The preparation and spectroscopic characterization of [Co(NH3)(5)(CH3)](2+) is reported. This coordination cation was obtained as the analytically pure nitrate salt and was characterized in solution by C-13 and Co-59 NMR, UV-visible spectrophotometry, and cation exchange HPLC. The position of the first absorption band of the visible absorption spectrum of [Co(NH3)(5)(CH3)](2+) (lambda(max) (epsilon(max), M(-1) cm(-1)) : 481 nm (50)) is very similar to that of the isoelectronic coordination cation [Co(NH3)(6)](3+), thereby demonstrating that the coordinated methyl carbanion has ligand field properties similar to those of the isoelectronic ammonia molecule. Similarly the Co-59 chemical shift value (delta = 7370 ppm) is close to that of the hexaamminecobalt(III) cation. The C-13 NMR spectrum consists of a single peak at delta = 3.2 ppm, which is broad (Delta v(1/2) = 18.4 Hz) due to scalar coupling to the Co-59 nucleus. An estimate of the scalar one-bond cobalt-carbon coupling constant is (1)J(Co-C) = 105(5) Hz confirming the existence of a Co-C sigma-bond. With respect to the nature of the ligands the title compound is the simplest model compound for the vitamin B-12 coenzyme.