Inorganic Chemistry, Vol.34, No.1, 214-217, 1995
Kinetics of H-2 Dissociation from Some Iridium-Dihydrogen Complexes with Structurally Different (H-2) Ligands
The variable-temperature H-1 NMR Spectra permit the determination of Delta H degrees (-7.1 kcal/mol) and Delta S degrees (-16.0 eu) for formation of trans-IrH(H-2)Cl-2(PPr3i)(2) from IrHCl2(PPr3i)(2) and H-2 in toluene-d(8). The (H-2) ligand of the related complex trans-IrH(H-2)Cl-2(PCY3)(2) was characterized by a T-1min value of 3.4 ms at 200 MHz using T-1 measurements for dissolved H-2 in the presence of IrHCl2(PCy(3))(2). Analysis of the variable-temperature H-1 NMR spectra and T-1 measurements resulted in estimation of the rate, Delta H-double dagger and Delta S-double dagger for H-2 dissociation from both trans- and cis-IrH-(H-2)Cl-2(PPT3i)(2) and IrH2(H-2)X(PPr3i)(2) (X = Cl, Br) in toluene-d(8). The rate of H-2 loss decreases dramatically from 10(5) to <7 S-1 (234 K), with elongation of the B-H bond in me (H-2) ligand from 0.74 to 1.1 Angstrom. The height of the Mf barrier is a direct consequence of the Ir-H-2 bond strength.