INTERSYSTEM CROSSING DYNAMICS IN THE IRON(III) SPIN-CROSSOVER COMPOUNDS [FE(ACPA)(2)]PF6 AND [FE(SAL(2)TR)]PF6

The high-spin --> low-spin relaxation dynamics of the Fe(III) spin-cro ssover complexes [Fe(Sal(2)tr)]PF6 (H(2)Sal(2)tr = Bis(salicylaldimino )triethylenetetramine and [Fe(acpa)(2)]PF6 (Hacpa = N-(1-acetyl-2-prop ylidene)-2-pyridyl-methylamine) are discussed within the theory of non adiabatic multiphonon relaxation. A Huang-Rhys factor S of approximate to 25, estimated on the basis of average metal-ligand bond length dif ferences Delta r(HL) Of approximate to 0.12 Angstrom, explains the obs erved low-temperature tunneling rate constants k(HL)(T-->0) Of approxi mate to 10(2) s(-1) as well as the thermally activated process at T > approximate to 100 K semiquantitatively. The results obtained for the Fe(III) compounds are compared to those for Fe(II) spin-crossover comp ounds.