DIHYDROGEN VERSUS DIHYDRIDE - RELATIVISTIC EFFECTS ON THE RELATIVE STABILITIES OF NONCLASSICAL AND CLASSICAL ISOMERS OF M(PH3)(3)H-4 (M=FE,RU,OS)

A nonlocal, quasirelativistic density functional method, NL-SCF+QR, ha s been used to study the geometries and relative stabilities of the cl assical (I) and nonclassical (II) isomers of M(PH3)(3)H-4 with M = Fe, Ru, and Os. According to the energy calculations based on optimized g eometries, the dihydrogen complex (II) is found to be most stable for Fe and Ru whereas the hydride complex (I) is the isomer of lowest ener gy for Os. These findings are in accordance with experimental observat ions. It was further found that relativistic effects are responsible f or the changeover in the order of stability between I and II in going from M = Fe and Ru to M = Os. Without relativity all three metal cente rs would prefer conformation II. The observed preference for a dihydri de complex (I) in the case of the heavier congener osmium is explained by a relativistic destabilization of the 5d orbital which makes the m etal center more basic.