New NiRh carbonyl clusters with unprecedented structural and electronic features: synthesis and X-ray structure of [NEt4]3[Ni10Rh(CO)19]·C2H5CN, [NMe3CH2Ph]3[Ni6Rh3(CO)17] and [NEt4]3[Ni9Rh3(CO)22]

The reaction of the [Ni6(CO)12]2− dianion with [Rh(COD)Cl]2 (COD=cyclooctadiene) affords in sequence the new [Ni10Rh(CO)19]3− (1) and [Ni9Rh3(CO)22]3− (2) trianionic clusters with good selectivity. The related [Ni6Rh3(CO)17]3− (3) trianionic cluster has been initially isolated among the degradation products of [Ni9Rh3(CO)22]3− (2) under carbon monoxide and has been more conveniently synthesized by condensation of [Rh(CO)4]− with Ni(CO)4 in excess. All these new bimetallic NiRh carbonyl clusters have been isolated in the solid state as tetrasubstituted ammonium salts and were characterized by elemental analysis, spectroscopy and X-ray diffraction studies. The structure of [Ni10Rh(CO)19]3− (1) consists of a distorted and incomplete Rh-centered Ni9(μ9-Rh) icosahedral moiety, capped by the tenth Ni atom on one of the interlayer triangular faces. The [Ni10Rh(CO)19]3− (1) trianion is moderately stable to oxidation and, as inferable from IR monitoring of the resulting reaction mixture, is irreversibly degraded upon exposure to an atmosphere of carbon monoxide to a mixture of Ni(CO)4, [Rh(CO)4]− and [Ni5(CO)12]2−. The [Ni9Rh3(CO)22]3− (2) trianion displays a three layer metal frame identical to that previously found in the [H4−xNi9Pt3(CO)21]x− (x=2–4) clusters. However, it features two less valence electrons. The structure of [Ni6Rh3(CO)17]3− (3) is closely related to that of [Ni9Rh3(CO)22]3− (2) and may be formally derived from that of the latter by substitution of a Ni3(CO)3(μ-CO)3 moiety with a face bridging carbonyl group. Notably, [Ni6Rh3(CO)17]3− (3) is isoelectronic but not isostructural with the known [Ni6Ir3(CO)17]3− congener. Both [Ni6Rh3(CO)17]3− (3) and [Ni9Rh3(CO)22]3− (2) are readily degraded by carbon monoxide at atmospheric pressure to colourless solutions containing Ni(CO)4 and [Rh(CO)4]− mixtures. Their reaction with [Rh(COD)Cl]2 leads to the previously reported [Ni6Rh5(CO)21]3− trianion and other yet uncharacterised NiRh carbonyl clusters. All above NiRh clusters do not display protonation–deprotonation behaviour and chemically reversible redox propensity; electrochemical experiments failed in all cases to show the occurrence of electrochemically reversible redox changes.