ETA-2-DIHYDROGEN ON THE BRINK OF HOMOLYTIC CLEAVAGE - TRANS-[OS(H=H)H(PET2CH2CH2PET2)2]+ HAS SPECTROSCOPIC AND CHEMICAL-PROPERTIES BETWEEN THOSE OF THE ISOELECTRONIC COMPLEXES TRANS-[OSH(PPH2CH2CH2PPH2)2(ETA-2-H2)]+ AND REH3(PPH2CH2CH2PPH2)2

The new, octahedral complex trans-[OsH(dppe)2(eta-2-H-2)]+, 10s, dppe = PPh2CH2CH2PPh2, prepared by addition of H+ to cis-OsH2(dppe)2, has a rapidly spinning dihydrogen ligand with an unusually long (0.99 angstrom) H-H distance according to the T1 NMR method. Additional evidence for this is a short T1 value for the H-2 ligand in the complex trans-[OsH-(dppe-d20)2(eta-2-H-2)]+ and a large 1J(H,D) value (25.5 Hz) for trans-[OsH(dppe)2(eta-2-HD)]+, 10s-d1. The known complex trans-[Os(H...H)H(depe)2]+, 20s, depe = PEt2CH2CH2PEt2, has properties (delta-H, J(H,D), T1-1) between those of 10s and ReH3(dppe)2, 3Re, a true pentagonal, bipyramidal trihydride with r(H...H) approximately 2 angstrom; e.g., the J(H...D) coupling for trans-[Os(H...D)D(depe)2]+, 20s-d2, is the unusual value of approximately 11.5 Hz, whereas for 3Re-d2 it is 0 Hz, as expected. Significantly the 11.5-Hz value varies by +/- 1 Hz depending on the solvent and temperature. VT NMR spectra of 20s-d2 conclusively show that J(H...D) is not lost but is averaged by intramolecular H/D atom exchange; at 325 K, J(av)(H,D) = 3.8 Hz. There are three possible answers to the question as to whether there is homolytic cleavage of H-2 in 20s: (1) no H-H cleavage so that 20s, like 10s, has a stretched, rapidly spinning dihydrogen ligand with r(H-H) = 1.2 angstrom; (2) complete H-H bond breaking to give an unusual trihydride with two closely spaced hydrides, 1.4-1.5 angstrom apart; and (3) a rapidly equilibrating mixture (DELTA-G double-ended dagger less-than-or-equal-to 9 kcal mol-1) of Os(H)(eta-2-H-2) and Os(H)3 tautomers. The rapid equilibrium model (3) with the Os(H)3 tautomer in greater abundance fits all available data for 20s and other complexes of Os with small J(H...D) couplings. The equilibrium shifts toward the dihydrogen form with an increase in temperature or on going from acetone-d6 to CD2Cl2 as signaled by changes in delta(H-2), T1(H-2), and J(H,D) values. An alternative answer (2) cannot be ruled out since closely spaced hydrides might move closer together with these changes in conditions and produce the same spectral changes. Also included are X-ray diffraction structural data for the three complexes, their infrared, chemical, and electrochemical properties as well as differences in NMR properties: rates of intramolecular H-atom exchange, chemical shifts, and coupling constants of the isotopomers 10s-d1, 20s-d1, 20s-d2, and 3Re-d2. 10s retains the H-H bond because it has lower energy 5d electrons than 20s or 3Re, and the dppe ligands are small enough to allow an undistorted octahedral coordination geometry. 3Re gives shorter T1 times for hydride and phosphorus nuclei than expected solely from dipolar relaxation by neighboring protons.