Ab initio calculations at the MP2 level with effective core potentials have been used to study the relative stabilities of classical and nonclassical isomers of 14 second- and third-row transition-metal polyhydride complexes with the formula ML8-n Hn, where n = 4-7 and L = PH3. Results show that the model complexes are divided into two groups. One group has as its most stable structure a classical one with the maximum coordination number, while the other has as its most stable structure a nonclassical one with an octahedral geometry. Through the detailed analyses of valence-electron densities, a model is proposed to explain the highly stable six-coordinate octahedral structure for those transition-metal polyhydride complexes which prefer a nonclassical isomer. From the model, we can make a general conclusion about these ML8-n Hn complexes. When twice the ionization enthalpy of an electron in the M-H bond is greater than sum of the ionization enthalpies of an electron in the H-H bond and one in the metal d orbital, a classical isomer is definitely preferred. Otherwise, a nonclassical isomer with an octahedral structure is adopted. 1992, American Chemical Society. All rights reserved.
