Synthesis and chemical characterization of the new anions [Rh3(μ3-E)2(CO)6]- (E = S or Se). Crystal and molecular structures of [PPh4][Rh3(μ3-S)2(CO) 6] and [NMe4][Rh3(μ3-Se)2(CO) 6]

Reactions of [Rh4(CO)12] or [Rh6(CO)16] in tetrahydrofuran with [SCN]-, [SeCN]-, or with polysulphides give, together with the decanuclear clusters [Rh10E(CO)22]2-, the trinuclear anions [Rh3(μ3-E)2(CO)6]- (E = S or Se). The latter species have been isolated and investigated by X-ray analysis. The salt [PPh4][Rh3(μ3-S)2(CO) 6] (1) crystallizes in the orthorhombic space group Pbc21, (no. 29) with a = 13.529(2), b = 16.965(2), c = 28.365(6) Å, and Z = 8; [NMe4][Rh3(μ3-Se)2(CO) 6] (2) crystallizes in the orthorhombic space group Ccm21 (no. 36) with a = 14.029(2), b = 10.889(1), c = 12.748(2) Å, and Z = 4. Both structures were solved by conventional Patterson and Fourier methods and refined on the basis of 3 438 and 1 032 significant independent counter data, respectively, to final conventional R values of 0.042 for compound (1) and 0.026 for (2). Both anions consist of a Rh3 triangle bicapped by triply bridging chalcogenide atoms and each rhodium atom, bearing two terminal CO groups, displays an almost square-planar geometry. The Rh-Rh distances are long [mean values 3.053 and 3.135 Å for compounds (1) and (2), respectively] and suggest very weak, if any, metal-metal interaction. The two anions can therefore be considered as trinuclear rhodium(I) aggregates rather than triangular clusters.