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Nitrogen Nuclear Magnetic Resonance Spectroscopy as a Probe of Bonding, Bending and Fluxionality of the Imido Ligand

DOI: 10.1039/DT9920001663

Source and publish data:

Journal of the Chemical Society, Dalton Transactions p. 1663 - 1668 (1992)

Update date:2022-08-02

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Authors:

Bradley, Donald C.Bradley, Donald C.

Hodge, Stephen R.Hodge, Stephen R.

Runnacles, John D.Runnacles, John D.

Hughes, MarkHughes, Mark

Mason, JoanMason, Joan

Richards, Raymond L.Richards, Raymond L.

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Article abstract of DOI:10.1039/DT9920001663

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, 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 the linear ligating nitrogen in N2 and NO ligands; the shielding is higher than in nitrides, with low-energy n(N) ---> ?* 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 ?* (MN) or ?* (MN), are higher lying than the ?* (NN) and ?* (NO) LUMOs.Another difference in the imido ligand is the closer parallelism of the nitrogen and the metal shielding, both of which increase with (? + ?)-acceptor ability of the coligands (increasing the ligand-field splitting).In The (thf = tetrahydrofuran) series, similarly, the nitrogen shielding increases in the sequence X = OMe < Me < H < F < Cl < NO2, with increase in (? + ?)-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 ?-donor ligands such as oxo and fluoro but differs from that of ?-acceptor nitrogen ligands, such as N2, N=NR or NO.

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