Understanding Antiferromagnetic and Ligand Field Effects on Spin Crossover in a Triple-decker Dimeric Cr(II) Complex

Two possible explanations for the temperature-dependence of spin crossover (SCO) behavior in the dimeric triple-decker Cr(II) complex ([(C5Me5)Cr(P5)Cr(C5Me5)]+) have been offered. One invokes variations in antiferromagnetic interactions between the two Cr(II) ions, while the other posits the development of a strong ligand-field effect favoring the low-spin ground state. We perform multireference electronic structure calculations based on multiconfiguration pair-density functional theory to resolve these effects. We find quintet, triplet, and singlet electronic ground states, respectively, for the experimental geometries at high, intermediate, and low temperatures. The ground-state transition from quintet to triplet at intermediate temperature derives from increased antiferromagnetic interactions between the two Cr(II) ions. By contrast, the ground-state transition from triplet to singlet at low temperature is attributable to increased ligand-field effects, which dominate with continued variations in antiferromagnetic coupling. This study provides quantitative detail for the degree to which these two effects can act in concert for the observed SCO behavior in this complex and others subject to temperature dependent variations in geometry.