Intersystem Crossing Dynamics in the Iron(III) Spin-Crossover Compounds [Fe(acpa)2]PF6 and [Fe(Sal2tr)]PF6

The high-spin → low-spin relaxation dynamics of the Fe(III) spin-crossover complexes [Fe(Sal2tr)]PF6 (H2Sal2tr = Bis(salicylaldimino)triethylenetetramine) and [Fe(acpa)2]PF6 (Hacpa = N-(1-acetyl-2-propylidene)-2-pyridylmethylamine) are discussed within the theory of nonadiabatic multiphonon relaxation. A Huang−Rhys factor S of ≈25, estimated on the basis of average metal−ligand bond length differences ΔrHL of ≈ 0.12 Å, explains the observed low-temperature tunneling rate constants kHL(T→0) of ≈ 102 s-1 as well as the thermally activated process at T > ≈100 K semiquantitatively. The results obtained for the Fe(III) compounds are compared to those for Fe(II) spin-crossover compoun