Ligand, field spectra

Ligand field spectra and chemical bonding in Cr3+-containing oxidic solids. D. Reinen, Struct. Bonding (Berlin), 1969, 6, 30-51 (34). 

The ligand properties of a cyclic dithioether, 1,4-dithiane monosulphoxide (DTMSO), have been studied by physical measurements . The infrared spectra indicate that the metal cation coordinates to the oxygen lone pair electrons of DTMSO. Both infrared and ligand-field spectra show the presence of octahedral ions MfDTMSO) in the compounds M(DTMS0)g(C104) and M(DTMSO)g(BF4) . In the case of M = Cu these ions are distorted from the regular octahedral structure. 

The intensities of polarized ligand field spectra ns-n ) of V(ethyl-dtp)3 doped into the corresponding indium(III) compoimd exhibit a relatively small temperature dependence. The source of the large intensities of the d-d transition is the static distortion of the ligand field n ) and not vibronic effects to any appreciable extent. 

Measurements of visible and ultraviolet spectra have become of great importance in the study of nickel(II) complexes, as the spectra of octahedral complexes have been very thoroughly assigned. The basic principles for the interpretation of ligand-field spectra have been laid down by Orgel... 

Very different redox potentials were recently reported for the two stable conformers of a hexaaminecobalt(III) complex with a cage ligand (see Fig. 11.3). Two isomers with very different properties were isolated one is yellow (first d-d transition at 480 nm) with a potential of ° = 0.0 V, the other is blue (600 nm) with E = 0.84 V. The crystal structure of the yellow compound was solved (Fig. 11.3(a))[346], for the blue compound no diffracting crystals were isolated. That structure was solved by a combination of molecular mechanics, the simulation of the ligand field spectra (MM-AOM) and the redox potential (MM-Redox), see Fig. 11.3(b)13101. 

We now extend the structural basis set for cobalt(III) hexaamines with one additional structure with relatively long Coin-N bonds. The [Co(tmen)3]3+ cation (tmen = 2,3-dimethylpropane-2,3-diamine) is a highly strained species with long Com-N bonds because of the four methyl substituents (see Fig. 17.12.1). The structure of the cation has been determined by an X-ray diffraction study, and the conformation in the crystal has been defined as 065 (see Section 17.3 for the nomenclature of the conformers). Due to the elongation of the Com-N bonds to 1.997 A, there is a remarkable shift in the ligand field spectra (the first d-d transition ( Ai- ) is at 515 nm vs 470 nm for [Co(en)3p ) and the redox potential (-0.18 V vs +0.28 V)[56>231]. 

For macrocyclic and cage ligands the size of the ligand cavity (often referred to as the hole size) and its shape are important features that have been used to rationalize stabilities, redox potentials, electron transfer rates and ligand field spectra. Some of these applications will be studied in the following lessons (see Chapters 9, 10, 11). 

Bis (4-methylamino-3-penten-2-onato) cobalt (II) forms orange crystals which melt at 169-171° and which are stable in air for several hours. The complex is soluble in most organic solvents, but its solutions are susceptible to rapid oxidation by air. Traces of moisture in solution may hydrolyze the complex. Room-temperature magnetic moments of the crystalline solid (4.22 B.M.) and of the complex in toluene solution (4.36 B.M.), together with ligand field spectra, demonstrate a tetrahedral stereochemistry in both phases. ... 

Ligand Field Spectra. Ligand field (d,d) transitions are overlap forbidden and in centrosymmetric complexes also Laporte forbidden. For these reasons even the spin-allowed LF absorption bands are generally weak (10 < 10 )(16). There are many... 

Binding of a paramagnetic, redox-inactive [Cr(CN)6]3- anion to specific sites of a blue copper protein, amicyanin, has been used in NMR-spectroscopic studies of the protein structure in solutions.285Ab initio calculations of the ligand-field spectra of [Cr(CN)6]3 have been performed and the results compared with those for cyano complexes of the other first-row transition metals.286 The role of Cr—C—N bending vibrations in the phosphorescence spectra... 

The ligand field spectra of orthoaxial chromium(iii) complexes have been studied by calculations utilizing the angular overlap model. A two-dimensional spectrochemical series of ligands has been established for Cr ". Certain observations do point towards a chemical significance for such parameters. ... 

D2d symmetry. The high-spin [MS ]2 complexes in solution are very sensitive to oxygen and display typical electronic ligand field spectra. On the basis of the 10 Dq transitions in the near-IR spectra of these complexes, the thiophenolate ligand is placed between Cl" and hexamethylphosphoric triamide (HMPA) in the spectrochemical series. 

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