Chemical and electrochemical oxidation of mer/fac-Cr(CO)3(.eta.1-L-L)(.eta.2-L-L) containing a pendant donor atom: ESR studies of the cations mer-[Cr(CO)3(.eta.1-L-L)(.eta.2-L-L)]+ and trans-[Cr(CO)2(.eta.2-L-L)2]+ (L-L = bidentate group 15 ligand