NATURE OF BONDING IN PHOSPHAZOYLIDES - A COMPARATIVE-STUDY OF N2H4, NPH4, AND P2H4

Structures, energies, and electronic properties of the ylides H3NNH (2), H3PPH (5), H3PNH (8), and H3NPH (10) of hydrazine (1), diphosphine (4), and aminophosphine (7) have been studied with ab initio molecular orbital theory by use of a split valence plus polarization basis set and incorporating electron correlation and zero-point energy corrections. The results show all ylides to be equilibrium structures. The Staudinger tautomer H3PNH (8) is 29.7 kcal/mol less stable than aminophosphine H2NPH2 with a 50.0 kcal/mol barrier for hydrogen migration. The very short P-N bond of 8 is ascribed to result mainly from the large Coulombic interaction between the NH and PH3 groups, augmented with a partial triple bond contribution that results from pi and sigma-interactions. The properties of the H3PPH ylide 5 are similar to the Staudinger complex 8 but less pronounced. The chemical bonding in these "hypervalent" molecules 5 and 8 is dominated by ionic bonding and conjugative stabilization with little if any d-orbital participation.