STRETCHED MOLECULAR-HYDROGEN COMPLEXES OF OSMIUM(II) - A QUANTUM-CHEMICAL STUDY OF THE INFLUENCE OF THE TRANS LIGAND ON GEOMETRIES, SPIN-SPIN COUPLING-CONSTANTS, BONDING, AND CHARGE-DISTRIBUTIONS

A theoretical study of the stretched molecular hydrogen complexes [Os( NH3)(4)L(z)(eta(2)-H-2)]((z+2)+)(L(z) = (CH3)(2)-CO, H2O, CH3COO-, Cl- , H-, C5H5N, and CH3CN) is reported. Using SCF and MP2 methods in conj unction with effective core potentials and basis sets of triple-zeta q uality on Os and double-zeta on the ligand atoms the geometries, HD sp in-spin coupling constants, and binding energies have been calculated and compared with the available experimental data. The calculated H-H distances are remarkably uniform: all fall in the range 1.30-1.40 Angs trom and correspond to stretched eta 2-H-2 complexes, i.e. no cis-dihy drides have been found. The predicted H-H distance in the acetate comp lex is consistent with the observed distance of 1.34 Angstrom in the e thylenediamine derivative. A unique feature of these complexes is the crucial role that electron correlation has on the H-H bond lengths; th is is a consequence of the unusual potential energy surfaces that are extraordinarily flat with respect to the H-H stretch. The calculated O s-H bond lengths and stretching frequencies are also consistent with e xperiment. The calculated HD spin-spin coupling constants (J(HD)) are of the same order of magnitude as those observed, yet the trend in the latter with changing trans ligand L(z) is not adequately reproduced b y the calculations, although a reasonable correlation between experime ntal J(HD) and calculated H-H distance is demonstrated. In order to el ucidate the nature of the Os-H-2 bonding, population analyses as well as an analysis of the binding energy of the Cl- complex have been carr ied out and these further emphasize the importance of electron correla tion. At the simplest level, the effect of the trans ligand L(z) on th e properties of the complexes can be related to its spectrochemical co nstant, which correlates with H-H distance and binding energy as well as HD spin-spin coupling constant.