Optical charge-transfer transitions

Much of chemistry occurs in the condensed phase solution phase ET reactions have been a major focus for theory and experiment for the last 50 years. Experiments, and quantitative theories, have probed how reaction-free energy, solvent polarity, donor-acceptor distance, bridging stmctures, solvent relaxation, and vibronic coupling influence ET kinetics. Important connections have also been drawn between optical charge transfer transitions and thennal ET. 

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). 

In this paper we will describe and discuss the metal-to-metal charge-transfer transitions as observed in optical spectroscopy. Their spectroscopic properties are of large importance with regard to photoredox processes [1-4], However, these transitions are also responsible for the color of many inorganic compounds and minerals [5, 6], for different types of processes in semiconductors [7], and for the presence or absence of certain luminescence processes [8]. 

In this chapter we have shown that optical metal-to-metal charge-transfer transitions are of large importance in many fields and that they occur very generally. Not only their direct, but also their indirect influence is of great importance. A more unified approach in the different areas of research, and a stronger interaction between the different approaches is desirable. 

No bands were reported by Brown et al. corresponding to charge-transfer transitions, and their absence below 50 kK. was confirmed by Allen et al. Since the lowest Laporte-allowed band for the PdCle- anion is found close to 30 kK. (24), this is as would be expected from optical electronegativity considerations. 

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. 

Optical charge transfer (CT) is commonly observed in un-symmetrical molecules or molecular complexes in which there are sites of distinctly different ionization energies and electron affinities. The origin and properties of optical charge transfer transitions provide the basis for this account. A convenient place to begin chemically is with mixed-valence compounds and two examples are shown below (1-3). In the first (eq 1), the sites of different oxidation states are held in close... 

General Discussion—Optical Charge-Transfer Transitions... 

The nickel in urease is nonmagnetic and appears to be in the oxidation state Ni(II). The broad optical absorption spectrum is influenced by ligands to the metal (Fig. 1). The spectrum obtained in the presence of the competitive inhibitor mercaptoethanol, after correction for Rayleigh scattering by the protein (31), shows absorption peaks at 324,380, and 420 nm, with molar absorption coefficients of 1550,890, and 460 A/-1 cm-1, respectively. These were assigned to sulfur-to-nickel charge transfer transitions. The spectrum is changed by addition of other inhibitors, such as acetohydroxamic acid (Fig. IB). Similar... 

Additional information has been obtained from single crystal, polarized optical and ESR spectroscopic studies924 on poplar plastocyanin, which have allowed a correlation of the electronic structure of the blue copper active site with its geometric structure. In summary, the three dominant absorption bands at 13 350, 16 490 and 17 870 cm-1 were assigned to CysS- Cu (d 2-,2 charge-transfer transitions. The methionine makes only a small contribution, due to the long Cu—S(Met) bond (2.9 A) and the poor overlap of the methionine sulfur orbitals with the dx y orbital of copper. Histidine-Cu charge transfer contributes to the weaker absorptions at 21 390 and... 

In this paragraph we will discuss two types of optical transitions in the transition metal ions first the intraconfigurational dn transitions, second the interconfigurational charge-transfer transitions in complexes with a d° central metal ion. 

---