The crystal and molecular structures of five complexes [M2m4(C≡ CR)8(PPh3)2] (M = Rh, Ir; m = Cu, Ag; R = Ph, Fc (Ir/Cu only)) are described. Each consists of an approximately octahedral arrangement of the six metal atoms, with atoms M occupying opposite apical positions and atoms m forming an approximate plane. Four alkynyl groups are σ-bonded to each M. Two of these, one from each M, also interact by an asymmetric π-bonding mode with each atom m. Each M is also coordinated to one PPh3 ligand. The M–m distances range between 2.7698(9) and 3.2050(9) Å, although average values fall within the tight ranges 2.86(8) and 2.90(5) (m = Cu) or 3.06(5), 3.09(9) Å (m = Ag). Formally non-bonding M ··· M distances range between 4.1511(8) and 4.654(1) Å, while the shorter m ··· m separations range between 2.659(1) and 2.918(2) Å, which suggests that only weak intermetallic bonding exists. Both the geometries of these complexes and their intense colours suggest that the M2m4 cluster formulation is a major contributor to the structure, rather than a more simplistic, formally zwitterionic assemblage of two [M(C≡ CR)4(PPh3)]2− centres held together by the four m+ cations.
Bruce, M. I., Zaitseva, N. N., Skelton, B., Somers, N., & White, A. (2003). Crystal and Molecular Structures of Some Alkynyl-Group 9/Group 11 Complexes [M2m4(C2R)8(PPh3)2](M = Rh, Ir; m = Cu, Ag; R = Ph, Fc (Ir/Cu only)). Australian Journal of Chemistry, 56(5), 509-516. https://doi.org/10.1071/CH02094