Osmium Nitrido Complexes : Electronic Structure and their Relation to their Reactivity

Institute of Chemical Sciences, Devi Ahilya University Indore, Khandwa Road, Post Box No. 105, Indore-452 001 Manuscript received 10 October 1995, accepted 31 January 1996 The nature of the chemical bonding in the Os=N group, the mutual influence of ligands and relation of electronic structure to the reactivity of nitrido complexes of osmium are examined by CNDO/2 molecularorbital calculations on [OsNC15]2-, [OsNX4H2O)), [OsNX4]- (X = Cl, Br) and [OsNCI3(PH3)2]· The computed trends for Os-N, trans-Os-L (L = Cl, H2O) and cis-Os-X bond strengths in the complexes, as measured by Wiberg indices, charge distributions and orbital populations, suggest that the nitrido ligand is strongest π-donor ligand and the donor interactions of the N3- ligand are greatly dependent on the nature of the ligands found in the cis- and trans-positions. The strength of the trans-Os-Cl bond is weaker than that of the cis-Os-CI. The calculations confirm the experimental observations that the trans-halide atoms in the [OsNX5]2- are easily replaced by water. The Os-N and Os-X Wiberg indices are in linear correlation with Os-N and Os-X vibrational stretching frequencies. Reactivities of osmium nitrido complexes are discussed on the basis of charge on nitrido ligand, energy and orbital contributions of lowest unoccupied molecular orbital. For these anionic osmium nitrido complexes, results favour substitution reactions and addition reactions over nucleophilic attack at the nitrido ligand. Triphenylphosphine and sulphide ions do not directly attack the nitride ligand in [OsNX4) but attack the nitride ligand in [OsNX3L2) (X = Cl, Br; L = PPh3, AsPh3) which are obtained by the reaction of [OsNX4]- with L