Skeletal growth by condensation of small metal fragments on a carbido carbonyl cluster. Synthesis of the anions [Rh15C2(CO)(24)X-2](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 Rh-I derivatives yie lds an uncharacterized intermediate (1) from which, by reaction with halide s, the new carbido-carbonyl cluster anions [Rh15C2(CO)(24)X-2](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 determin ed by single-crystal X-ray diffractometry. The two anions are isostructural and contain a Rh15C2 cluster that can be described as a C-2v centered and tetracapped pentagonal prism. The carbide atoms occupy octahedral interstic es, and the halide ligands span basal edges of the prism. The CO ligands ha ve been found partially ordered in the dibromo derivative and totally order ed in the diiodo one, adopting an unprecedented and peculiar asymmetric ste reogeometry. The room-temperature solution C-13 NMR spectrum indicates that in compound 2c a fraction of the carbonyl ligands is fluxional, probably t hose found disordered in the solid state structure of 2b.