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

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 ΔMJ = ±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 ΔMJ = ± 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.