Other Charge-Transfer Transitions

The deep violet color of pentaphenylbismuth and certain other pentaarylbismuth compounds has been the subject of considerable speculation. It has been shown by x-ray diffraction (173) that the bismuth atom in pentaphenylbismuth is square—pyramidal. WeU-formed crystals are dichromic, appearing violet when viewed in one plane but colorless in another plane. The nature of the chromophore has been suggested to be a charge-transfer transition by excitation of the four long equatorial bonds ... 

This paper surveys several aspects of metal-to-metal charge-transfer transitions. Species of interest originate from non-molecular and molecular solids and from solutions. The parallel in the different approaches is stressed. In addition to the spectroscopy of these transitions, their influence or role in other phenomena is also discussed. 

Similar vivid colorations are observed when other aromatic donors (such as methylbenzenes, naphthalenes and anthracenes) are exposed to 0s04.218 The quantitative effect of such dramatic colorations is illustrated in Fig. 13 by the systematic spectral shift in the new electronic absorption bands that parallels the decrease in the arene ionization potentials in the order benzene 9.23 eV, naphthalene 8.12 eV, anthracene 7.55 eV. The progressive bathochromic shift in the charge-transfer transitions (hvct) in Fig. 13 is in accord with the Mulliken theory for a related series of [D, A] complexes. 

Charge-transfer transitions, involving the transfer of an electron between two orbitals one of which is associated predominantly with the ligand and the other with the metal. 

This treatment is based on the inadequate three-parameter theory of the free ion on the one hand and does not account for covalency effects on the other. Indeed, three main absorption bands are found, the third of which is of low intensity (two electron jump) and located in the UV-region. It is only observed in cases where it is not masked by charge-transfer transitions. In addition, some peaks of very weak intensity are found in the visible region around the first two main bands (Fig. 1, 2 Table 2). While A is fixed by the energetic position of the first main band, B may be calculated from the second (or third) main band by the formula ... 

All of the calculations described here use local functionals that do not include exact exchange. The best choice of functional for calculating MCD spectra is still an open question. In our experience, the MCD parameters produced with different local functionals do not vary much (83). The exception to this rule is functionals that are asymptotically correct. Asymptotically correct functionals tend to give excitation energies that differ from those produced by standard local functionals, particularly for Rydberg and charge-transfer transitions. The MCD parameters produced by asymptotically correct functionals tend to follow a similar pattern when compared with results of other local functionals. A few studies have considered hybrid functionals (42,43). This work found that hybrid functionals were superior in some applications. More work in this area is needed. 

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