A series of methylcobalt(III) compounds with solely classical ligands such as NH3, H2O, NO2-, CN-, and 1,2-ethanediamine (en) has been prepared by ligand substitution of the pentaamminemethylcobalt(III) cation. The new compounds have been characterized by H-1, C-13, and Co-59 nuclear magnetic resonance spectroscopy and by absorption spectroscopy. Single-crystal X-ray structure determinations at 122.0(5) K were performed on [Co(NH3)(5)(CH3)]S2O6 (1), trans-[Co(en)(2)(NH3)(CH3)]S2O6 (2), and [Co(NH3)(6)]-mer,trans-[Co(NO2)(3)(NH3)(2)(CH3)](2)-trans-[Co(NO2)(4)(NH3)(2)] (3). Compound 1 is monoclinic, P2(1)/n, with cell dimensions a = 6.4856(9) Angstrom, b = 18.0720(11) Angstrom, c = 9.7854(14) Angstrom, and beta = 98.280(13)degrees. Compound 2 is orthorhombic, P2(1)2(1)2, with cell dimensions a = 12.211(3) Angstrom, b = 10.107(3) Angstrom, and c = 11.514(3) Angstrom. Compound 3 is monoclinic, P2(1)/n, with cell dimensions a = 8.390(2) Angstrom, b = 8.970(3) Angstrom, c = 20.650(9) Angstrom, and beta = 91.29(2)degrees. Virtually identical Co-C distances in the range 1.980-1.984 Angstrom are found in the three compounds. The methyl group exerts a strong trans influence. The largest relative elongation of the bond to the trans ligand is 0.170 Angstrom in the trans-[Co(en)(2)(NH3)(CH3)](2+) cation. The structural trans influence appears to decrease with the overall charge of the coordination compound. Estimates of the scalar one-bond cobalt-carbon coupling constants, (1)J(Co-C), in the pentacyanomethylcobalt(III) anion was obtained by line shape analysis, yielding values of 127, 95, and 62 Hz for the cis cyano groups, the trans cyano group, and the methyl group, respectively.