Whereas there are hundreds of known iron(II) spin-crossover compounds, only a handful of cobalt(II) spin-crossover compounds have been discovered to date, and hardly an in depth study on any of them exists. This review begins with an introduction into the theoretical aspects to be considered when discussing spin-crossover compounds in general and cobalt(II) systems in particular. It is followed by case studies on [Co(bpy)3]2+ and [Co(terpy)2]2+ (bpy = 2,2′-bipyridine, terpy = 2,2′:6′,2″-terpyridine) presenting and discussing results from magnetic susceptibility measurements, X-ray crystallography, optical spectroscopy, and EPR spectroscopy.
  • Density-functional theory investigation of the geometric, energetic and optical properties of the cobalt(II)tris(2,2'-bipyridine) complex in the high-spin and in the Jahn-Teller active low-spin state
    A. Vargas, M. Zerara, E. Krausz, A. Hauser and L.M. Lawson Daku
    Journal of Chemical Theory and Computation, 2 (2006), p1342-1359
    DOI:10.1021/ct6001384 | unige:3638 | Abstract | Article HTML | Article PDF
State-of-the-art generalized gradient approximation (GGA) (PBE, OPBE, RPBE, OLYP, and HCTH), meta-GGA (VSXC and TPSS), and hybrid (B3LYP, B3LYP*, O3LYP, and PBE0) functionals are compared for the determination of the structure and the energetics of the D3 [Co(bpy)3]2+ complex in the 4A2 and 4E trigonal components of the high-spin 4T1g( t52g e2g ) state and in the low-spin 2E state of octahedral 2Eg( t62g e1g) parentage. Their comparison extends also to the investigation of the Jahn−Teller instability of the 2E state through the characterization of the extrema of C2 symmetry of this spin state's potential energy surface. The results obtained for [Co(bpy)3]2+ in either spin manifold are very consistent among the functionals used and are in good agreement with available experimental data. The functionals, however, perform very differently with respect to the spin-state energetics because the calculated values of the high-spin/low-spin energy difference ΔEelHL vary between −3212 and 3919 cm-1. Semilocal functionals tend to give too large ΔEelHL values and thus fail to correctly predict the high-spin state as the ground state of the isolated complex, while hybrid functionals tend to overestimate the stability of the high-spin state with respect to the low-spin state. Reliable results are, however, obtained with the OLYP, HCTH, B3LYP*, and O3LYP functionals which perform best for the description of the isolated complex. The optical properties of [Co(bpy)3]2+ in the two spin states are also analyzed on the basis of electronic excitation calculations performed within time-dependent density functional response theory. The calculated absorption and circular dichroism spectra agree well with experimental results.



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