Reactions between [(CpIrCl2)-Ir-*](2) and diaryl-substituted diazabutadienes (dab) ArN=CR'-CR'=NAr (R' = H: Ar = 4-MeC6-H-4, 2,6-Me2C6H3, 2,6-(i) Pr2C6H3; R' = Me: Ar = 4-MeC6H4) proceed straightforwardly to give the dab complexes (CpIr)-Ir-*(dab)Cl+Cl-. Sterically more encumbered dab ligands (R' = Me: Ar = 2,6-Me2C6H3, 2,6-(i) Pr2C6H3, 2,4,6-Me2C6H3) instead give products Cp-* IrClCH2C(=NHAr)CMe=N(Ar)Cl-+(-) in which the dab ligand has undergone C-H activation at one of methyl groups of the dab bridge. The C-H cleavage reaction is reversible in the sense that thermally, the cyclometalated Ir complex decomposes to regenerate the free ligand. (C) 2002 Elsevier Science B.V. All rights reserved.