The microcrystalline sample of a parent complex, [CoCl2(py)4], showed a single-molecule magnet (SMM) behavior with an effective activation barrier, Ueff/kB, of 16 K for reversal of the magnetism in the presence of a dc field of 3 kOe. Pyridine ligands having 2–4 diazo moieties, DYpy; Y = 2, 3l, 3b, and 4, were prepared and confirmed to be quintet, septet, septet, and nonet in the ground state, respectively, after irradiation. The 1:4 complexes, CoCl2(DYpy)4; Y = 2, 3l, 3b, and 4 in frozen solutions after irradiation showed the magnetic behaviors of SMMs with total spin multiplicity, Stotal = 17/2, 25/2, 25/2, and 33/2, respectively. Hysteresis loops depending on the temperature were observed and the values of coercive force, Hc, at 1.9 K were 12, 8.4, 11, and 8.1 kOe for CoCl2(CYpy)4; Y = 2, 3l, 3b, and 4, respectively. In dynamic magnetic susceptibility experiments, ac magnetic susceptibility data obeyed the Arrhenius law to give Ueff/kB values of 94, 92, 93, and 87 K for CoCl2(CYpy)4; Y = 2, 3l, 3b, and 4, respectively, while the relaxation times for CoCl2(CYpy)4; Y = 2 and 3l, obtained by dc magnetization decay in the range of 3.5–1.9 K slightly deviated downward from Arrhenius plots on cooling. The dynamic magnetic behaviors for CoCl2(CYpy)4 including [CoCl2(py)4] and CoCl2(C1py)4 suggested that the generated carbenes interacted with the cobalt ion to increase the relaxation time, τq, due to the spin quantum tunneling magnetization, which became larger with increasing Stotal of the complex.