SYNTHESIS, STRUCTURE, AND BONDING OF THE UNUSUAL MU-SIGMA,SIGMA-ALLENYLIDENE COMPLEX )(MU-SIGMA,SIGMA-C=C=CPH(2))(CO)(2)(PCY(3))(2)]BF4

The binuclear complex [Rh(mu-OOCCH3)(CO)(PCy(3))](2) (1) reacts with 1 ,1-diphenyl-2-propyn-1-ol to give )(mu-eta(1):eta(2)-C2C(OH)R(2))(CO)( 2)(PCy(3))(2)] (2), which affords )(mu-sigma,sigma-C=C=CPh(2))(CO)(2)( PCy(3))(2)]BF4 . 1/2CH(2)Cl(2) (3) by protonation with HBF4 . OEt(2). The molecular structure of 3 can be described as a coordinatively unsa turated (30 valence electrons according to the EAN rule) dinuclear spe cies, containing a single rhodium-rhodium bond (d(Rh-Rh) = 2.723(1) An gstrom). Additionally, the metal centers are bridged by an allenyliden e ligand and a carboxylate group. The C-alpha-C-beta and C-beta-C-gamm a distances of the former are 1.30(1) and 1.32(1) Angstrom, while the C-alpha-C-beta-C-gamma angle is 174.4(12)degrees. The electronic struc ture ofthe model cations u-OOCH)(mu-sigma,sigma-C=C=CH2)(CO)(2)(PH3)(2 )](+) (5) and [Rh-2(mu-OOCH)(mu-C=CH2)(CO)(2)(PH3)(2)](+) (6) have bee n studied by means of approximate EHT MO calculations. The interaction of the vinylidene ligand with the bimetallic unit is similar to that of the allenylidene. For both cases the unsaturated eta(1)-carbon liga nd has a net acceptor behavior. In the allenylidene ligand the highest negative charge is located on the C-alpha carbon atom; this charge de creases toward the C-gamma carbon atom. In contrast, for the complex 6 , the C-alpha bridging carbon atom is the least negatively charged.