Skeletal Growth by Condensation of Small Metal Fragments on a Carbido Carbonyl Cluster. Synthesis of the Anions [Rh15C2(CO)24X2]3- (X = Cl, Br, I) and Molecular Structure of the Bromo and Iodo Derivatives

The reaction of [Rh12C2(CO)24]2- (as K+ salt) with RhI derivatives yields an uncharacterized intermediate (1) from which, by reaction with halides, the new carbido-carbonyl cluster anions [Rh15C2(CO)24X2]3- (X = Cl, 2a; Br, 2b; I, 2c) have been obtained. The molecular structures of 2b and 2c as [N(n-Pr)4]+and [NEt4]+ salts, respectively, have been determined by single-crystal X-ray diffractometry. The two anions are isostructural and contain a Rh15C2 cluster that can be described as a C2v centered and tetracapped pentagonal prism. The carbide atoms occupy octahedral interstices, and the halide ligands span basal edges of the prism. The CO ligands have been found partially ordered in the dibromo derivative and totally ordered in the diiodo one, adopting an unprecedented and peculiar asymmetric stereogeometry. The room-temperature solution 13C NMR spectrum indicates that in compound 2c a fraction of the carbonyl ligands is fluxional, probably those found disordered in the solid state structure of 2b.