Assessment of the T1 Criterion for Distinguishing between Classical and Nonclassical Transition-Metal Hydrides: Hydride Relaxation Rates in Tris(triarylphosphine)osmium Tetrahydrides and Related Polyhydrides

Article abstract of DOI:10.1021/ja00011a019

[OsH₄(PTol₃)₂] (1, Tol = ρ-tolyl) and a series of related isotopomers [OsH_xD_4-x{P(Tol-o-d₂)₃}₃] (x = 1, 2, 3, and 4) were prepared and hydride spin-lattice relaxation times, 7, determined as a function of temperature for each one. From these measurements the contributions to the relaxation rates of the hydride ligands from the other hydrides and from the ortho protons of the triarylphosphine ligands were determined to be 62% and 33%, respectively. Taking account of these, as well as of the contributions from the phosphorus nuclei (2%) and osmium nucleus (<0.5%), the calculated relaxation rate, 4.17 s^-1 (T₁ (min) = 238 ms at 500 MHz) agrees to within 3% with the measured one, 4.29 s^-1. Similar calculations were performed on 33 other classical hydrides for which the relaxation rates at 500 MHz ranged from 1.67 to 9.09 s^-1M (T₁min) at 500 MHz from 670 to 110 ms). For the majority of these, the calculated and observed relaxation rates agree to within 10%, with contributions from dipole-dipole interactions with nuclei other than metal-coordinated protons accounting for more than 60% of the observed relaxation rates. Such contributions, typically, have been neglected in previous interpretations of T₁(min). For nonclassical hydrides, i.e. those containing η²-H₂ ligands, agreement between observed and calculated relaxation rates was less satisfactory. Even in such cases dipole-dipole interactions with nuclei other than metal-coordinated protons account for up to 25% of the observed relaxation rates emphasizing that, in general, such interactions cannot be ignored. There is an overlap between the range of T₁(min) values found for classical and nonclassical polyhydrides. For several polyhydrides of undetermined structure, such as [OsH₅(PTol₃)₃]⁺, [RuH₄(PCy₃)₃], and [RuH₆(PCy₃)₂] (including some that had previously been assigned nonclassical structures on the basis of the T₁ criterion ), T₁ (min) is shown to be consistent with both classical and nonclassical structures.