The 1:2 mixtures of Co(p-tolsal)(2), p-tolsal = N-p-tolylsalicylideniminato, and diazo-pyridine ligands, DXpy; X = 1, 2, 31, 3h, and 4, in MTHF solutions were irradiated at cryogenic temperature to form the corresponding 1:2 cobalt-carbene complexes Co(p-tolsal)(2)(CXpy)(2), with S-total = 5/2, 9/2, 13/2, 13/2, and 17/2, respectively. The resulting Co(p-tolsal)(2)(CXpy)(2), X = 1, 2, 31, 31,, and 4, showed magnetic behaviors characteristic of heterospin single-molecule magnets with effective activation barriers, U-eff/kB, of 40, 65, 73, 72, and 74 K, for reorientation of the magnetic moment and temperature-dependent hysteresis loops with a coercive force, H-c, of similar to 0, 6.2, 10, 6.5, and 9.0 kOe at 1.9 K, respectively. The relaxation times, tau(Q), due to a quantum tunneling of magnetization (QTM) were estimated to be 1.6 s for Co(p-tolsal)(2)(C1py)(2), similar to 2.0 X 10(3) s for Co(p-tolsal)(2)(C2py)(2), and >10(5) s for Co(p-tolsal)(2)(CXpy)(2); X = 3b, 3I, and 4. In heterospin complexes, organic spins, carbenes interacted with the cobalt ion to suppress the QTM pathway, and the tau(Q) value increased with increasing the S-total values.