Bis(hexafluoroacetylacetonato)copper(II) {Cu(hfac)(2)} formed with diazodi-4-pyridylmethane yellow-green plates (a) and dark green bricks (b) in a 1:1. complex. X-ray crystal and molecular structure analysis of a single crystal of a revealed that the two pyridyl nitrogens of two different ligand molecules are coordinated with Cu(II) in a trans configuration to produce a one-dimensional chain. Complex b, contains solvent molecules. The temperature dependence of molar magnetic susceptibility revealed that, while the complexes are dilute Cu(II) paramagnets before irradiation, they become ferromagnetic chains due to 3d spin of Cu(II) and 2p spins of di-4-pyridylcarbene generated by photolysis of the diazo group. The field dependence of magnetization revealed that the apparent spin quantum number of the ferromagnetic chains amounts to S = ca. 33.6 in complex b. A theoretical approach revealed that the ferromagnetic interaction between the neighboring Cu(II) and the carbene center along the chain amounts to J/k(B) = +66.8 K. A 1:2 complex of Cu(hfac)(2) with diazophenyl-4-pyridylmethane (2) was prepared as a reference, and the photogenerated carbenes in the complex were found to interact with Cu(II) ions in a ferromagnetic fashion.