The two key Mössbauer parameters, isomer shift and quadrupole splitting (see Figure ), are altered by several temperature-dependent effects. The second-order Mössbauer effect is a relativistic effect leading to a decrease in the isomer shift at elevated temperatures. An effect that may influence both the isomer shift and quadrupole splitting measured experimentally is that different electronic states may be thermally accessible at a given temperature due to the energetic proximity of different configurations or multiplicities, or due to even smaller energy gaps introduced by spin-orbit coupling. A well-known branch of chemistry where these effects are exploited explicitly are spin crossover complexes. Herein, we focus on an effect that mainly influences the quadrupole splitting: the influence of vibrational movements of a molecular complex that are inherently temperaturesensitive.