Optical dimer excitations and exchange parameters in (Et4N)(3)Cr2F9: Firstobservation of the (4)A(2) -> (2)A(1) transition

The synthesis, crystal growth, and polarized optical absorption spectra in the visible and near-UV of (Et4N)(3)Cr2F9 are reported. In the energy range 25800-27700 cm(-1) the (4)A(2) --> (2)A(1) (O notation) ligand field trans ition can be resolved in detail for the first time in any Cr3+ compound. Th is allows the determination of the antiferromagnetic ground-state exchange splitting with great accuracy: J = 25.9 cm(-1) and j = 0.27 cm(-1) using th e Hamiltonian H = J(S-A.S-B) - j(S-A.S-B)(2), where j leads to deviations f rom the regular Lande pattern. The temperature dependence of the magnetic s usceptibility is nicely reproduced by these parameters. A comparison with C s3Cr2Cl9 and Cs3Cr2Br9 reveals an exponential dependence of the ground-stat e splitting upon the Cr-Cr distance in the [Cr2X9](3-) dimers, This is the result of a dominant sigma -type orbital exchange pathway along the Cr-Cr a xis.