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The photochemistry of many transition metal complexes is governed by a multitude of electronically excited states, coupled by various mechanisms. For the transition metal complex HMn(CO) 3 (dab) (dab ) 1,4-diaza-1,3-butadiene) the photoreactivity (cleavage of the Mn-H bond) and electronic absorption spectra are characterized on the basis of quantum mechanical first-principles calculations. In a first step, the A′ ground (singlet) and the three lowest electronically excited (triplet) potential curves along the Mn-H bond distance are computed using the CASSCF/CCI method. Two of the excited states are found to be bound and are of the metal-to-ligand charge transfer type, whereas the third, ligand-to-ligand charge transfer state is repulsive. In the relevant energy region, two avoided crossings are observed, indicative for strong nonadiabatic couplings. In a second step, the UV/vis photochemistry of the complex is investigated by means of nuclear wave packet dynamics. We solve the nonadiabatically coupled, time-dependent Schrödinger equation in a diabatic representation for different initial conditions to determine both photodissociation yields and electronic absorption spectra. In particular, the effect of the nonadiabatic couplings on the electronic absorption spectrum and on the photoreactivity is investigated.