By means of the electron spin resonance (ESR) technique, we have investigated the electronic structures of the tridentate imino nitroxyl diradical complex with copper(II) (Cu-bisimpy), which has a square planar structure and a ground quartet state with an extremely strong ferromagnetic exchange interaction, and its related compounds (bisimpy = 2,6-bis(1'-oxyl-4',4',5',5'-tetramethyl-4',5'-dihydro-1'H-imidazol-2'-yl )pyridine). It was clarified that Cu-bisimpy had unique magnetic orbitals, compared with the biradical ligand (bisimpy), a zinc(II) biradical complex (Zn-bisimpy) and a copper(II) terpyridine complex (Cu-tpy) (tpy = 2,2';6',2 ''-terpyridine). Multifrequency ESR spectroscopy provided a reliable set of magnetic parameters of Cu-bisimpy, which has a small g anisotropy (g(x) = 2.02, g(y) = 2.01, g(z) = 2.08) and small hyperfine coupling with Cu (vertical bar A(x)vertical bar = 42.0 MHz, vertical bar A(y)vertical bar <= 14 MHz, vertical bar A(z)vertical bar = 153 MHz) but huge zero-field splitting (D = +17.4 GHz, E = -1.0 GHz). The maximum principal axis of zero-field interaction (Z(ZF)) is perpendicular to the z axis for the g and A tensors, which is normal to the molecular plane. These characteristics of the magnetic properties prove that the substantial spin transfer from the d(x)(2)-(2)(y) Orbital of copper to the n-orbitals of the ligand is caused by a a-type covalent bonding effect between the central metal and the ligand nitrogens. The covalent bonding effect produces carbene configurations on the nitrogen atoms of the imino nitroxyl radicals. The carbene configuration was concluded to be the main reason for the strong ferromagnetic coupling in Cu-bisimpy.. Multifrequency electron spin resonance spectroscopy clarified the unique electronic structure of a square planar copper(II) complex with an imino nitroxyl diradical, which undergoes a strong ferromagnetic interaction caused by a covalent bonding effect.