The electronic spectrum of μ-oxo-bis[pentammine chromium(III)] ion

The single crystal polarized spectra of μ-oxo-bispentammine chromium(III)] chloride and perchlorate salts are measured over the temperature range 300 to 8 K. All sharp bands are predominantly z polarized where z is the CrOCr axis. The sharp and intense near ultraviolet absorptions are assigned as symmetric double excitations of t_2g electrons allowed through a vibronically exhange — induced electric dipole mechanism. This new mechanism can explain the observed z polarization, temperature-independent intensity and relatively high values of the oscillator strengths, f ≈ 10^?4?10^?3.The pair energies calculated from H= ∑_ikj1^Ja(ik)b(jl)⁵a(ik)^.5b(jl) do not agree with the observed values