Crystal and molecular structure ofcis-dicarbonyltetraphosphinechromium(O), [(CO)2(PH3)4Cr]

The crystal and molecular structure ofcis-dicarbonyltetraphosphinechromium(O), [(CO)₂(PH₃)₄Cr] has been determined by X-ray diffraction methods. The compound crystallizes in the monoclinic system: space groupC2/c,a = 6·968(6),b = 11·98(1),c = 12·87(1) Å, = 99·80(8) °,V= 1059(3) ų,Z=4,D_m = 1·534(5),D_x = 1·531(5) gcm^–3, diffraction symmetry 2/m. The structure was solved by conventional methods, and refined by least-squares techniques to an agreement factorR = 0·078. The chromium atoms lie on rotation diads, and thus the molecules must have at least the symmetry 2(C₂); they were found to have, within the limits of accuracy, the even higher symmetrymm2 (C_2v). There are two distinct independent Cr-P distances in the molecule, the Cr-P bonds where phosphorus istrans to carbonyl [2·338(4) Å] being significantly longer than those where phosphorus istrans to phosphine [2·282(4) Å]. This difference may be attributed to a difference in the back-bonding capabilities of the two types of ligands.We wish to thank Professor Dr E. O. Fischer for his stimulating interest in this work. Our thanks are due also to the Leibniz Rechenzentrum der Bayerischen Akademie der Wissenschaften for use of their computing facilities. This work would not have been possible without a generous grant from the Deutsche Forschungsgemeinschaft which is gratefully acknowledged.