Structural characterization in solid state and solution of the hydridocarbonyl cluster anion [Re4(\u3bd-H)4(CO)13]12

The salt [AsPh4]2[Re4(\u3bd-H)4(CO)13], obtained by pyrolysis of [AsPh4][ReH2(CO)4] in ethanol, has been investigated by X-ray analysis. The crystals are monoclinic, space group P21/n, with a = 12.630(3), b = 38.062(6), c = 12.916(3) \uc5, \u3b2 = 105.94(2)\ub0 and Z = 4. The refinements, performed by full-matrix least-squares on the basis of 3389 significant reflections, gave final agreement indices R and Rw of 0.035 and 0.040, respectively. The anion contains a tetrahedron of rhenium atoms [Re-Re in the range 3.048(1)-3.126(1) \uc5], bearing thirteen terminal carbonyl groups and four edge-bridging hydride ligands. The variable temperature 1H NMR investigation showed three hydridic resonances (ratio 1:2:1) at 153 K, in agreement with the Cs symmetry of the solid state structure. At higher temperatures, two different degenerate exchange processes concerning the hydride ligands are operative. The first, frozen only below 153 K, equalizes the three hydrides bridging the edges of the basal triangle. The second, frozen below 188 K, exchanges all the hydrides, leading to a single hydride resonance at room temperature. Band shape analysis provided an estimate of the rate constants and of the activation parameters of the two processes: intrabasal exchange, Ea = 30.4 \ub1 1.3 kJ/mol, \u394H\u2021 = 29.0 \ub1 1.3 kJ/mol, \u394S\u2021 = -8.1\ub17.5 J/K mol; overall scrambling, Ea, = 42.7\ub10.6 kJ/mol, \u394H\u2021 = 40.7\ub10.6 kJ/mol, \u394S\u2021 = -16.6 \ub1 2.6 J/K mol. \ua9 1992