An NMR Method for Distinguishing Classical from Nonclassical Structures in Transition-Metal Polyhydrides

Article abstract of DOI:10.1021/ja00221a005

A T₁ method is described for distinguishing classical hydrides with terminal M-H bonds from nonclassical hydrides with H-H as well as M-H bonds.The method can still be used even where the hydrides are fluxional as is the case for transition-metal polyhydrides.The temperature dependence of the T₁ shows a minimum for several cases studied.This allows the H-H distance in the H2 ligand to be estimated and the structural type determined.ReH7(PPh3)2 is shown to be nonclassical with the probable structure Re(H2)H5PPh3)2. ^- is shown to be classical and ^2- seems to be classical, however.Other hydrides shown to be nonclassical are Re(H2)H5(dpe), FeH2(H2)L3, RuH2(H2)L3, and ⁺; the Re examples are the first d² and the Os example the first d⁴ dihydrogen complex.Complexes shown to be classical are the following: IrH5(PCy3)2, ReH5(PPh3)3, OsH4(P<*>p-tolyl<*>3)3, MoH4(PMePh2)4, and, more surprisingly, ⁺, H2Fe(CO)4, ⁺, and WH6(PMe2Ph)3.Typical distance estimates (r_eff) obtained are the following: ⁺, 1.04C Angstroem; ReH5(H2)(PPh3)2, 1.20C Angstroem; ReH5(H2)(dpe), 1.25C Angstroem; MoH4(PMePh2)4, 1.72C Angstroem, where C is a calibration constant (as yet undetermined but ca. 0.9).The ordering of the r_eff values observed seems to parallel the relative lability order of the complexes studied, the more labile species being associated whit shorter r_eff values.