New bimetallic Ni&z.sbnd;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&z.sbnd;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&z.sbnd;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&z.sbnd;Rh carbonyl clusters. All above Ni&z.sbnd;Rh clusters do not display protonation behaviour. [Copyright &y& Elsevier]