Crystal structure of (5-pentamethylcyclopentadienyl)(triphenylphosphine)(ethylene)rhodium(I).A case of important p-component in a metal-olefin bond

Abstract: The crystal and molecular structure of (sS-CsMes)(PPh3)Rh(C2H4w)a s determined from x-ray intensity data measured by a single-crystal automated diffractometer. Crystals are monoclinic, space group P21/n, with a = 12.180 (I), b = 23.416 (2), c = 9.177 (1) A, /3 = 96.60 (2)'. The structure was solved by Patterson and Fourier methods and refined by least squares to a conventional R value of 0.037, based on 2359 observed reflections. In the molecule, of approximate C, symmetry, the rhodium atom is coordinated to a CsMes ring, a triphenylphosphine molecule, and the ethylene ligand, with the approximate mirror plane passing through the Rh and P atoms and the middle point of the ethylene C-C bond. The geometry of the CsMes ligand is discussed also with reference to a partial localization of electron charge within the ring. The ethylene ligand, whose hydrogen atoms were clearly located in a Fourier difference map, is far from being planar and its conformation, which is characterized by a twist of 20' of the CH2 groups about the C-C bond axis and by an almost tetrahedral geometry of the C atoms, suggests an important T component in the metal-ethylene bond.