Coordination compounds sensitization

Electron correlation is often very important as well. The presence of multiple bonding interactions, such as pi back bonding, makes coordination compounds more sensitive to correlation than organic compounds. In some cases, the HF wave function does not provide even a qualitatively correct description of the compound. If the weight of the reference determinant in a single-reference CISD calculation is less than about 0.9, then the HF wave function is not qualitatively correct. In such cases, multiple-determinant, MSCSF, CASPT2, or MRCI calculations tend to be the most efficient methods. The alternative is... 

Photocatalytic systems with light-sensitive coordination compounds and possibilities of their spectroscopic sensitization — an overview. H. HennigandD. Rehorek, Coord. Chem. Rev., 1985,61,1 (281). 

In the last example, a serious handicap is the extreme sensitivity of the calculations to the parameterization of the metal atoms. In a paper concerning the spin states of metal dimer complexes (38) as studied by EHT, heavy manipulation of the original theory was needed. In the field of transition metal coordination compounds self-consistent charge (SCC) calculations (of the type already mentioned for electronegative atoms) are essential to obtain the diagonal elements Hu. 

As regards other coordination compounds of silver, electrochemical synthesis of metallic (e.g. Ag and Cu) complexes of bidentate thiolates containing nitrogen as an additional donor atom has been described by Garcia-Vasquez etal. [390]. Also Marquez and Anacona [391] have prepared and electrochemically studied sil-ver(I) complex of heptaaza quinquedentate macrocyclic ligand. It has been shown that the reversible one-electron oxidation wave at -1-0.75 V (versus Ag AgBF4) corresponds to the formation of a ligand-radical cation. Other applications of coordination silver compounds in electrochemistry include, for example, a reference electrode for aprotic media based on Ag(I) complex with cryptand 222, proposed by Lewandowski etal. [392]. Potential of this electrode was less sensitive to the impurities and the solvent than the conventional Ag/Ag+ electrode. 

Energy transfer from singlet or triplet states of suitable organic molecules can cause excitation of the central metal ion in a coordination compound. Photosensitization of Cr(III) complexes by biacetyl leads to aquation reaction of Cr(NH3)5 (NCS)2 ion. The aquation of (NH3) is hundred times more than that of (NCS) for the sensitized reaction, whereas it is only 66 times on direct excitation of Cr(IlI). This shows that energy... 

CTTS transitions in coordination compounds result in a radial movement of electron density from the metal to the surrounding solution medium. The energies of these transitions generally are very sensitive to environmental parameters such as solvent polarity, temperature and the presence of salts.104 This sensitivity has been used in a diagnostic sense to identify CTTS bands in the spectra of anionic cyanide complexes105 and 1,2-dithiolene complexes of Ni, Pd and Pt.106 Hydrated cations such as Cr2+(aq) and Fe2+ (aqj exhibit absorption bands that are sometimes referred to as CTTS in character. Since the solvent occupies the first coordination sphere of the metal, however, the distinction between CTTS and CTTL transitions in these systems becomes obscured. 

Evidence forcoball(l) was first obtained from the electrolytic reduclion of cyano-compounds and some of the reduced species have been isolated. There arc also many cohaltfl) coordination compounds of Ihe organomelallic class carbonyl, isonitriles, and unsaturated hydrocarbon derivatives. The oxidation state cobalt(O) may be represented in the eyano-Lompound which has been formulated as Ks[Cu (CN)b. It has been prepared as an air-sensitive brown-violet compound by reducing a liquid ammonia solution of Kj Co(CN)f,l with an excess of K tnctal. The only other known coball(O) species arc organomelallic compounds. 

Coordination compounds and coordination chemistry are involved in a number of these steps. Chemical sensitizing agents, antifoggants and stabilizers are discussed separately. This discussion concerns silver halide emulsion precipitation and physical ripening methods. 

Again, the precise roles of coordination-compound chemical sensitizers, in most cases, are not understood. In fact, their effects may have little to do with their own coordination chemistry. Many simple salts of gold and other noble metals are effective sensitizers. They also may be added to solutions during silver halide precipitation to produce doped emulsions that have special properties. A variety of compounds that can act as ligands to metal ions are also effective alone as chemical sensitizers, the result of complicated oxidation-reduction, ion replacement and adsorption reactions on the silver halide grain surface. These include polyamines, phosphines and thioether- or thiol-containing compounds. The chemistry of these materials with the silver halide surface is discussed in the reference literature. 

Coordination compounds have the potential to be used in both diagnostic and therapeutic applications, but only the former have been extensively developed so far. This reflects the differing nature of the two types of application. In diagnosis it is only necessary to detect the location of the radionuclide in the body. Since extremely sensitive devices are available for the detection of ionizing radiation, useful diagnostic information may be obtained with relatively low patient... 

Organometallic chemistry does have its own distinctive flavour. Organometallic compounds are often (but not invariably) air- and moisture-sensitive and their handling requires much care and skill. In the interplay between kinetic and thermodynamic considerations, organometallic chemistry falls somewhere between inorganic and organic chemistry. Most organometallic compounds of the transition elements can also be classified as coordination compounds, and are dealt with in more detail in Chapter 8. (See also Section 10.5.)... 

Many PSPs are composed of probe dyes, such as polycyclic aromatic hydrocarbons (e.g., pyrene) and coordination compounds (e.g., platinum por-phryins and ruthenium(II) polypyridyl complexes) immobilized in various gas permeable polymer films such as silicon polymer, organic glassy polymers (e.g., poly(methylmethacrylate), polystyrene), fluorinated polymers, or cellulose derivatives such as ethyl cellulose [9,10]. As probe molecules interact with polymer matrices directly, the properties of PSPs strongly depend on the properties of polymer matrices. The oxygen permeability of polymer matrix is an especially important factor for highly sensitive PSP. 

Models for the static spectral sensitization of coordination compounds are discussed. Among them, ion pairs with oppositely charged metal complexes, where intervalence charge-transfer interactions occur, appear to be promising exarrples. The spectra and the electron transfer kinetics,of mixed-valence ion pairs of ihe type M /fMCNjgJ4-, where M + = Fe3 Cu2 UO V02 are discussed in detail. 

The aim of spectral sensitization of a coordination compound M is, broadly speaking, the photochemical conversion of the ca Jalyt ical ly inactive coordination compound into a product C which may act as a catalyst or a photoassistor in a subsequent photoinduced catalytic or photoassisted cycle. 

The general pathways of spectrally sensitized photoreactions of coordination compounds as well as organorreta 11 ics is best described by the Scheme 1 given Delow. t... 

Figure 1. Simplified Jablonski-type diagram of photoinduced catalytic (I), photoassisted (II), and sensitized photoreaction (III) as well as catalysed photolysis (IV). (MLn i + L) represents coordinatively unsaturated species, free ligands as well as ligands and/or coordination compounds with changed formal oxidation number generated by photo-redox and/or photodissociation reactions.

Liu prepared a sandwich type coordination compound (103) from porphyrin and phthalocyanine with the assistance of a microwave. The resulted compounds showed good solubility in conventional organic solvents. The photoelectric conversion properties have been tested with a Gratzel type cell. The results revealed that the sandwich type compound showed better photo-electric conversion efficiency than the corresponding monomeric porphyrin or phthalocyanine precursors. The short-circuit photocurrent of the solar cell with this sandwich type compound as sensitizer, was, as high as 691.31 A cm-2, which was much better, than those of porphyrin or phthalocyanine monomers [100]. 

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