Synthesis, Structure, and Bonding of the Unusual μ-σ,σ-Allenylidene Complex [Rh2(μ-OOCCH3)(μ-σ,σ-CCCPh2)(CO)2(PCy3)2]BF4

The binuclear complex [Rh(μ-OOCCH 3 )(CO)(PCy 3 )] 2 ( 1 ) reacts with 1,1-diphenyl-2-propyn-1-ol to give [Rh 2 (μ-OOCCH 3 )(μ-η 1:η 2 -C 2 C(OH)R 2 )(CO) 2 (PCy 3 ) 2 ] ( 2 ), which affords [Rh 2 (μ-OOCCH 3 )(μ-σ,σ-C C CPh 2 )(CO) 2 (PCy 3 ) 2 ]BF 4 · 1 / 2 CH 2 Cl 2 ( 3 ) by protonation with HBF 4 ·OEt 2 . The molecular structure of 3 can be described as a coordinatively unsaturated (30 valence electrons according to the EAN rule) dinuclear species, containing a single rhodium−rhodium bond ( d (Rh−Rh) = 2.723(1) Å). Additionally, the metal centers are bridged by an allenylidene ligand and a carboxylate group. The C α −C β and C β −C γ distances of the former are 1.30(1) and 1.32(1) Å, while the C α −C β −C γ angle is 174.4(12)°. The electronic structure of the model cations [Rh 2 (μ-OOCH)(μ-σ,σ-C C CH 2 )(CO) 2 (PH 3 ) 2 ] + ( 5 ) and [Rh 2 (μ-OOCH)(μ-C CH 2 )(CO) 2 (PH 3 ) 2 ] + ( 6 ) have been 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 η 1 -carbon ligand has a net acceptor behavior. In the allenylidene ligand the highest negative charge is located on the C α carbon atom; this charge decreases toward the C γ carbon atom. In contrast, for the complex 6, the C α bridging carbon atom is the least negatively charged.