The solutions of 1:4 complexes of Co(X)(2)-(D1py)(4), X = Cl-, and NCO-and D1py = phenylpyridyldiazomethane, were photolyzed under cryogenic conditions, and their magnetic properties were investigated by direct current (DC) and alternating current (AC) magneto/susceptometries. After irradiation, the resulting cobalt-carbene complexes, Co-(X)(2)(C1py)(4), exhibited the behaviors of heterospin single-molecule magnets (SMMs) strongly depending on the axial ligands. In Co(X)(2)(C1py)(4): X = Cl- and NCO-, the effective activation barriers, U-eff, for the reorientation of the magnetic moment and the resonant quantum tunneling time, tau(Q), characteristic to SMM properties were estimated to be 91 and 130 K, and 4 x 10(3) and 2 x 10(5) s, respectively. The tau(Q) of Co(NCS)(2)(C1py)(4) with U-eff = 89 K was found to be 6 x 10(2) s. In Co(X)(2)(C1py)(4): X = Cl- and NCO-, temperature-dependent hysteresis loops were also observed below the blocking temperature (T-B = 3.2 and 4.8 K, respectively) and the coercive forces, He, of 7.0 and 20 kOe at 1.9K, respectively, were obtained. In a series of 1:4 complexes of Co(X)(2)(C1py)(4), X = NCS-, Cl-, and NCO-, the axial ligands strongly affected the heterospin SMM properties, and the NCO- ion having the large magnitude of the ligand-field splitting in a spectrochernical series, gave the largest Ueff and He and the longest cc?,