Molybdenum complexes spectra

The electronic spectra of niobium(IV) and -(V) and zirconium(IV) complexes 126,127) have been reported but not interpreted. The spectrum of Nb(ethyl-dtp)4 is of particular interest since the compound is probably 8-co-ordinate. Discussion of the spectrum of binuclear molybdenum complexes 130,131) employed the molecular orbital model of Blake, Cotton and Wood for MO2O3LX complexes s). 

The [Fe3S4(SEt)4]3 undergoes conversion into the [4Fe-4S] cluster in acetonitrile with FeCl2 alone or in the presence of NaSEt.752 The molybdenum complex [Mo3S4(SCH2CH2S)3]2 contains the Mo3S44+ core, and shows similarity in its resonance Raman spectrum with [3Fe-4S] proteins.812... 

In solution the bridging fluorines are rapidly transferred between adjacent silicons, such that the 29Si spectrum features a triplet due to two equivalent fluorines (Vsi-F = 127 Hz). This triplet did not change to a doublet of doublets at the low-temperature limit in toluene-ds solution, and hence freezing out of the fluorine transfer between vicinal silicons could not be achieved. When the temperature was raised, all six fluorines became equivalent, causing the silicon resonance to be a septet ( /si-F = 43 Hz), presumably due to exchange by correlated rotation178 of the silicon substituents about the Si-benzene bonds. This rotational process was stopped when a molybdenum complex was formed (152), presumably as a result of increased steric hindrance for rotation. 

The positions of the photoluminescence (i.e., phosphorescence) and its corresponding excitation bands are nearly identical to those of the M0O4 complex in CaMo04. The energetic positions of the peak maxima of the excitation and photoluminescence spectra of five- and six-coordinated oxo-molybdenum complexes are known to exist at lower energy levels, i.e., at approximately 400-333 nm (25,000-30,000 cm ) for the excitation and at 714-588 nm (14,000-17,000 cm" ) for the photoluminescence, respectively 130). Hence, the photoluminescence spectrum shown in Fig. 26 can be attributed to the presence of an isolated four-coordinate molybdenum oxide species. 

Various kinds of the theoretical spectroscopies for the transition metal complexes were also reviewed. For the excitation spectrum of Cr02Cl2, the SAC-Cl method simulated accurate spectrum. For tetraoxo metal complexes, the systematic studies explained the spectral differences when the central metal was substituted. In the analysis of the NMR chemical shift, not only the optically allowed states but also the magnetically allowed states are important. In the molybdenum complexes, the inverse of the d-d excitation energy is proportional to the experimental chemical shift. The photofragmentation reaction of Ni(CO)4 was also studied and the reaction mechanism was clarified. 

In the electron spectrum of the molybdenum complex, there is an intense metal-ligand charge transfer band (MLCT) from the d orbital on the 71 orbital of phenanthroline. After the formation of the adduct with triethylaluminum, the MLCT band becomes shifted to higher energies. This constitutes evidence for stronger back-bonding in the adduct than in the [Mo(CO)2 (phen)(PPh3)2] complex. 

Fig. 1 illustrates the types of reaction pathway which have been proposed to be of importance in the decomposition reaction. In practice, where additional processes such as reforming of DME may possibly occur, the reaction scheme could be much more complex than that shown. For example, on the basis of the product spectrum over molybdenum oxycarbide dimers reported in NaY,20 the following pathway has... 

Of course, not all dissolved ions produce colored solutions, and therefore not all ions in solution can be quantified by colorimetry. Noncolored solutions can sometimes, however, be converted to colored solutions by introducing chromophore species which complex with (i.e., attach themselves to) the target ion to produce a colored solution, which may then be measured by UV/visible colorimetry. An important archaeological example of this is the determination of phosphorus in solution (which is colorless) by com-plexation with a molybdenum compound, which gives a blue solution (see below). The term colorimetry applies strictly only to analytical techniques which use the visible region of the spectrum, whereas spectrophotometry may be applied over a wider range of the electromagnetic spectrum. 

Upon purification of the XDH from C. purinolyticum, a separate Se-labeled peak appeared eluting from a DEAE sepharose column. This second peak also appeared to contain a flavin based on UV-visible spectrum. This peak did not use xanthine as a substrate for the reduction of artificial electron acceptors (2,6 dichlor-oindophenol, DCIP), and based on this altered specificity this fraction was further studied. Subsequent purification and analysis showed the enzyme complex consisted of four subunits, and contained molybdenum, iron selenium, and FAD. The most unique property of this enzyme lies in its substrate specificity. Purine, hypoxanthine (6-OH purine), and 2-OH purine were all found to serve as reductants in the presence of DCIP, yet xanthine was not a substrate at any concentration tested. The enzyme was named purine hydroxylase to differentiate it from similar enzymes that use xanthine as a substrate. To date, this is the only enzyme in the molybdenum hydroxylase family (including aldehyde oxidoreductases) that does not hydroxylate the 8-position of the purine ring. This unique substrate specificity, coupled with the studies of Andreesen on purine fermentation pathways, suggests that xanthine is the key intermediate that is broken down in a selenium-dependent purine fermentation pathway. ... 

Molybdenum(i ) and Tungsten(iv) Complexes.—An absorption maximum in the electonic spectrum of WOCI2 at 345 nm has been assigned to and... 

Hunt s group (50, 51) have pioneered the application of the Cl source to organometallics such as the iron tricarbonyl complex of heptafulvene, whose electron impact spectrum shows (M—CO)+ as the heaviest ion, in contrast to the methane Cl spectrum with the ion as base peak. Boron hydrides (52) and borazine (53) have also been studied. The methane Cl spectrum of arenechromium and -molybdenum (54) show protonation at the metal giving a protonated parent or molecular ion. Risby et al. have studied the isobutane Cl mass spectra of lanthanide 2,2,6,6-tetramethylheptane-3,5-dionates[Ln(thd)3] (55) and 1,1,1,2,2,3,3-heptafluoro-7,7-dimethyl-4,6-oetanedione [H(fod)] lanthanide complexes (56). These latter complexes have been suggested as a means of analysis for the lanthanide elements. 

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