New bimetallic NiRh carbonyl clusters: synthesis and X-ray structure of the trigonal antiprismatic [NiRh 5(CO) 14] 3− and body-centered cubic [NiRh 14(CO) 28] 4− cluster anions

Abstract

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 [NiRh 5(CO) 14] 3− and [NiRh 14(CO) 28] 4− have been isolated and characterised. The [NiRh 14(CO) 28] 4− tetraanionic cluster has been also obtained from reacting [Ni 2Rh 12(CO) 25] 4− and [NiRh 13(CO) 25] 5− with [Rh(CO) 2Cl] 2, whereas [NiRh 5(CO) 14] 3− has been more conveniently obtained by reduction with alkali metals or hydroxides of [NiRh 5(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 [NiRh 5(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 [NiRh 5(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 [NiRh 14(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, [NiRh 14(CO) 28] 4− features two less valence electrons and displays a shrinked cubic moiety. [NiRh 14(CO) 28] 4− is slowly degraded by carbon monoxide and halide ions to give [NiRh 13(CO) 25] 5− and other yet uncharacterised NiRh carbonyl clusters. All above NiRh clusters do not display protonation behaviour.