Electronic and Molecular Structure of High-Spin d⁴ Complexes:  Experimental and Theoretical Study of the [Cr(D₂O)₆]²⁺ Cation in Tutton's Salts

Variable-temperature spectroscopic and crystallographic studies on the chromium(II) Tutton's salts, (MI)2Cr(X2O)6(SO4)2, where MI = ND4+, Rb+, or Cs+, and X = H or D, are reported. Inelastic neutron scattering (INS) and multifrequency EPR experiments facilitate a rigorous definition of the ground-state electronic structure from 1.5 up to 296 K, which is unprecedented for a high-spin d4 complex. Modeling of the INS data using a conventional S = 2 spin Hamiltonian reveals a dramatic variation in the axial and rhombic zero-field-splitting parameters. For the ammonium salt, D and E are -2.454(3) and 0.087(3) cm(-1) at 10 K and -2.29(2) and 0.16(3) cm(-1) at 250 K, respectively. A temperature variation in the stereochemistry of the [Cr(D2O)6]2+ complex is also identified, with an apparent coalescence of the long and medium Cr-O bond lengths at temperatures above 150 K. The corresponding changes for the rubidium and cesium salts are notable, though less pronounced. The experimental quantities are interpreted using a 5Ee Jahn-Teller Hamiltonian, perturbed by anisotropic strain. It is shown that good agreement can be obtained only by employing a model in which the anisotropic strain is itself temperature dependent. A new theoretical approach for calculating variable-temperature EPR spectra of high-spin d4 complexes, developed within the 5Ee coupling model, is described. Differences between spin-Hamiltonian parameters determined by INS and EPR are consistent with those of the different time scales of the two techniques.