Oxotris(oxalato)niobate(V) as counterion in cobalt(II) spin-crossover systems

This work is devoted to the investigation of the thermally induced spin-crossover behavior from a high-spin state (HS, S = 3/2) at higher temperatures to a low-spin phase (LS, S = 1/2) at lower temperatures of the six-coordinate cobalt(II) complex in the compound [Co(terpy)(2)](3)[NbO(C2O4)(3)]center dot 3CH(3)OH center dot 4H(2)O (2). The crystal structure of 2 together with that of its counterion as tetraphenylarsonium(V) salt (AsPh4)(3)[NbO (C2O4)(3)]center dot 9H(2)O (1) are also included. The spin-crossover process was followed by the thermal variation of the chi T-M product between 2.0 and 400 K under the warming mode, with the LS configuration being achieved at T <= 200 K and the LS -> HS interconversion being incomplete at 400 K. X-band EPR measurements both in solution and in the solid state were used to characterize the LS state. In order to substantiate the phenomenon in 2, an analytical expression was presented based on the combination of the spin-orbit coupling and both first- and second-order Zeeman effects for a d(7) electronic configuration. The best-fit parameters were C-vib = 0.0249, lambda = -163 cm(-1), alpha = 1.06, Delta =1532 cm(-1), with a fixed value of g(LS) = 2.11 determined from the fit of the EPR spectrum at 10 K. (C) 2016 Elsevier Ltd. All rights reserved.