A case of solid-state isomerism for a rhodium carbonyl cluster anion. Synthesis, characterization in solution and X-ray crystallographic structures of the salts [nMe4]3[Rh11(CO)23]·Me 2CO and [nMe4]3[Rh11(CO)23]·C 6H5Me

Two crystal species, [NMe4]3[Rh11(CO)23]·Me 2CO (1) and [NMe4]3[Rh11(CO)23]·C 6H5Me (2), containing two isomers of the same anion, have been isolated and investigated. The salt (1) crystallizes in the monoclinic space group P21/n, with a = 12.275(4), b = 19.987(5), c = 24.007(5) Å, β = 104.47(3)°, and Z = 4; the salt (2) gives monoclinic crystals, space group P21/c, with a = 19.545(2), b = 12.597(2), c = 24.151(2) Å, β = 92.62(1)°, and Z = 4. The structures were solved and refined on the basis of 7 421 [(1)] and 3 531 [(2)] significant counter data, converging to final R values of 0.032 and 0.041, respectively. The metallic skeleton (of ideal D3h symmetry) is the same in the two species, based on three face-to-face condensed octahedral units, sharing a common edge, coincident with the ideal three-fold axis. The carbonyl stereochemistry however is different in (1) and (2), as well as the pattern of metal-metal bonds. Complexes (1) and (2) have identical i.r. spectra in solution and variable-temperature 13C n.m.r. spectra show that the ligands are completely fluxional, even at -90 °C.