ANALYSIS OF D-D TRANSITIONS IN [CR(CN)(NH3)(5)](2-SPECTRA AND ANGULAROVERLAP MODEL-CALCULATIONS() AS INFERRED FROM POLARIZED OPTICAL)

The polarized single-crystal absorption spectra of [Cr(CN)(NH3)(5)]Cl( ClO4) were measured for interconfigurational quartet (t(2g)(3) --> t(2 g)(2)e(g)) and intraconfigurational doublet transitions (t(2g)(3) --> t(2g)(3)). The band splittings of the spin-allowed transitions, (4)A(2 g) --> T-4(2g), T-4(1g), were derived to 600-1000 cm(-1) from a Gaussi an band deconvolution of the sigma and pi spectra. At low temperature (77 K), more than 45 sharp bands could be detected in the intercombina tion band region of the lowest doublets ((2)E(g), T-2(1g)), which were completely assigned to the five possible zero-phonon transitions and corresponding vibronic sidebands. The vibrational frequencies due to t he electronic ground state were obtained from the infrared spectrum an d by measuring the emission lines under resonant excitation, providing us with a sufficient data base for a normal coordinate treatment of t he pentaamminecyano cation. The d-energy level scheme is rationalized using the angular overlap model (AOM). Geometric parameters were obtai ned from an X-ray analysis of the chloride-perchloride salt, which sho ws a cation of approximately 4mm (C-4v) site symmetry, consistent with a linearly coordinated cyano group. The AOM parameters evaluated for the CN- ligand show strong sigma-antibonding (e(sigma) = ca. 7500 cm(- 1)) together with unusually larger pi-bonding (e(pi) = ca. -900 cm(-1) ), which both contribute to the extremely high value of the respective ligand held parameter Dq. The large splitting of the lowest doublet s tate, (2)E(g) (O-h), has been shown to originate from a low-symmetry e ffect in the treatment of interelectronic repulsion. The attributed te tragonal orbital expansion parameter (tau = 0.99) was calculated to be similar to values recently obtained for related pentaammineaniono com plexes.