- Ab initio calculations with effective core potentials have been used to study the relative stability of classical and nonclassical isomers of the model complexes, IrH5L2 (L = Be, PH3, and PMe3). The examination of different levels and techniques of electron correlation suggests that the second-order Moller-Plesset (MP2) method will provide a reliable prediction of the stability of one isomer over the other. The conventional substitution of a PR3 ligand by PH3 in quantum chemical calculations has also been examined. The small error (ca. 2 kcal/mol) caused by this replacement suggests that it is a reasonable choice in this class of transition metal polyhydride complexes. Our results on the electronic structural difference between classical and nonclassical isomers suggest that the nonclassical isomer is preferred for complexes with strong acceptor ligands and contracted central-metal d orbitals. 1992, American Chemical Society. All rights reserved.