Exploring the Ligand Chemical Space of the [Cr(TpmR)2]2+ Spin-Crossover Family

The electronic and steric factors that control the spin-crossover behavior in the [Cr(Tpm)2]2+ family (Tpm = tris(pyrazolyl)methane) have been investigated by means of density functional theory calculations. Systematic functionalization of the ligand framework reveals that both electronic and steric effects strongly influence the spin-state energetics and the corresponding transition temperatures. Substitution at different positions of the pyrazolyl rings affects the ligand field through distinct pathways, including resonance-mediated electronic effects and steric modulation of the metal-ligand bond lengths. The analysis of the electronic structure and ligand-field splitting through the relevant d-based molecular orbitals provides a molecular-level understanding of the observed trends and identifies the key factors in controlling the stability of the low- and high-spin states. These results allow for the construction of structure-property relationships within the [Cr(Tpm)2]2+ family and provide guidelines for the rational design of new Cr(II)-based spin-crossover compounds.