In the solid state, tris(3,5-di-tert-butylcatecholato)molybdenum(VI) forms a dimer with seven-coordinate molybdenum and bridging catecholates. NMR spectroscopy indicates that the dimeric structure is retained in solution. The molybdenum center has a high affinity for Lewis bases such as pyridine or pyridine-N-oxide, forming seven-coordinate monomers with a capped octahedral geometry, as illustrated by the solid-state structure of (3,5-(t)Bu(2)Cat)(3)Mo(py). Structural data indicate that the complexes are best considered as Mo(VI) with substantial pi donation from the nonbridging catecholates to molybdenum. Both the dimeric and the monomeric tris(catecholates) react rapidly with water to form free catechol and oxomolybdenum bis(catecholate) complexes. Monooxomolybdenum complexes are also obtained, more slowly, on reaction with dioxygen, with organic products consisting mostly of 3,5-di-tert-butyl-1,2-benzoquinone with minor amounts of the extradiol oxidation product 4,6-di-tert-butyl-1-oxacyclohepta-4,6-diene-2,3-dione. The pyridine-Noxide complex reacts on heating (with excess pyO) to form initially (3,5-(t)Bu(2)Cat)(2)MoO(Opy) and ultimately MoO3(Opy), with quinone and free pyridine as the only organic products. The decay of (3,5-(t)Bu(2)Cat)(3)Mo(Opy) shows an accelerated, autocatalytic profile because the oxidation of its product, (3,5-(t)Bu(2)Cat)(2)MoO(Opy), produces an oxo-rich, catecholate-poor intermediate which rapidly conproportionates with (3,5-(t)Bu(2)Cat)(3)Mo(Opy), providing an additional pathway for its conversion to the monoproduct. The tris(catecholate) fragment Mo(3,5-(t)Bu(2)Cat)(3) deoxygenates Opy in this nonclassical oxygen atom transfer reaction slightly less rapidly than does its oxidized product, MoO(3,5-(t)Bu(2)Cat)(2).