Metal (4f)−ligand (Cl 3p) bonding in LnCl₆^3- (Ln = Ce to Yb) complexes has been studied on the basis of 4f→4f and Cl,3p→4f charge-transfer spectra and on the analysis of these spectra within the valence bond configuration interaction model to show that mixing of Cl 3p into the Ln 4f ligand field orbitals does not exceed 1%. Contrary to this, Kohn−Sham formalism of density functional theory using currently available approximations to the exchange-correlation functional tends to strongly overestimate 4f−3p covalency, yielding, for YbCl₆^3-, a much larger mixing of Cl 3p→4f charge transfer into the f¹³ ionic ground-state wave function. Thus, ligand field density functional theory, which was recently developed and applied with success to complexes of 3d metals in our group, yields anomalously large ligand field splittings for Ln, the discrepancy with experiment increasing from left to the right of the Ln 4f series. It is shown that eliminating artificial ligand-to-metal charge transfer in Kohn−Sham calculations by a procedure described in this work leads to energies of 4f−4f transitions in good agreement with experiment. We recall an earlier concept of Ballhausen and Dahl which describes ligand field in terms of a pseudopotential and give a thorough analysis of the contributions to the ligand field from electrostatics (crystal field) and exchange (Pauli) repulsion. The close relation of the present results with those obtained using the first-principles based and electron density dependent effective embedding potential is pointed out along with implications for applications to other systems.

The ground and excited state properties of the Cr³⁺ ion doped into the cubic host lattices Cs₂NaYCl₆ and Cs₂NaYBr₆ have been studied using density functional theory. A new symmetry based technique was employed to calculate the energies of the multiplets ⁴A_2g, ⁴T_2g, ²E_g, and ⁴T_1g. The geometry of the CrX^{3 -Â} ₆ cluster was optimized in the ground and excited states. A Madelung correction was introduced to take account of the electrostatic effects of the lattice. The experimental Cr–X distance in the ground state can be reproduced to within 0.01 Ã… for both chloride and bromide systems. The calculated d–d excitation energies are typically 2000–3000 cm^–1 too low. An energy lowering is obtained in the first ⁴T_2g excited state when the octahedral symmetry of CrX^{3 -Â} ₆ is relaxed along the eg Jahn–Teller coordinate. The geometry corresponding to the energy minimum is in excellent agreement with the ⁴T_2g geometry derived from high-resolution optical spectroscopy of Cs₂NaYCl₆:Cr³⁺. It corresponds to an axially compressed and equatorially elongated CrX^{3 -Â} ₆ octahedron.

The luminescence of [CrX₆]^3– X=Br^–, Cl^– has been studied through density functional theory (DFT) using both deMon and ADF codes. Multiplet energies⁴A₂,²E,⁴T₂, and⁴T₁ have been expressed as energies of non-redundant single determinants and calculated as in Ref. [1]. The influence of the metal ligand distance on the multiplet energies has been investigated. Of particular interest to this work is the Jahn-Teller effect distortion. We found that the system moves to a more stable geometry when the axial bond length is compressed and the equatorial one elongated in agreement with the experimental value.

The absorption spectra of the ferrate (VI) ion (FeO^2-₄) in K₂MO₄ (M = S, Se, Cr, Mo) host lattices consist of a series of relatively weak bands at low energy, which can be assigned to transitions within the partially filled 3d shell and some intense bands at higher energy, which are assigned to ligand-to-metal charge transfer transitions (LMCT). In the near-infrared (NIR) region sharp lines are observed belonging to the spin-forbidden spin-flip transitions ³A₂→ ¹E and ³A_{2 Â} â†’ ¹A₁. The lowest excited state is the ¹E state, serving as initial state for ¹E → ³A₂ sharp-line luminescence at around 6200 cm^-1. Another luminescence is observed centered at 9000 cm^-1, which is assigned to the ³T₂ → ³A₂ transition. It is rather broad and three orders of magnitude weaker than the ¹E luminescence at 30K as a result of efficient non-radiative relaxation processes to the ¹E state. The temperature dependence of the total intensity and the lifetime of the ¹E → ³A₂ luminescence is understood within a complex scheme of radiative and non-radiative processes.

The two title compounds were synthesized and investigated with the inelastic-neutron-scattering (INS) technique. They contain mixed YbMBr₉^3- (M=Cr³⁺, Ho³⁺) dimers as discrete units, and the magnetic excitations of mixed Yb³⁺-Cr³⁺ and Yb³⁺-Ho³⁺ dimers could thus be observed. The Yb³⁺-Cr³⁺ dimer has three INS transitions, for which anisotropic exchange, as well as zero-field splitting of Cr³⁺, has to be included in the exchange Hamiltonian. For the Yb³⁺-Ho³⁺ dimer the effect of the exchange interaction manifests itself as a broadening and a splitting of the crystal-electric-field levels of the isolated Ho³⁺ ion. Taking into account the full (2J + 1) ground-state multiplet of Ho³⁺, as well as anisotropic exchange, gives a satisfactory description of this dimer.

Quantum chemical calculations based on density functional theory have been performed on ruthenocene. Excellent agreement is obtained with groundâ€� and excitedâ€�state properties derived from optical spectroscopy. In particular, the energies of the first d–d excitations, the unusually large Stokes shift, the structural expansion of Ru(cp)₂ and the substantial reduction of the Ruâ€�cp force constant in the first triplet excited state are almost quantitatively reproduced. The lowestâ€�energy excitation is found to have substantial charge transfer character.

The electronic ground state of isolated Cr³Â ⁺ introduced into the title compounds has been investigated with electron spin resonance and electronâ€�nuclear double resonance spectroscopy. Simultaneously a multiple scattering(MS) Xα study of the (CrCl₆)³Â ^−cluster has been performed. The experimental results agree with a cubic Cr site. They further show evidence for strong quadrupoleinteraction at the anion neighbor nuclei and for observably different covalency in the two hosts. Rather good agreement is found between the predictions of the MS Xα model and the experimental superhyperfine interaction constants but not with the Crâ€�hyperfine structure constant. It is suspected that the second neighboring Cs play a nonâ€�negligible role in the electronic structure of the cluster.