The structure of (dibenzylideneacetone)(pentamethylcyclopentadienyl)rhodium(I)

The structure of (dibenzylideneacetone)(pentamethylcyclopentadienyl)rhodium(I), Rh(C₅Me₅)(dba), (dba = PhCHCHCOCHCHPh, Ph = C₆H₅, Me = CH₃), has been determined from three-dimensional X-ray data collected by counter methods. The structure has been refined by least-squares techniques to a final R index on F of 0.035 based on 2100 observations above background. The material crystallizes in the monoclinic space group C⁵_2hP2₁/a, with four molecules in a cell of dimensions a=14.348(12), b=14.063(13), c=11.393(10) », β=104.42(3)°. The observed and calculated densities are 1.35(3) and 1.41 g/cm³. The compound is monomeric. The Rh atom is bonded to the C₅Me₅ ring on one side and to the dba molecule through the two olefinic double bonds on the other. The C₅ and Me₅ portions of the C₅Me₅ ring are planar, but not coplanar as the Me group are bent away from the Rh atom by 0.10 » relative to the C₅ plane. The dba molecule is in the s-cis,s-cis conformation with the two double bonds in a plane which is parallel to the planes of the C₅Me₅ ring. The rest of the dba molecule is nonplanar with the CO group pointing away from Rh. The olefinic double bonds have lengthened on coordination to 1.411(9) ». The H atoms on the terminal C atoms of the dba molecule are in very close contact, the refined H?H distance being 1.83(9) ». It is suggested that the strain imposed on the coordinated dba molecule by this interaction might contribute to its ease of dissociation from the complex in solution and hence account for the catalytic properties of the complex.