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 [Ni-6(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)(2)Cl](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 [Co-6(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 [Rh-7(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 [Rh-15(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. (C) 2003 Elsevier Science B.V. All rights reserved.