Molybdenum complexes electronic 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). 

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. 

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. ... 

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. 

The four-coordinate complexes [Mo(SBu,)4] and [Mo(NR2)4] are also diamagnetic. The UV-PE spectrum of [Mo(SBu )4] exhibits a low ionization potential at 6.8 eV which has been assigned to the ionization of electrons with predominant molybdenum Adz2 character, on the basis of discrete variational-Xa MO calculations on the model compounds [Mo(SH)4] and [Mo(SMe)4].210 For [Mo(NR2)4] (R = Me, Et), the UV-PE spectrum contains a low energy ionization at 5.3 eV which has been attributed to ionization from the molybdenum Adx2-y orbital. This assignment was based on Fenske-Hall calculations on [Mo(NMe2)4].2U... 

Redox processes are fairly common in the formation of Z —CO— complexes of transition metals, and an example is given in Eq. (9). In this reaction, titanium is oxidized from the + 2 to the +3 state, thus becoming a better Lewis acid, and the molybdenum dimer is reductively cleaved, thus developing Z —CO— donor character (59). A characteristic low-frequency Z —CO— band is observed in the IR spectrum, and a crystal structure is available. A proposed mechanism for the redox process, based on CO mediated electron transfer, is discussed in Section IV,C. 

X-ray line spectra arc also produced by electronic tranaitions. In this case, however, the electrons involved are those in the innermost orbitals. Thus, in contrast to ultraviolet and visible emissions, the X-ray spectrum for an element is independent of its environment. For example, the emission spectrum for molybdenum is the same regardless of whether the sample being excited is molybdenum metal, solid molybdenum sulfide, gaseous molybdenum hexafluoride, or an aqueous solution of an anionic complex of the metal. 

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