QUANTUM EXCHANGE COUPLING - A HYPERSENSITIVE INDICATOR OF WEAK-INTERACTIONS

Os(H)(3)ClL2 (L = (PPr3)-Pr-i) forms a 1:1 adduct with L' = PEt3, NH3, MeCN, acetone, methanol, and THF. The case L' = PEt3 permits the dear est identification of adduct structure as pentagonal bipyramidal. For NH3 and MeCN, the respective kinetics of L' loss are measured as Delta H-double dagger = 20.7(3) and 17.6(3) kcal/mol and Delta S-double dag ger = 16(1) and 14.7(9) cal/(mol K). For acetone, methanol, and THF, t he following respective Delta H degrees and Delta S degrees values for L' binding are measured: Delta H degrees = -10.4(1), -6.66(8), and -5 .8(2) kcal/mol; Delta S degrees = -41.8(5), -25.5(3), and -33(1) cal/( mol K). Decoalesced H-1 NMR spectra are reported for several of these Os(H)(3)ClL2L' species, and they show a variety of examples of quantum exchange coupling among the hydride ligands. The values of J(e)x are higher when L' is a more weakly-binding ligand. The quantum exchange c oupling constants of Os(H)(3)XL2 (X = Cl, Br, I, OCH2CF3, OCH(CF3)(2)) in CD2Cl2, in toluene, and in methylcyclohexane show an unprecedented decrease of J with increasing temperature, which is attributed to wea k formation of Os(H)(3)Cl(solvent)L-2 adducts at low temperature. For L' = CO, adduct formation leads to liberation of coordinated H-2. Exce ss L' = MeCN or NH3 slowly leads to formation of [Os(H)(3)L'L-2(2)]Cl; the X-ray structure for L' = NH3 is reported. Crystal data (-171 degr ees C): a = 11.561(4) Angstrom, b = 14.215(5) Angstrom, c = 8.851(3) A ngstrom, alpha = 97.51(2)degrees, beta = 107.73(2)degrees, gamma = 104 .47(2)degrees, with Z = 2 in space group <P(1)over bar>. The potential energy was calculated for exchange of 2H of OsH3X(PH3)(2)L (X = Cl wi th L = no ligand and PH3, X = I with L = no ligand) using effective co re potential ab initio methods at the MP2 level. The site exchange is found to be energetically easier for Cl than for I, in agreement with experiment. The hydride site exchange in the seven-coordinate species OsH3Cl(PH3)(3) (a model for coordination of either ligand or solvent t o Os) is found to be easier than that in the 16-electron species. No d ihydrogen ligand is located on the reaction path for site exchange. Th e current theory which relates quantum exchange to a tunneling effect was used for calculating J(ex) as a function of temperature. The dynam ic study was done using several sets of coordinates, in particular the rotation angle phi and the internuclear distance r between the exchan ging H. The vibrational levels have been calculated and the symmetry o f each level assigned within the permutation group in order to determi ne the nature of the nuclear spin function associated with each level. It is found that the rotation, phi, gives rise to the largest tunneli ng effect but that r cannot be neglected. The influence of the tempera ture, J(ex)(T), was included by a Boltzmann distribution. The results are in qualitative agreement with experiment in that quantum exchange coupling is larger in the case of Cl than in the case of I. Additional ligand L increases the value of the quantum exchange coupling mostly by lowering the activation energy for pairwise exchange.