STRUCTURE AND SOLUTION BEHAVIOR OF A SERIES OF CLASSICAL AND NONCLASSICAL RHENIUM HYDRIDE COMPLEXES

A series of nonclassical rhenium dihydrogen complexes, Re(H2)(CO)n(PMe3)5-n+ (n=1-3), Re(H2)(H)2(CO)-(PMe3)3+, and ReH(H2)(CO)(NO)(PPri3)2+ and their isotopomers are prepared by the protonation of the corresponding mono-, di-, or trihydride or deuteride complexes at low temperature and are characterized by NMR methods. The stability, structure, and solution behavior are studied. Due to a subsequent conversion of the nonclassical Re-(H2)(CO)(PMe3)4+ compoUnd into the previously reported corresponding classical dihydride complex ReH2(CO)-(PMe3)4+ an isolable protonation product is obtained in the case of Re(H2)(CO)(PMe3)4+. The other representatives of dihydrogen complexes are very labile and decompose at temperatures above 230 K. Re(H2)(H)2(CO)(PMe3)3+ exists as an equilibrium mixture with its classical tautomer ReH4(CO)(PMe3)3+. A variable-temperature study of this transformation reveals the following thermodynamic parameters: DELTAH = -1.05 +/- 0.1 kcal/mol and DELTAS = -2.3 +/- 0.6 eu. In addition using dynamic NMR methods the exchange process between the dihydrogen and the hydride positions of the nonclassical tautomer Re(H2)(H)2(CO)(PMe3)3+ is investigated. A kinetic isotope effect (k(H)/k(D) = 2 at 18 3 K) is observed. Thus, H-H bond cleavage participates in the rate-determining step. Possible mechanisms are discussed on the basis of kinetic and spectroscopic data. ReH(CO)2(PMe3)3, [ReH2(CO)(PMe3)4][BPh4], and ReH3(CO)(PMe3)3 were characterized by X-ray structure analyses: ReH(CO)2(PMe3)3, space group P2(1)2(1)2(1), a = 14.249(8) angstrom, b = 16.067(6) angstrom, c = 16.466(7) angstrom, Z = 8 (two independent molecules), R = 0.069, R(w) = 0.077 for 3234 reflections with F > 6sigma(F); [ReH2(CO)(PMe3)4] [BPh4], space group P2(1)2(1)2(1), a = 10.000(5) angstrom, b = 13.197(6) angstrom, c = 30.891(13) angstrom, z = 4, R = 0.064, R(w) = 0.073 for 3562 reflections with F > 6sigma(F); ReH3(CO)-(PMe3)3, space group Pna2(1), a = 20.423(4) angstrom, b = 11.475(3) angstrom, c = 15.444(4) angstrom, Z = 8 (two independent molecules), R = 0.057, R(w) = 0.057 for 2993 reflections with F > 6sigma(F).