Polarized crystal spectra of tris(ethylenediamine) chromium(III) ion

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Abstract
The low-temperature, single-crystal, polarized spectra of the Cr(en)3 3+ complex in two crystals, 2[Cr(en)3]Cl3. KCl. 6H2O (I) and [Cr(en)3]-Cl3. 3·5 H2O (II) are presented. Transitions to the spin-forbidden 2 E, 2 T 1, and 2 T 2 states and the spin-allowed 4 T 2 and 4 T 1 states have been observed and analysed. An interference effect has been noted between the sharp 2 T 2 state and the broad continuum-like 4 T 2 state. The spin-orbit trigonal-field components of the 2 E and 2 T 1 states have been experimentally determined, and theoretically justified by means of a complete crystal-field spin-orbit calculation. The R 1 and R 2 lines in crystal I (π polarization) are shown to have different linewidths at 1·6 K; this is ascribed to a life-time broadening process for which the radiationless transition rate has been calculated. The much broader linewidths observed in crystal II are the result of random Stark fields from the statistically absent waters of hydration. Satellite bands are observed around the R 1 and R 2 lines in crystal I at 1·6 K. It is suggested that they result from different conformations of the ethylenediamine ligands which are stabilized by hydrogen-bonding in the crystal. Both the emission spectrum and the temperature dependence of the R 1 and R 2 absorption lines point to a ground state phonon mode of ∼ 19 cm-1.