The nitrogen-15 nuclear magnetic resonance criterion of bond angle in diazenido-complexes

Abstract

Nitrogen-15 n.m.r. spectroscopy provides a useful criterion of linear or bent geometry, which may differ from solid to solution, at the ligating nitrogen (N_α) of the diazenido (M–NNR) ligand (as also in nitrosyls M–NO); as shown by parameters determined for diazenides of Mo, W, Re, Fe, Ru, Rh, Os, Ir, and Pt. Both N_α and N_β are strongly deshielded, sometimes by 400 p.p.m. or more, in the doubly bent compared with the singly bent ligand. The deshielding on bending is linked to low-energy n(N)→π* paramagnetic circulations when there is non-bonding electron density on the nitrogen; thus protonation or alkylation of N_β in a singly bent diazenide to form a hydrazide(2–), or of N_α in a doubly bent diazenide to form a diazene ligand, greatly increases the nitrogen shielding. Coupling constants to the metal, also, are reduced on bending of the α-nitrogen (with development of lone-pair-electron density with s character), increasing again on protonation of N_α. The singly bent ligand is particularly responsive to the nature of the co-ordination site, the shielding of both nitrogens decreasing from six-co-ordinate {MN₂R}⁶ to five-co-ordinate {MN₂R}⁸ 18-electron complexes (with increase in back bonding). Both nitrogens are more shielded (and the NN coupling constant reduced) in four-co-ordinate (16-electron){MN₂R}⁸ complexes, with π donation to the metal. Useful parallels may be drawn with nitrogen n.m.r. parameters in related organic groups: arenediazonium, diazo, and diazene.