Absorption spectra coordination compounds

The crystal structures of transition metal compounds and minerals have either cubic or lower symmetries. The cations may occur in regular octahedral (or tetrahedral) sites or be present in distorted coordination polyhedra in the crystal structures. When cations are located in low-symmetry coordination environments in non-cubic minerals, different absorption spectrum profiles may result when linearly polarized light is transmitted through single crystals of the anisotropic phases. Such polarization dependence of absorption bands is illustrated by the spectra ofFe2+ in gillespite (fig. 3.3) and of Fe3+in yellow sapphire (fig. 3.16). 

Perylenediimides represent another class of photoactive dyes which are characterized by their strong fluorescence emission and facile electrochemical reduction. Recently, a supramolecular bis(phthalocyanine)-perylenediimide hetero-triad (compound 42) has been assembled through axial coordination [47]. Treatment of perylenediimide 43, which has two 4-pyridyl substituents at the imido positions, with 2.5 equiv. of ruthenium(II) phthalocyanine 44 in chloroform affords 42 in 68% yield (Scheme 3). This array shows remarkable stability in solution due to the robustness of the ruthenium-pyridyl bond. Its electronic absorption spectrum is essentially the sum of the spectra of its molecular components 43 and 44 in... 

The intensities of the absorption bands of coordination compounds have been studied [61,112,82,178]. A superposition model has been applied to the analysis of the spectrum of Er3+ in YGG and LuGG and the best-fit parameters obtained are given in Tables 8.21 and 8.22. 

Dendrimer 1 (Fig. 5) is a classical example of a dendrimer built around a metal complex core. In this compound, the 2,2 -bipyridine ligands, that constitute the first coordination sphere of the Ru ion, carry branches containing 1,3-dimethoxybenzene and 2-naphthyl chromophoric imits separated by aliphatic connectors (10). Since the interchromophoric interactions are weak, the absorption spectrum of 1 is substantially equal to the summation of the spectra of [Ru(bpy)s], which is characterized by a broad spin-allowed Ru bpy metal-to-ligand (MLCT) band around 450 nm (11), and of the chromophoric groups contained in the branches, which show very intense bands in the near UV region. 

There exists a large literature on the spectroscopic properties of copper(ll) compounds. This is due to the simpHcity of the d electron configuration, the wide variety of stereochemistries that copper(ll) compounds can adopt, and the flu-xional geometric behavior that they sometimes exhibit [1]. The electronic and geometric properties of a molecule are inexorably linked and this is especially true with six-coordinate copper(II) compounds which are subject to a Jahn-Tel-ler effect. However, the spectral-structural correlations that are sometimes drawn must often be viewed with caution as the information contained in a typical solution UV-Vis absorption spectrum of a copper(ll) compound is limited. 

It is not always possible to make a simple prediction of color directly from the absorption spectrum, in large part because many coordination compounds contain two or more absorption bands of different energies and intensities. The net color observed is the color predominating after the various absorptions are removed from white light. 

It is extremely common for coordination compounds also to exhibit strong charge-transfer absorptions, typically in the ultraviolet and/or visible portions of the spectrum. These absorptions may be much more intense than d-d transitions (which for octahedral complexes usually have e values of 20 L moF cm or less) molar absorp-tivities of 50,000 L mole cm or greater are not uncommon for these bands. Such absorption bands involve the transfer of electrons from molecular orbitals that are primarily ligand in character to orbitals that are primarily metal in character (or vice versa). For example, consider an octahedral d complex with cr-donor ligands. The ligand electron pairs are stabilized, as shown in Figure 11-15. 

In complexes such as Cr(CO)6 which have both o-donor and Tr-acceptor orbitals, both types of charge transfer are possible. It is not always easy to determine the type of charge transfer in a given coordination compound. Many ligands give highly colored complexes that have a series of overlapping absorption bands in the ultraviolet part of the spectrum as well as the visible. In such cases, the d-d transitions may be completely overwhelmed and essentially impossible to observe. 

Fraunfelder and co-workers [65-68] have found the same relationship in the reduction kinetics of CO and O2 ligand coordination bonds with the complex-forming Fe ions of the heme group myoglobin upon photodissociation of these bonds induced by laser photolysis of 10 s duration. The formation of Fe-CO and Fe-02 bonds after photolysis was registered with a spectrophotometer by restoration of the mother compound absorption spectrum in the wide range of times (10 -10 s) and temperatures (2-300 K). Before photolysis the six-coordinate Fe ion is in the heme plane. The coordination bond break causes not only the ligand shift but also... 

Crystal field theory was developed, in part, to explain the colors of transition-metal complexes. It was not completely successful, however. Its failure to predict trends in the optical absorption of a series of related compounds stimulated the development of ligand field and molecular orbital theories and their application in coordination chemistry. The colors of coordination complexes are due to the excitation of the d electrons from filled to empty d orbitals d-d transitions). In octahedral complexes, the electrons are excited from occupied t2g levels to empty Cg levels. The crystal field splitting Ao is measured directly from the optical absorption spectrum of the complex. The wavelength of the strongest absorption is called Amax and it is related to Ao as follows. E = hv, so Ao = hv = Because en-... 

The square planar coordination compounds [Cu(acac)(tmen)][C104] and [Cu(acac) (tmen)][BPh4] have been used to probe the nucleophilicity and hence the donor power of ionic liquids. The cationic complex shows a good correlation between the donor number (DN) of a solvent and the absorption maximum of the lowest d-d-transition in the absorption spectrum [104], It has also been used to determine the donor numbers of anions in solutions [126], Studies of the solvent polarity of [C4mim]+, [C8mim]+, [C4mmim]+ and [C8mim]+ ionic liquids with [PF6], [OTf]- and... 

---