UNUSUAL MAGNETIC PROPERTIES OF SOME SIX-COÖRDINATE COBALT(II) COMPLEXES¹ —ELECTRONIC ISOMERS

Spin-crossover in cobalt(II) imine complexes

“…Whereas for the high-spin species essentially one spin-allowed transition corresponding to the 4 T 1 (t 2g 5 e 2 g ) → 4 T 2 (t 2g 4 e 3 g ) transition in the near infrared is expected, several such transitions in the visible are to be expected for the low-spin species, the lowest ones corre- sponding to the 2 E(t 6 2g e 1 g ) → 2 T 2 (t 5 2g e 2 g ), 2 T 1 (t 5 2g e 2 g ) transitions. In addition to the ligand-field transitions one may expect metal to ligand charge transfer (MLCT) transitions at comparatively low energies [17].…”

“…The coordination motif of the first cobalt(II) complex with the tri-dentate terimine ligand [2] found to exhibit spin-crossover constitutes the basic motif for a whole class of cobalt(II) spincrossover compounds. The most prominent member is the [Co(terpy) 2 ] 2+ complex in combination with a number of different anions [13].…”

“…In octahedral symmetry, the two states involved are the 2 T 2 (t 5 2g ) state as low-spin state and the 6 A 1 (t 3 2g e 2 g ) state as high-spin state. The first cobalt(II) compound to show spin-crossover, namely [Co(L) 2 ]X 2 , where L = a tridentante terimine ligand, was presented by Stoufer et al [2] in 1961, the spin transition occurring between the 2 E(t 6 2g e 1 g ) and the 4 T 1 (t 5 2g e 2 g ) state. This was followed by the discovery of spin-crossover from the low-spin 1 A 1 (t 6 2g ) state to the high-spin 5 T 2 (t 4 2g e 2 g ) state in the iron(II) compound [Fe(phen) 2 (SCN) 2 ] by Madeja and König [3] in 1963.…”

The foundation of modern spin-crossover

“…5 König and Madeja's papers were not the first statement of the spin-crossover effect. While the idea had been discussed previously, 6 the first rigorous treatment had come in 1964 from 70 Ewald, Martin et al 7 That study had used spin-crossover to explain temperature-dependent magnetic moments in a series of iron(III) complexes dating from the 1930s. None-the-less, König and Madeja's extension of the rationale to iron(II) compounds is still significant, since iron(II) compounds have played the dominant role in spin-crossover research over the last 20 years.…”

“…1). The iron(III) complexes studied by Ewald and Martin, 7 and cobalt(II) compounds that were also discovered in the 1960s, 6 all exhibit spin-crossover as a gradual 10 thermal equilibrium spanning tens or hundreds of degrees.…”