(2,3,7,8,12,13,17,18-Octamethylcorrolato [Mn(OMC)], has been characterized by several physical measurements. In the presence of nitrogenous bases the complex exists as a valence tautomer [Mn-II(OMC(.+))] as demonstrated by H-1 NMR and EPR. Complete resonance assignment in the NMR spectrum has been achieved by systematic substitution of the peripheral substituents. The crystal structure of the first example of a tetracoordinated tetrapyrrolic macrocycle Mn(III)) complex, (7,13-dimethyl-2,3,8,12,17,18-hexamethylcorrolato)manganese(III), [Mn-7,13-Me(2)-HEC)], is also reported. Crystal data with Cu K alpha (lambda = 1.541 78 Angstrom) at 293 K are as follows : C33H39MnN4, a = 4.671(2) Angstrom, b = 28.31(2) Angstrom, c = 20.882(6) Angstrom, beta = 94.60(3)degrees, V = 2753(2) Angstrom(3), Z = 4, monoclinic, space group P2(1)/n, 4088 data, R1 = 0.0563 for 4088 observed reflections with I > 2 sigma(I). The analysis reveals a high degree of planarity of the macrocycle and the existence of strong overlap between the pi systems with the formation of an infinite stack of molecules.