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 π and σ 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.