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 theMn-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 non-adiabatic couplings. In a second step, the UV/VIS photochemistry of the complex is investigated by means of nuclear wave packet dynamics. We solve the non-adiabatically 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 role of the non-adiabatic couplings on the electronic absorption spectrum, and on the photoreactivity is investigated.