A novel catecholato complex, Nb-V(tpp)(cat)(Hcat), where cat and Heat are two distinct catechol ligands (a bidentate catecholate dianion and a monodentate catecholate monoanion, respectively) and tpp is 5,10,15,20-tetraphenylporphyrin dianion, has been isolated in the reaction of NBV(tpp)(O)(AcO) with catechol, where AcO is an acetato ligand. Its molecular structure has been determined by X-ray crystallography. Crystal data : monoclinic, space group P2(1)/c, Z = 4, a = 14.592(3) Angstrom, b = 23.46(1) Angstrom, c = 14.415(4) Angstrom, beta = 100.95(2)degrees, R = 0.079. The molecular structure was confirmed to have no oxo ligand, and in this respect it is unique among the niobium(V) porphyrins. The bidentate and monodentate ligands are located on the same side of the porphyrin ring. The heptacoordinate niobium atom is bonded to the four gyrrole nitrogen atoms and to three oxygen atoms in the catechols. The niobium atom is displaced by 1.02 Angstrom from the mean plane of the four nitrogen atoms. The structure of the complex in solution and the mechanism of the Nb = O cleavage were investigated by means of H-1-NMR spectroscopy. The bidentate catechol is oriented in C-s symmetry with respect to the porphyrin plane, and the monodentate catechol is located perpendicularly to both the bidentate catechol and the porphyrin plane. Two intermediates with the bidentate catechol were observed after addition of 2 equiv of catechol to Nb(tmp)(O)(AcO) at -30 degrees C, where tmp denotes the 5, 10, 15, 20-tetramesitylporphyrin dianion. These intermediates were determined to be Nb(tmp)(cat)(OH) and Nb(tmp)(cat)(AcO). Thus, the Nb = O bond of Nb(tmp)(O)(AcO) was easily cleaved to create the two intermediates. We propose a unique route to the Nb = O cleavage that involves an intramolecular electron transfer from the catechol ligand coordinated at the first stage through a ligand exchange with AcO. Both protonation and electron transfer to the Nb = O moiety play important roles in the Nb = O cleavage.