Alkylcobalt(III) Schiff base B-12 model complexes with secondary alkyls or a bulky diamine in the equatorial position were synthesized and characterized. Structures have been first determined by X-ray diffraction analysis for i-C4H9Co(salen)(gamma-pic) (I), n-C3H7Co(salen)(gamma-pic) (II) and C2H5Co(SB) (III), where salen = N,N’-ethylenebis-(salicylideneamine) dianion; SB = 1,1,2,2-tetramethyl-N,N’-ethylenebis(salicylideneamine) dianion, gamma-pic = gamma-picoline. Crystal data for I(CoC26N3O2H30) : space group P2(1)/c with a = 6.661(5) Angstrom, b = 18.612(2) Angstrom, c = 19.533(3) Angstrom, beta = 98.93(1)degrees, V = 2392.10 Angstrom(3), D-calcd = 1.320 g . cm(-3), Z = 4, and R = 0.048 for 4469 measured reflections. Crystal data for II (CoC25N3O2H28) : space group P2(1)/c, a = 9.609(6) Angstrom, b = 19.169(8) Angstrom, c = 12.995(9) Angstrom, beta = 106.9(7)degrees, V = 2290.4 Angstrom(3), D-calcd = 1.332 g . cm(-1), Z = 4, and R = 0.048 for 4358 measured reflections. Crystal data for III (CoC22N2O2H27) : space group P2(1)/c, a = 8.318(3) Angstrom, b = 21.579(2) Angstrom, c = 11.572(2) Angstrom, beta = 93.35(1)degrees, V = 2073.7 Angstrom(3), D-calcd = 1.314 g . cm(-1), Z = 4, and R = 0.060 for 3954 measured reflections. The crystal structure data reveal that complexes I and II display six-coordinate octahedral geometry; their Co-C, Co-N bond lengths, as well as the Co-C-C angles, are very close to those in 5’-deoxyadenosylcobalamin. Complex III is one of the very few compounds having five-coordinate square pyramidal geometry and observed instability of the Co-C bond.