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]·Me2CO and [NMe4]3[Rh11(CO)23]·C6H5Me

Abstract

Two crystal species, [NMe₄]₃[Rh₁₁(CO)₂₃]·Me₂CO (1) and [NMe₄]₃[Rh₁₁(CO)₂₃]·C₆H₅Me (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 D_3h 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.