NEW ASPECTS OF RHODIUM(III) DIMETHYL-SULFOXIDE CHEMISTRY - SYNTHESIS AND MOLECULAR-STRUCTURE OF [NET(4)]-[CIS-RHCL4)(CH3)2SO(2] AND CHEMICAL BEHAVIOR IN AQUEOUS-SOLUTION OF [RHCL(N))(CH3)2SO((6-N)](3-N) (N=3 OR 4) COMPLEXES

A careful reinvestigation of the synthesis of the long known complex [H(dmso)2][trans-RhCl4(dmso-S)2] 2a has resulted in the isolation of the unprecedented cis isomer [H(dmso)2][cis-RhCl4(dmso-S)2] 2b. The new isomer has been characterized spectroscopically and the crystal structure of the anion determined as the tetraethylammonium salt. Crystal data: monoclinic, space group P2(1)/c, Z = 4, a = 10.500(4), b = 17.054(3), c = 13.304(5) angstrom, beta = 114.19(2)degrees. As in the case of the neutral isomers, mer,cis- and mer,trans-[RhCl3(dmso-S)2(dmso-O)] 1a and 1b, respectively, the anionic trans derivative is thermodynamically unstable and isomerizes to the cis isomer in dmso solution with first order kinetics; the reverse cis to trans isomerization is promoted by visible light. A comparative investigation of the chemical behaviour of the two pairs of isomers 1 and 2 in aqueous solution, determined by H-1 and C-13 NMR spectroscopy, has also been carried out. In both cases dissociation of the ligand trans to an S-bonded dmso ligand is the first step.