDE COMPLEXES - A THEORETICAL VIEW

The factors governing the stabilities of classical and non-classical isomers in transition metal polyhydride complexes are summarized in this review. Diagonal lines in the Periodic Table, which divide the classical (left-hand side of the line) and non-classical (right-hand side of the line) isomers, were defined for neutral (through Ru and Ir) and cationic (between Tc/Ru and Os/Ir) complexes with only phosphine ligands. The carbonyl ligand was found to stabilize significantly the non-classical isomers due to its strong pi-accepting ability. The strong ''sigma''- and ''pi''-donating ability of the Cp ligand makes most of its polyhydride complexes adopt classical structures. Tn this review, we also discuss some unique structural features in several types of polyhydride complex, such as the orientation of eta(2)-H-2 ligands in non-classical structures and the distortions in the ''four-legged piano-stool'' complexes (MH(n)L(4-n)Cp) and the pseudo-pentagonal-bipyramidal complexes (MH(n)L(6-n)Cp).