Transition Metal Polyhydride Complexes. 4. Highly Stable Nonclassical Isomers with Octahedral Structures

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 ML_8-n H_n, 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 ML_8-n H_n 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.