ORGANOMETALLIC RHODIUM(III) AND RHODIUM(I) COMPLEXES WITH 1,3-DIMETHYLLUMAZINE(DML) - THE CRYSTAL-STRUCTURE OF [(ETA(2)-(O-4,N-5)DML)(ETA(5)-C(5)ME(5))CLRH](PF6)

The crystal structure determination of the new rhodium(III)-complex [( DML)(C(5)Me(5))ClRh](PF6), DML = 1,3-dimethyllumazine, reveals O-4,N-5 coordination of the metal to DML with not very different bond lengths of 213.0(2) pm (Rh-N) and 219.2(2) pm (Rh-O). This result stands in c ontrast to the previously reported structure of a cationic dihydrobiop terin bound to [Mo(O)CL(3)](-) which exhibited a very unsymmetrical ch elate arrangement. Chemical and electrochemical two-electron reduction of the Rh(III) complex led to a very labile Rh(I) species (DML)(C(5)M e(5))Rh which is distinguished by an intense charge transfer band in t he visible. The results confirm the characterization of DML as a weak sigma donor/pi acceptor ligand with a rather low-lying pi orbital.