for distinguishing between classical and nonclassical transition-metal

J. Am. Chem. SOC. 1991, 113, 4173-4184

4173

Assessment of the T I Criterion" for Distinguishing between Classical and Nonclassical Transition-Metal Hydrides: Hydride Relaxation Rates in Tris( triary1phosphine)osmium Tetrahydrides and Related Polyhydrides c(

Peter J. Desrosiers,"Vb Lisheng Cai,18 Zerong Lin,'* Rosalie Jack Halpern*.l'

and

Contribution from the Departments of Chemistry, The University of Chicago, Chicago, Illinois 60637, and The University of the Virgin Islands. Charlotte- Amalie, St. Thomas, US.Virgin Islands 00802. Received September 4, 1990 Abstract: [OsH,(PTo13)J (1, To1 = p-tolyl) and a series of related isotopomers [0~H,D,(P(Tol-o-d,),]~] ( x = 1, 2, 3, and 4) were prepared and hydride spin-lattice relaxation times, TI,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 (