Chromium complexes, absorption symmetry

Low-spin Cr(II) complexes are octahedral with tetragonal distortion (distorted from Oi, to Z>4/, symmetry). They show two absorption bands, one in the visible and one in the near-infrared region, caused by this distortion. In a pure octahedral field, there should be only one d-d transition (see Chapter 11 for more details). Cr(II) also forms dimeric complexes with Cr — Cr bonds in many complexes. The acetate, Cr2(OAc)4, is an example in which the acetate ions bridge between the two chromiums, with significant Cr—Cr bonding resulting in a nearly diamagnetic complex. 

Transient absorption measurements have recently been recorded from the organometallic species chromium hexacarbonyl in ethanol solution [94], Absorption of a 65-fs, 310-nm excitation pulse was followed by measurement of excited-state absorption of a 65-fs, 480-nm probe pulse. The data shown in Figure 14 indicate a rapid nonexponential decay at short times followed by a gradual exponential rise. The slower feature was observed previously [95] and is known to correspond to the solvent complexation of Cr(CO)5 to yield Cr(CO)j(MeOH). The initial feature, which is observed at other probe wavelengths as well, is believed to correspond to the initial ligand loss reaction. Note that this case is different from ICN in that the initially excited wavepacket is not on the side of the Sj potential but rather (as is clear from the molecular symmetry) on a local potential maximum. The wavepacket must then spread that is, dissociation along either direction is equally likely. The rapid nonexponential decay was analyzed in terms of classical kinematics along a dissociative potential. 

This work lead to an interest in the luminescence of lower symmetry chromium ammine complexes. In these complexes, the 2Eg state is split into two components by the lower symmetry ligand field. The tetragonal ligand field parameters for many of these compounds were well known or easily available from optical spectroscopy, but the splitting of the 2Eg as measured by the absorption and emission spectroscopy... 

In aqueous solution, these effects have been clearly observed in the case of the chromium (III) and indium systems. As the indium system has been the object of Raman investigations (2, 3) with conflicting results, it is of special interest that we find three new peaks at 1136 K, 1050 K and 975 K, i.e. corresponding to a sulphato complex of C3V symmetry. The 1050 K absorption cannot be caused by HSO as this ion has an absorption band at 1200 K too, which was not observed. For solutions of other systems of such a composition that the first complex should dominate only a broadening of the z/ -band was observed, together with a very feeble v absorption. These systems included the sodium and the cerium ones. 

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