X-Ray crystal structures of two isomers of the hydrido-carbonyl anion [re7(μ-H)C(CO)21]2-. the problematic existence of triply bridging hydrides in clusters containing interstitial main-group elements

The X-ray crystal analyses of two different salts of the hydrido-carbido carbonylrhenate anion [Re7HC(CO)21]2-, obtained by protonation of [Re7C(CO)21]3-, have shown the presence of two structural isomers, differing in the location of the hydride ligand. The [NEt4]+ salt (isomer a) crystallizes in the monoclinic system, space group P21/c (no. 14), with a = 10.168(3), b = 43.282(8), c = 11.505(3) Å, β = 91.72(2), and Z = 4. The [NEt4]+[C7H7]+ mixed salt (isomer b) crystallizes in the monoclinic system, space group P21 (no. 4), with a = 12.007(4), b = 16.725(5), c = 12.004(4) Å, β = 105.79(2)°, and Z = 2. The gross features of the two structures are those of the parent anion [Re7C(CO)21]3-, where a carbon-centred monocapped octahedron of metal atoms is surrounded by 21 terminal carbonyl ligands, three for each metal. In both isomers, the hydride ligand has been indirectly located, on the basis of potential energy computations, as doubly bridging, on an edge of the basal triangle in isomer (2a), in accord with the idealized Cs symmetry of the anion, and on an inter-layer edge of the octahedron in the less symmetric (C1) isomer (2b). Carbon-13 and 1H n.m.r. analyses at low temperatures have also shown the presence in solution of two isomers, rapidly exchanging at room temperature. The ratio between the two isomers is temperature dependent and for this equilibrium ΔH⊖ = -982 cal mol-1 and ΔS⊖ = -1.7 cal-1 K-1 mol-1 have been estimated. A possible correspondence between the two solid-state and the two solution isomers is discussed. Simple geometric considerations, showin the 'non-innocent' nature of triply bridging hydrides in clusters containing interstitial main-group elements, X, are also presented. Depending on the metal size and on the geometry of the cage, anomalous H ⋯ X contacts can arise; in particular, for the wide family of octahedral carbido clusters, the lack of genuine examples of μ3-H co-ordination suggests a repulsive character of this H ⋯ C interaction.