Nitrogen nuclear magnetic resonance spectroscopy as a probe of bonding, bending and fluxionality of the imido ligand

Nitrogen-14 and -15 NMR studies of imido (NR)-ligands have been made for 37 complexes of Ta, Mo, W, Re and Os, including bent NR ligands, with evidence of bent–linear fluxionality in solution, in concert with an OR or a second NR ligand. A striking difference from diazenido (NNR) or nitrosyl ligands is the small difference in nitrogen shift for linear and bent NR ligands, the latter appearing across the whole range for NR ligands of the metals studied (with R = H, Me, Et, But, CH2But, aryl or SiMe3) in complexes with co-ordination numbers ranging from four to seven, and OR, Cl, F, NHR, NR2, N(SiMe3)2, S2CNEt2, phosphine, diphosphine or oxo coligands. This range, δN–90 to 156, resembles that of linear ligating nitrogen in N2 and NO ligands; the shielding is higher than in nitrides, with low-energy n(N)→π* paramagnetic circulations, and lower than in bridging imides and amides. The deshielding on bending is much smaller than for NNR or NO ligands because the imido lowest-unoccupied molecular orbitals (LUMOs), mainly π*(MN) and σ*(MN), are higher-lying than the π*(NN) and π*(NO) LUMOs. Another difference in the imido ligand is the closer parallelism of the nitrogen and the metal shielding, both of which increase with (σ+π)-acceptor ability of the coligands (increasing the ligand-field splitting). In the [WCl4(NC6H4X-4)(thf)](thf = tetrahydrofuran) series, similarly, the nitrogen shielding increases in the sequence X = OMe < Me < H < F < Cl < NO2, with increase in (σ+π)-acceptor ability of the aromatic group. The overall pattern of imido-nitrogen shielding, including the periodicity of the metal dependence (the shielding increasing down the group of the metal, but decreasing across the row) thus resembles that of other π-donor ligands such as oxo and fluoro but differs from that of π-acceptor nitrogen ligands, such as N2, NNR or NO.