THE ROLE OF PI-BONDING FOR TRIGONAL LEVEL SPLITTINGS IN CHELATE COMPLEXES .3. THE LOWEST ELECTRONIC STATES IN CR(ACAC)3

The 2Eg→ 4A2gpolarized emission spectra of Cr(III)-doped Ga(acac)3were measured down to 1.9 ? in the region of the zero-phonon transitions. Zero-field ground-state splitting could be optically resolved to 1.1–1.2 cm−1for the four inequivalent Cr(acac)3molecules in the host crystal. Polarization ratios and dipole oscillator strengths for D3* symmetry show 2Ȧ (2Eg) as the lowest emitting state. A large trigonal 2Egsplitting of about 250 cm−1was obtained from absorption measurements. These results were rationalized in terms of an extended angular overlap model, which considers specific π interactions between metal d and chelate-ring π orbitals and the detailed angular geometry simultaneously. From calculations for Cr(acac)3the π-bonding effects were found to be the main source for the sign and magnitude of the observed excited-state splittings, whereas the ground-state splitting is very sensitive to the angular geometry and the anisotropic spin-orbit coupling. Theoretical results obtained by introducing the Zeeman operator are consistent with the experimental findings obtained from Zeeman spectra reported previously. We have also checked the concept of misdirected valency, which considers the orientation of the oxygen lone pairs with respect to the metal-ligand bonding axes. However, with the introduction of σ-π mixing as reported for other acac−complexes, a level sequence was calculated, which is in variance with our polarized spectra. © 1990, American Chemical Society. All rights reserved.