摘要：

A statistically disordered crystal structure and detailed molecular configuration of Co4(CO)12 have been unambiguously determined from a three-dimensional single-crystal X-ray photographic analysis. The compound crystallizes with four tetrameric molecules in an orthorhombic unit cell of symmetry Pccn and dimensions a = 8.99 (2) , b = 11.70 (2) , and c = 17.28 (4) . A three-dimensional X-ray photographic investigation of twinned apparent orthorhombic crystals of Rh4(CO)12 has also been completed. The successful analysis of the structure of Rh4(CO)12 was accomplished by the assumption of a simple twinning mechanism and the application of derived mathematical relations which made possible a breakdown of the observed F2 data for the twin composite into the appropriate F coefficients for the single-crystal component. The results of the analysis conclusively showed (a) that the individual twin component contains four tetrameric molecular species in a centrosymmetric monoclinic unit cell of dimensions a = 9.24 (2) , b = 12.02 (2) , c = 17.74 (3) , and β = 90°, with symmetry P21/c, and (b) that the twinning mirror plane parallel to the unique b axis and perpendicular to either the a or c axis results in apparent orthorhombic D2h-mmm Laue symmetry. Both structures solved by the heavy-atom technique (complicated in each case by the pseudomirror symmetries imposed by metal atom positions) were refined isotropically by full-matrix least squares to conventional discrepancy factors R1(F) = 0.126 for Co4(CO)12 and R1(F) = 0.096 for Rh4(CO)12, based on 529 and 962 nonzero observed photographic data, respectively. Despite the different ways in which these complexes crystallize, the molecular configurations of the two polynuclear metal carbonyls are strikingly similar and have idealized C3v-3m symmetry. Each molecule consists of an apical M(CO)3 group coordinated by the three metal-metal bonds to a basal M3(CO)9 fragment containing three chemically identical M(CO)2 groups linked to each other by metal-metal bonds and bridging carbonyl groups. A detailed structural comparison of Co4(CO)12 and Rh4(CO)12 is given.

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