Analysis of d–d Transitions in [Cr(CN)(NH3)5]2+ as Inferred from Polarized Optical Spectra and Angular Overlap Model Calculations

Abstract

Abstract The polarized single-crystal absorption spectra of [Cr(CN)(NH3)5]Cl(ClO4) were measured for interconfigurational quartet (t2g3 → t2g2eg) and intraconfigurational doublet transitions (t2g3 → t2g3). The band splittings of the spin-allowed transitions, 4A2g → 4T2g,4T1g, were derived to 600—1000 cm−1 from a Gaussian band deconvolution of the σ and π spectra. At low temperature (77 K), more than 45 sharp bands could be detected in the intercombination band region of the lowest doublets (2Eg, 2T1g), which were completely assigned to the five possible zero-phonon transitions and corresponding vibronic sidebands. The vibrational frequencies due to the electronic ground state were obtained from the infrared spectrum and by measuring the emission lines under resonant excitation, providing us with a sufficient data base for a normal coordinate treatment of the pentaamminecyano cation. The d-energy level scheme is rationalized using the angular overlap model (AOM). Geometric parameters were obtained from an X-ray analysis of the chloride–perchloride salt, which shows a cation of approximately 4mm (C4v) site symmetry, consistent with a linearly coordinated cyano group. The AOM parameters evaluated for the CN- ligand show strong σ-antibonding (eσ = ca. 7500 cm−1) together with unusually larger π-bonding (eπ = ca. −900 cm−1), which both contribute to the extremely high value of the respective ligand field parameter Dq. The large splitting of the lowest doublet state, 2Eg (Oh), has been shown to originate from a low-symmetry effect in the treatment of interelectronic repulsion. The attributed tetragonal orbital expansion parameter (τ = 0.99) was calculated to be similar to values recently obtained for related pentaammineaniono complexes.