Magnetic Properties of Some High-Spin Complexes of Iron(II)

Spin‐State Energetics of FeII Complexes – The Continuing Voyage Through the Density Functional Minefield

“…[4] It was first reported in the 1930s by Cambi and Cagnasso [5] for d 5 Fe III dithiocarbamates, over a decade after the death of Werner. In 1964, the serendipitous discovery of the first d 6 Fe II complexes showing a thermal SCO, [Fe(phen) 2 (NCE) 2 ] (phen = 1,10-phenanthroline; E = S, Se), was reported, [6] and since then, many hundreds of new ligand systems that support SCO have been discovered. [1] Thermal SCO is characterised by the ST temperature, T 1/2 , which corresponds to the point where half the SCO centres are HS and half are LS.…”

“…The perhaps best-known mononuclear examples are octahedral complexes with d 4 , d 5 , d 6 , and d 7 configurations. These species will be high-spin (HS, S = 2, 5/2, 2, and 3/2, respectively) with weak-field ligands or low-spin (LS, S = 1, 1/2, 0, and 1/2, respectively) with strong-field ligands.…”

The foundation of modern spin-crossover

“…1). 1 When X -= Cl -, Br -, I -or N 3 -the compounds obey the Curie law, 2 exhibiting effective magnetic moments ( eff ) that are constant between 110 K and room temperature at 5.0-5.3 BM †. However, different behaviour was 15 observed when X -= NCS -or NCSe -.…”

“…A similar drop in  eff was also shown by the related complex cis-[Fe(NCS) 2 (bipy) 2 ] (bipy = 2,2'-20 bipyridyl). 1 Such behaviour is anomalous according to the Curie law, which predicts that  eff should be invariant with temperature for samples composed of discrete paramagnetic molecules, like these. 2 Baker and Bobonich noted that the drop in  eff for the thiocyanate and selenocyanate complexes is reminiscent of 25 antiferromagnetic coupling between the paramagnetic iron centres, but were unable to explain the data in more detail.…”

“…4 They simulated the magnetic susceptibility curves using a van Vleck equation for a system with an S = 0 magnetic ground state and a fixed S = 2 excited state. 2 Although it could not model the complete 1 The data points for each compound are linked by spline curves for clarity. 45 temperature range, this approach reproduced the magnetic moments reasonably well in the region of the transitions.…”