On the spin equilibrium in bis(2,2′,2″-terpyridine) cobalt(II) salts

Optical investigation of spin-crossover in cobalt(II) bis-terpy complexes

“…4, confirm the conclusions obtained form the analysis of the optical spectra. The curve corresponding to [Co-(terpy) 2 ](ClO 4 ) 2 shows the typical behaviour for a cobalt(II) spin-crossover complex, with vT close to 0.4 cm 3 mol À1 K at lower temperature (spin-only value for the low-spin state) and around 2.4 cm 3 mol À1 K at room temperature, indicating a high-spin fraction of around 80% [4,5]. The curve for [Co(terpy) 2 ](PF 6 ) 2 is characteristic for a high-spin compound at low temperatures.…”

“…Red, transparent crystals of square pyramidal shape were obtained, which are dichroic in the case of [Co(terpy) 2 ](PF 6 ) 2 . As the presence of water molecules in the unit cell may influence the geometry and the local electronic structure of the title complexes [4,5], the synthesis was performed in water free solvents in order to synthesise the compounds without any lattice water. Elemental analyses confirmed the composition without any lattice solvent molecules.…”

“…Absorption max- ). The three more intense peaks have alternatively been assigned either to metal to ligand charge-transfer (MLCT) transitions [7] or to d-d transitions [5,7]. The broad band between 13 500 and 15 500 cm À1 has unanimously been attributed to the transitions from the 2 A 1 component of the low-spin 2 E ground state in compressed D 2 d symmetry to low-symmetry and spin-orbit levels arising from the 2 T 1 and 2 T 2 ligand-field states of octahedral parentage [5], using the energy level diagram of Liehr [9].…”

“…The [Co(terpy)] 2+ complex (terpy = 2,2 0 :6 0 ,2 00 -terpyridine) is one of the first and most important cobalt(II) spin-crossover complexes discovered to date [4,5], for which the transition occurs from the 2 E low-spin manifold to the 4 T 1 high-spin manifold. It has a two tridentate coordination motif.…”

“…The anhydrous [Co(terpy) 2 ] (ClO 4 ) 2 , for instance, is fully in the low-spin state at low temperatures and shows a transition to the high-spin state with a thermal population $80% at room temperature [4,5]. The various hydrated forms, on the other hand, show different degrees of partial spin transitions with remnant high-spin fractions at low temperature, or temperature independent populations of the two spin states.…”