TRANSITION-METAL POLYHYDRIDE COMPLEXES .3. RELATIVE STABILITIES OF CLASSICAL AND NONCLASSICAL ISOMERS

Ab initio calculations with effective core potentials have been used to study the relative stabilities of classical and nonclassical isomers of 18-electron polyhydride transition-metal complexes. Systematic calculations on ML(7-n)H(n) and ML(8-n)H(n) (n = 2-7), where M = Mo, W, Tc, Re, Ru, Os, Rh, and Ir, and L = PH3 and CO, lead to the following conclusions. The trans influence of two H ligands is significantly destabilizing and influences the stability and structure of the isomers. A diagonal line in the Periodic Table through Ru and Ir divides the classical (left side of the line) and nonclassical (right side of the line) forms for neutral complexes without strong pi-accepting ligands. For monocationic hydride complexes the corresponding diagonal line shifts slightly toward early transition metals and crosses between Tc/Ru and Os/Ir. The stability of nonclassical complexes increases with an increase in the number of strong pi-accepting ligands or with an increasing contraction of the transition-metal d orbitals. The conclusion for cationic hydride complexes applies to neutral polyhydride transition-metal complexes with a chloride ligand because of the strong electron-withdrawing ability of chloride. Our calculations predict that several complexes previously identified as nonclassical isomers should be reclassified as classical isomers. The trends predicted here also lead to suggestions for finding new classical and nonclassical isomers.