Geometrical variations which occur when a quadridentate Schiff-base co-ordinates to a cobalt(II) atom are compared on the basis of the crystal structure analysis of the ligand NN′-(o-phenylene) bis(salicylideneamine)(I) and its Co^II derivative in its orthorhombic (II) and monoclinic (III) modifications. Crystals of (I) are monoclinic, space group P2₁/c, with cell parameters: a= 6.064(3), b= 16.541(7), c= 13.306(7)Å, β= 91.5(1)°. Crystals of (II) are orthorhombic, space group P2₁2₁2₁, with a= 16.755(7), b= 17.532(8), c= 5.362(3)Å, and of (III) are monoclinic, space group P2₁/nwith a= 10.681(5), b= 8.354(4), c= 18.185(8)Å, β= 105.3(1)°. A total of 1 277 (I). 1 113 (II), and 2 558 (III) independent reflexions were used : the structures were solved from diffractometer data by the heavy-atom method and refined to final R factors of 0.056 (I), 0.046 (II), and 0.041 (III). The enolimine form is established for (I) in the solid state. Upon co-ordination, with formation of (II) and (III), the geometrical data suggest that the contribution to the resonance of a ketamine form becomes as important as that of the enolimine. This is in agreementwith a π-orbital delocalization of the electronic charge over the planar complex molecule.