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 (Nα) 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 Nα and Nβ 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 Nβ in a singly bent diazenide to form a hydrazide(2–), or of Nα 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 Nα. 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.