Ferromagnetic exchange coupling was operative between nickel(II) spins in a dinuclear nickel(II) complex [{Ni(hfac)(2)}(2)(py(4)C)] with J/k(B) = +0.57(2)K (H = -2JS(1) . S-2), where hfac and py(4)C Stand for 1,1,1,5,5,5-hexafluoropentane-2,4-dionate and tetrakis(2-pyridyl)methane, respectively. X-ray crystal structure analysis revealed an approximate D-2d symmetry of the [Ni(py4C)Ni] core and accordingly the orthogonal arrangement between the two coordination basal planes. Mechanism of the ferromagnetic coupling was proposed on the basis of the orthogonal d sigma-type magnetic orbitals, which was supported by DFT calculation. (c) 2006 Elsevier B.V. All rights reserved.