High nuclearity carbonyl clusters of rhodium. Part 3. Crystal and molecular structure of hexadeca-μ-carbonyl-enneacarbonyl-polyhedro-tetradecarhodate(4-) in its tetraethylammonium salt

The title compound, [NEt4]4[Rh14(CO)25], crystallises in the tetragonal space group P4/ncc, with unit-cell dimensions a = l7.060(3), c = 27.220(6) Å, and Z = 4. The structure has been solved by Patterson and Fourier methods from X-ray single-crystal counter data and refined by least-squares methods to R = 0.046 for 1 224 significant reflections. The [Rh14(CO)25]4- anion contains a metal-atom cluster which can be described as a distorted fragment of a body-centred cubic lattice, of C4ν idealized symmetry, with one interstitial metal atom which exhibits eight short metal-metal bonds (mean 2.638 Å), four longer contacts (3.076 Å), and one very long interaction [3.372(4) Å] with the capping atom on the four-fold axis. The Rh-Rh surface interactions are very scattered, ranging from 2.724(2) to 3.325(2) Å. There are nine terminally bound carbonyls (mean Rh-C and C-O, 1.80 and 1.17 Å respectively) and 16 edge-bridging groups. Of these, 12 are symmetric (mean Rh-C and C-O, 1.99 and 1.19 Å) and four, on the Rh(3)-Rh(5) edges, are markedly asymmetric [Rh(3)-C(6) 1.91(2) and Rh(5)-C(6) 2.33(2) Å].