New Bimetallic Ni–Rh Carbonyl Clusters: Synthesis and X-ray Structure of the [Ni7Rh3(CO)18]3?, [Ni3Rh3(CO)13]3? and [NiRh8(CO)19]2? Cluster Anions

The reaction of the Ni₆(CO)₁₂]²⁻ dianion with Rh(COD)Cl]₂ (COD = cyclooctadiene) in acetone affords a mixture of bimetallic Ni–Rh clusters, mainly consisting of the new Ni₇Rh₃(CO)₁₈]³⁻ and Ni₈Rh(CO)₁₈]³⁻ trianions. A study of the reactivity of Ni₇Rh₃(CO)₁₈]³⁻ led to isolation of the new Ni₃Rh₃(CO)₁₃]³⁻ and NiRh₈(CO)₁₉]²⁻ anions. All these new bimetallic Ni–Rh carbonyl clusters have been isolated in the solid state as tetrasubstituted ammonium salts and have been characterised by elemental analysis, X-ray diffraction studies, ESI-MS and electrochemistry. The unit cell of the NEt₄]₃Ni₇Rh₃(CO)₁₈] salt contains two orientationally-disordered ν₂-tetrahedral Ni₇Rh₃(CO)₁₈]³⁻ trianions with occupancy factors of 0.75 and 0.25. Besides, their inner Ni₃Rh₃ octahedral moieties show two cis sites purely occupied by Rh atoms, two trans sites purely occupied by Ni atoms and the remaining two cis sites are disordered Ni and Rh sites with respective occupancy fraction of 0.5. At difference from the parent Ni₇Rh₃(CO)₁₈]³⁻, the octahedral Ni₃Rh₃(CO)₁₃]³⁻ displays an ordered distribution of Ni and Rh atoms in two staggered triangles. The NiRh₈(CO)₁₉]²⁻ dianion adopts an isomeric metal frame with respect to that of the PtRh₈(CO)₁₉]²⁻ congener. As a fallout of this work, new high-yield synthesis of the known Ni₆Rh₃(CO)₁₇]³⁻ and Ni₆Rh₅(CO)₂₁]³⁻, as well as other currently-investigated bimetallic Ni–Rh clusters have been obtained.