The anisotropic ligand polarization intensity mechanism in dihedral lanthanide(III) complexes

Abstract

The f-f electronic spectra of a D ₄ coordination compound [Eu^(III)(2,2′-bipyridine-N,N′-dioxide)₄]³⁺ and of D ₃ analogues indicate, on comparison, that only the ΔM_J = ±1 component of the quadrupolar Eu(III) ⁷ F ₀ → ⁵ D ₂ transition is active in the D ₄ case. The intensity of a quadrupolar metal-ion transition in a D ₄ complex is governed uniquely by a single rank-2 environmental ligand tensor, within the restriction to one-electron operators, and it generates a first order electric dipole moment solely for the ΔM_J = ± 1 transition component. A rank-2 ligand tensor cannot arise from a configurational interaction mechanism based upon a static point-charge or dipolar crystal field, nor from the dynamic ligand-polarization mechanism in the mean polarizability approximation. The extension of the latter mechanism to include the polarizability anisotropy of the ligand bonds gives a rank-2 ligand polarization field tensor F ⁽²⁾ ₀ which accounts for the ⁷ F ₀ → ⁵ D ₂ dipole strength of the D ₄ complex investigated.