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 (N=NR) 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, Bu(t), 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, delta(N), -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) --> pi* paramagnetic circulations, and lower than in bridging imides and amides. The deshielding on bending is much smaller than for N=NR or NO ligands because the imido lowest-unoccupied molecular orbitals (LUMOs), mainly pi*(MN) and sigma*(MN), are higher-lying than the pi*(NN) and pi*(NO) LUMOs. Another difference in the imido ligand is the closer parallelism of the nitrogen and the metal shielding, both of which increase with (sigma + pi)-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 (sigma + pi)-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 pi-donor ligands such as oxo and fluoro but differs from that of pi-acceptor nitrogen ligands, such as N2, N=NR or NO.