The Pentaamminemethylcobalt(III) Cation: Synthesis and Spectroscopic Characterization.
by Pauli Kofod,
Inorg. Chem., 1995, 34(10), 2768-70.

The preparation and spectroscopic characterization of [Co(NH3)5(CH3)]2+ was described. The cation was characterized in solution by 13C and 59Co 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+ (Imax = 481 nm, emax = 50M-1 cm-1) was very similar to that of the isoelectronic cation [Co(NH3)6]3+, demonstrating that the coordinated methyl carbanion has ligand field properties similar to those of the isoelectronic ammonia molecule. Similarly the 59Co chemical shift value (delta = 7370 ppm) was close to that of the hexaamminecobalt(III) cation. The 13C NMR spectrum consisted of a single peak at delta = 3.2 ppm, which was broad (Dn1/2 = 18.4 Hz) due to spin spin coupling to the 59Co nucleus. An estimation of the coupling constant was 1JCo-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 B12 coenzyme. Disputed! See J.Halpern and J.P.Maher, J.Amer.Chem.Soc.,1964,86,2311. "Pentacyanobenzylcobaltate(III). A new series of stable organocobalt compounds" where [MeCo(CN)5]3- was also reported.