Anomalous behaviour at the 6 A 1 -2 T 2 crossover in iron (III) complexes

Abstract

Transition metal complexes with configurations d 4 , d 5 , d 6 and d 7 are of two types—high-spin and low-spin— depending on the strength of the ligand field. Ligand field theory predicts that truly octahedral complexes ML 6 could exist in which both types are in thermal equilibrium at ordinary temperatures, but no completely unambiguous examples of this 6 crossover ’ situation have yet been put forward. The magnetic and spectral properties associated with nearly equi-energetic high- and lowspin states should be singularly sensitive to temperature, pressure, and minor chemical modifications of the ligating molecules. These matters are discussed in detail, for the configuration d 5 , with particular emphasis on the variation of total molecular energy with change in metal-ligand separation, and on a necessary inequality, namely ∆ (high-spin) < π < ∆ (low spin) (∆ == ligand field strength, π = mean pairing energy). Experimental data for certain FeS 6 -type com pounds, viz. iron (III) N-N -dialkylditliiocarbamates, are then reported, which reproduce qualitatively all the requirements so formulated. The reciprocal magnetic susceptibility passes through a maximum and then a minimum with increasing temperatures and also increases sharply with applied pressure; the electronic spectrum is temperature dependent; and different alkyl substituents in the ligand drastically affect the magnetism. The values of ∆ and π, as obtained from independent evidence, conform with the inequalities previously mentioned. The temperature dependence of the magnetism does not exactly conform with the predictions of the conventional van Vleck equation, but is considered to be tractable when vibrational partition functions are introduced into this equation. The pressure dependence of the magnetism leads to an estimate of the difference in molar volume of the two states, from which it is concluded that the Fe-S distances differ by about 0.07 Å, that in the low-spin state being the shorter.