New bimetallic NiRh carbonyl clusters: synthesis and X-ray structure of the trigonal antiprismatic [NiRh5(CO)14]3− and body-centered cubic [NiRh14(CO)28]4− cluster anions

The reaction of the [Ni6(CO)12]2 dianion with [Rh(COD)Cl]2 (COD /cyclooctadiene) in excess affords a complicate mixture of bimetallic Ni /Rh clusters, from which the new [NiRh5(CO)14]3 and [NiRh14(CO)28]4 have been isolated and characterised. The [NiRh14(CO)28]4 tetraanionic cluster has been also obtained from reacting [Ni2Rh12(CO)25]4 and [NiRh13(CO)25]5 with [Rh(CO)2Cl]2, whereas [NiRh5(CO)14]3 has been more conveniently obtained by reduction with alkali metals or hydroxides of [NiRh5(CO)15] . All these new bimetallic Ni /Rh carbonyl clusters have been isolated in the solid state as tetrasubstituted ammonium salts and were characterised by elemental analysis, spectroscopy and X ray diffraction studies. The structure of the diamagnetic [NiRh5(CO)14]3 trianion is identical to that of the homometallic [Co6(CO)14]4 tetraanion, the unique Ni atom being disordered over the 6 equiv. trigonal antiprismatic sites. The [NiRh5(CO)14]3 trianion is slowly and irreversibly degraded to a mixture of Ni(CO)4, [Rh(CO)4] and [Rh7(CO)16]3 upon exposure to an atmosphere of carbon monoxide. The [NiRh14(CO)28]4 tetranion displays a metal frame consisting in a hexacapped cube of rhodium atoms centered by the unique nickel atom. This metal frame has previously been found in the homometallic [Rh15(CO)30]3 cluster. However, at difference from the latter, [NiRh14(CO)28]4 features two less valence electrons and displays a shrinked cubic moiety. [NiRh14(CO)28]4 is slowly degraded by carbon monoxide and halide ions to give [NiRh13(CO)25]5 and other yet uncharacterised Ni /Rh carbonyl clusters. All above Ni /Rh clusters do not display protonation behaviour.