Electronic absorption spectra coordination compounds

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... 

We have examined (8), as did Brady and coworkers (3), the electronic absorption spectrum of the Saltman- piro ball. The spectrum (Figure 6) (8) shows a pattern of four weak bands with the lowest band at about 900 nm, in very good agreement with [Fe(III)06]oct coordination. The derived LF parameters are A<>ct = 11,260 cm" C/B = 3, and B = 815 cm" We can say with considerable confidence that most of the Fe(III) ions occupy octahedral coordination sites. There is no hint of bands attributable to [Fe(III)04]tet, and as these bands are intrinsically more intense than are those assigned to [Fe(III)06]oct> we can eflFectively rule out tetrahedral coordination in this synthetic model compound. 

The macrocyclic product [Ni(L54)]I is isolated through refluxing an aqueous solution of L52 and nickel(II) acetate with the subsequent addition of an aqueous solution of Nal (Eq. 2,15) [23], The diamagnetism and electronic absorption spectrum of [Ni(L54)]+ indicate that it has a square-planar structure. The peak with the maximal m/z in the mass spectrum of the iodide corresponds to the [Ni(L54)]+ cation. It should be noted that, in the mass spectra of a number of other macro-cyclic coordination compounds of the [ML]X type, the most intense peak in the high-mass number range is [ML]+ as, for example, in case of the nickel( II) complex with the 13-membered macrocycle derived from 3,7-diaza-l,9-diaminononane [24]. 

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 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. 

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-... 

Luminescent coordination compounds continue to attract considerable attention. Zink recently reported a new mixed-ligand copper(I) polymer that shows interesting photoluminescence (232). The complex [CuCl(L44)Ph3P] consists of a one-dimensional chain lattice of metal ions bridged by both Cl" ions and pyrazine molecules. The compound shows conductivity of less than 10-8 S cm 1. The absorption spectrum of the complex shows a band at 495 nm, which could be interpreted as the promotion of an electron from the valence band to the conduction band. On the basis of resonance Raman spectra, the lowest excited state in the polymer is assigned to the Cu(I)-to-pyrazine metal-to-ligand charge-transfer excited state. 

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