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, [CpML6nHn]*+ (n = 46, x = 02, and L = PH3), where M = Nb, Ta, Mo, W, Tc, Re, Ru, Rh, Os, and Ir. These systematic calculations reveal that a diagonal line in the periodic table through Ru and Ir atoms divides the classical forms (left side of the line) and nonclassical forms (right side of the line). Most of these polyhydrides adopt classical isomers with a pseudo-pentagonal-bipyramidal geometry, where the Cp ring occupies one of the two axial positions. In this pseudo-pentagonal-bipyramidal structure, the Cp-M-He (He: an equatorial hydrogen) angle is found to be about 110.0, significantly greater than the 90.0 expected for a regular pentagonal bipyramid. This unexpected large deviation results from a pseudo-second-order JahnTeller (SOJT) distortion, which allows a stronger interaction between the equatorial hydrogens' e1 set of orbitals and metal dxz and dyz orbitals. 1993, American Chemical Society. All rights reserved.
