The electronic structures of Ti(CatNSQ)(2) and Sn(CatNSQ)(2), where CatNSQ(2-) is the tridentate radical ligand (3,5-di-tert-butyl-1,2-semiquinonato 1(2-hydroxy-3,5-di-tert-butyl-phenyl)immine) were investigated with density functional (DF) calculations, using the local approximation for the exchange-correlation functional. The crystal structure of Sn(CatNSQ)(2) was solved. The complex crystallizes in the orthorhombic space group, C222(1), with Z = 8 in a unit cell of the following dimensions : a 19.580(5) Angstrom, b = 24.310(5) Angstrom, c = 23.690(5) Angstrom. The crystals are not isomorphous with similar M(CatNSQ)(2) (M = Ti, V) complexes previously reported. DF calculations showed that the triplet (S = 1) spin state is stabilized with respect to the first excited singlet (S = 0) state and the computed exchange coupling constant J is in semiquantitative agreement with the values obtained from magnetic susceptibility measurements. Using a symmetry-based multiplet structure decomposition in terms of states defined by a single determinant (single determinant method, SD) the energies of the excited singlet states were also computed in agreement with the experimental data. The calculations have shown that the main exchange mechanism between the organic radicals, responsible for the ferromagnetism of these complexes, is a superexchange pathway mediated by the 3d orbitals of Ti and the 4p empty orbitals of Sn. Magnetostructural correlations between the exchange coupling constant and the M-O and M-N bond distances have been established.