Charge transfer transitions complexes

Though thermally stable, rhodium ammines are light sensitive and irradiation of such a complex at the frequency of a ligand-field absorption band causes substitution reactions to occur (Figure 2.47) [97]. The charge-transfer transitions occur at much higher energy, so that redox reactions do not compete. 

The colors that we have described arise from d-d transitions, in which an electron is excited from one d-orbital into another. In a charge-transfer transition an electron is excited from a ligand onto the metal atom or vice versa. Charge-transfer transitions are often very intense and are the most common cause of the familiar colors of d-metal complexes, such as the transition responsible for the deep purple of permanganate ions, Mn04 (Fig. 16.33). 

The nickel(II) dithiocarbamate complexes are neutral, water-insoluble, usually square-planar, species, and they have been studied extensively by a range of physical techniques. The usual methods for the synthesis of dithiocarbamate complexes have been employed in the case of Ni(II), Pd(II), and Pt(II). In addition, McCormick and co-workers (330,332) found that CS2 inserted into the Ni-N bonds of [Ni(aziri-dine)4P+, [Nilaziridinelgf, and [Ni(2-methylaziridine)4] to afford dithiocarbamate complexes. The diamagnetic products are probably planar, but they have properties typical of dithiocarbamate complexes, and IR- and electronic-spectral measurements suggested that they may be examples of N,S-, rather than S,S-, bonded dithiocarbamates. The S,S-bonded complexes are however, obtained, by a slow rearrangement in methanol. The optically active lV-alkyl-iV(a-phenethyl)dithio-carbamates of Ni(II), Pd(II), and Cu(II) (XXIV) have been synthesized, and the optical activity was found to be related to the anisotropy of the charge-transfer transitions (332). 

The adsorption spectrum of aerosil containing admixture vanadium ions exhibits a maximum within the band 290 - 380 nm which was attributed by authors of [97] to the charge transfer transitions in oxygen-containing complexes of five valance vanadium = O -... 

Table 20. Charge-Transfer Transitions of

Table 21. Charge-Transfer Transitions for Various dx Ground States of Sandwich Complexes.

Fig. 2 Mulliken correlation of the ionization potentials (IP) of various enol silyl ethers with the charge-transfer transition energies (/jvct) of their EDA complexes with chloranil. Reproduced with permission from Ref. 36.

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. 

Table 27. Charge-transfer transitions of 4 d and 5d hexafluoro complexes...

The colours of these compounds are associated with charge-transfer transitions involving the dye moiety. Thus, when the steric and/or electronic effects of coordination cause variation in the energy of these transitions, then the induced spectral changes may be used to monitor complex formation. 

Charge transfer also occurs between ions in solution. The classical test for the Fe3+ ion in solution is to mix solutions of Fe3+ and thiocyanate ion SCN-, to form the [FeSCN]2"1" complex, and a deep blood-red colour forms. The colour originates from a charge-transfer transition between Fe3+ and SCN-. There was no red colour before mixing, confirming that the optical transition responsible for the colour did not originate from either constituent but from the new compound formed. 

In heterogeneous photoredox systems also a surface complex may act as the chromophore. This means that in this case not a bimolecular but a unimolecular photoredox reaction takes place, since electron transfer occurs within the lightabsorbing species, i.e. through a ligand-to-metal charge-transfer transition within the surface complex. This has been suggested for instance for the photochemical reductive dissolution of iron(III)(hydr)oxides (Waite and Morel, 1984 Siffert and Sulzberger, 1991). For continuous irradiation the quantum yield is then ... 

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