Selected Fe and Ru systems, whose photochemical behaviour is sensitive to numerous parameters, are presented. These systems, containing multiple species in equilibrium, are versatile enough to be adapted to special tasks and may also be used to model the phenomena and mechanisms occurring in nature. The role of various parameters is analysed and principal emphasis is given to the ligand sphere influence on the nature of the excited state and thereby on the photochemical mode. This is crucial in the case of Fe(II) complexes of the type , whereas in the carbolyl – cyclopentadienyl complexes, represented by , the nature of the excited state is of less importance than for pentacyanofer- rates(II). The photochemistry of the carbonyl – cyclopentadienyl complexes is more suscepti- ble to the impact of the medium and the role of the secondary processes is more significant.