SYNTHESIS OF 2 NOVEL PENTANUCLEAR RHODIUM CLUSTERS BEARING THE ALKYL-LIKE LIGANDS CH2COOME AND CH2CN - X-RAY STRUCTURE OF [PPH4]2[RH5(MU-CO)6(CO)8(CH2CN)]

The reaction of [PPh4][Rh(CO)4] (1) under CO with BrCH2COOMe or BrCH2CN in a 5:3 molar ratio gave high yields of the two novel clusters [PPh4]2[Rh5(CO)14-(CH2COOMe)] (3a) and [PPh4]2[Rh5(CO)14(CH2CN)] (3b), respectively, which appear to be the first examples of rhodium clusters bearing an alkyl-like ligand. The first step of the reaction is the rapid formation of [PPh4]-[RhBr(CO)2(CH2R)] (R = COOMe (2a), CN (2b)), which further react, much more slowly, with an excess of [Rh-(CO)4]-to yield the cluster compounds. 3a,b are indefinitely stable in the solid state under CO atmosphere. Crystals of 3b suitable for an X-ray diffraction study could be obtained. 3b is monoclinic, space group P2(1)/c with a = 12.488 (3) angstrom, b = 43.585 (9) angstrom, c = 23.222 (8) angstrom, beta = 91.46 (4)-degrees, Z = 8, and R = 0.037 for 8489 reflections having I greater-than-or-equal-to 3-sigma(I). The [Rh5(CO)14(CH2CN)]- anion contains an elongated trigonal-bipyramidal metallic framework bearing eight terminal and six edge-bridging carbonyls; the alkyl-like substituent is bonded to an apical Rh atom.