Chromophore octahedral symmetry

The UV-vis spectra of the trinuclear aza- and nitro-capped aminoethanethiol cobalt sarcophaginates are very similar to those for the initial [Co(Co(aet)3)2] cation and intermediate hexaimine complex. The absorption band at 18 180 cm- is due to the ig— d-d transition for a cobalt(III) ion in octahedral symmetry, and another one at 23 260 cm- was observed. These broad asymmetric bands have more than one contributing component, consistent with a deviation from octahedral symmetry and with an overlap of both terminal C0N3S3 and central CoSe chromophore transitions. The UV bands for these complexes are much more intense than the visible ones and were attributed to CT from ligand (sulphur atoms) to terminal and central cobalt(III) ions. The band at 28 570 cm- was assigned to the CT transition from the bridged sulphur to cobalt ion, because this one is not observed in the spectrum of the mononuclear complex [129]. 

In the ligand polarization mechanism for optical activity, the potential of the electric hexadecapole component, Hxy(x>-y>), produces a determinate correlation of the induced electric dipole moment in each ligand group which does not lie in an octahedral symmetry plane of the [Co Ng] chromophore (Fig. 8). The resultant first-order electric dipole transition moment has a non-vanishing component collinear with the zero-order magnetic moment of the dxy dxj yj transition in chiral complexes, and the scalar product of these two moments affords the z-component of the rotational strength, RJg, of the Aj -> Ti octahedral excitation. 

Now look at octahedral complexes, or those with any other environment possessing a centre of symmetry e.g. square-planar). These present a further problem. The process of violating the parity rule is no longer available, for orbitals of different parity do not mix under a Hamiltonian for a centrosymmetric molecule. Here the nuclear arrangement requires the labelling of d functions as g and of p functions as m in centrosymmetric complexes, d orbitals do not mix with p orbitals. And yet d-d transitions are observed in octahedral chromophores. We must turn to another mechanism. Actually this mechanism is operative for all chromophores, whether centrosymmetric or not. As we shall see, however, it is less effective than that described above and so wasn t mentioned there. For centrosymmetric systems it s the only game in town. 

It is not always possible to choose these independently. For example, in an octahedral chromophore the ns and nc ligator orbitals by symmetry transform into one another, whereas the 5-orbitals do not. 

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