The manganese tricarbonyl complex fac-Mn-(Br)(CO)(3)((Pr2Ph)-Pr-i-DAB) (1) ((Pr2Ph)-Pr-i-DAB = (N,N'-bis(2,6-di-isopropylphenyl)-1,4-diaza-1,3-butadiene)] was synthesized from the reaction of Mn(CO)(5)Br with the sterically encumbered DAB ligand. Compound 1 exhibits rapid CO release under low power visible light irradiation (560 nm) suggesting its possible use as a photoCORM. The reaction of compound 1 with TlPF6 in the dark afforded the manganese(1) tetracarbonyl complex, [Mn(CO)(4)((Pr2Ph)-Pr-i-DAB)][PF6] (2). While 2 is comparatively more stable than 1 in light, it demonstrates high thermal reactivity such that dissolution in CH3CN or THE at room temperature results in rapid CO loss and formation of the respective solvate complexes. This unusual reactivity is due to the large steric profile of the DAB ligand which results in a weak Mn-CO binding interaction.