Altering Charges on Heterobimetallic Transition-Metal Carbonyl Clusters

The homoleptic group 5 carbonylates [M(CO)₆ ]^- (M=Nb, Ta) serve as ligands in carbonyl-terminated heterobimetallic Ag_m M_n clusters containing 3 to 11 metal atoms. Based on our serendipitous [Ag₆ {Nb(CO)₆ }₄ ]²⁺ (4 a²⁺ ) precedent, we established access to such Ag_m M_n clusters of the composition [Ag_m {M(CO)₆ }_n ]^x (M=Nb, Ta; m=1, 2, 6; n=2, 3, 4, 5; x=1-, 1+, 2+). Salts of those molecular cluster ions were synthesized by the reaction of [NEt₄ ][M(CO)₆ ] and Ag[Al(OR^F )₄ ] (R^F =C(CF₃ )₃ ) in the correct stoichiometry in 1,2,3,4-tetrafluorobenzene at -35 °C. The solid-state structures were determined by single-crystal X-ray diffraction methods and, owing to the thermal instability of the clusters, a limited scope of spectroscopic methods. In addition, DFT-based AIM calculations were performed to provide an understanding of the bonding within these clusters. Apparently, the clusters 3⁺ (m=6, n=5) and 4²⁺ (m=6, n=4) are superatom complexes with trigonal-prismatic or octahedral Ag₆ superatom cores. The [M(CO)₆ ]^- ions then bind through three CO units as tridentate chelate ligands to the superatom core, giving overall structures related to tetrahedral AX₄ (4²⁺ ) or trigonal bipyramidal AX₅ molecules (3⁺ ).