Low oxidation state metal ions

It seems worth pointing out, that 137 and human semm albumin contain no pendant phosphines and the donor atoms in the complexes formed with rhodium can be only O (137) or O, N and perhaps S (HSA), which are not the most suitable for stabilizing low oxidation state metal ions. Still these macroligands gave active and stable catalysts with rhodium, which shows that perhaps in the high local concentration provided by the polymer even these hard donor atoms are able to save the metal ion against hydrolysis or other deterioration. 

G. (1999) Structural chemistry of uranium associated with Si, Al, Fe gels in a granitic uranium mine. Chem. Geol. 158 81-103 Allen, G.C. Kirby, C. Sellers, R.M. (1988) The effect of the low-oxidation-state metal ion reagent tris-picolinatovanadium(II) formate on the surface morphology and composition of crystalline iron oxides. J. Chem. Soc. Faraday Trans. I. 84 355-364... 

In recent years there has been a growing interest in the chemistry of carbon dioxide, stimulated by current anxieties about alternative petrochemical feedstocks. One aspect under active exploration involves carbon dioxide activation via coordination to a transition metal, and indeed transition metal ions do form C02 complexes.177 The number of simple and reasonably stable complexes is still relatively small and usually limited to low oxidation state metal ions. There are many systems where C02 is used as a reagent giving rise to systems which, while not true C02 complexes, may simplistically be viewed as the products of insertion into metal-ligand bonds, e.g. reaction (9), where if L = H, formates are produced if L = OH, carbonates or bicarbonates result L = NR2 yields dialkylcarbamates and if L = R, carboxylate products result. Much of this area has recently been reviewed and will not be considered further.149... 

Most transition metal ions can react via pathways described by Equation 4.4 a few biologically relevant examples include FenFen > FeraFenl or Cu Cu1 > CunCun oxidations. Two low-oxidation state metal ions can be a part of a dinuclear complex. Alternatively, a mononuclear complex may undergo oxidation, forming dinuclear complexes in the oxidized form. Two metal ions in Equation 4.4 can be different in this case, exemplified by Cu f e11 systems, heterodinuclear oxidation products form. 

Figure 3 Schematic representation of two common supramolecular motifs used in supramolecular polymerizations (a) UPy quadruple hydrogen bonding unit 1, its dimerization, tautomerization, and subsequent complexation with NaPy 2 (b) terpyridine 3 and 2 1 complexation with a low-oxidation-state metal ion M. As shown, each motif demonstrates a different binding mode.

One of the most frequently reported decontamination processes is the Lomi process (Low Oxidation State Metal Ions), which was originally developed by the British CEGB for removal of corrosion product deposits from the fuel bundles of the Steam Generating Heavy Water Reactor (SGHWR). Since Fe " always is the... 

The use of low oxidation state metal ions or their complexes as reductants has limited applicability in synthetic coordination chemistry. These reductants typically must be synthesized and the products of their oxidation often form complexes themselves, leading to complicated mixtures of by-products. 

Low oxidation state metal ions are used in selective reduction of coordinated ligands. In particular, Armorl2i has shown that chromium(II) chloride reduces the nitrosyl ligand in [Ru(NH3)5NO] +, Eq. 9.33 ... 

The intramolecular electron transfer leads to fast formation of semi-quinone and the lower oxidation state metal ion. The catalytic cycle is completed by fast reoxidation of the metal ion. Significant deviations from this model were observed at low dioxygen concentrations and it was suggested that another oxidation path becomes operative under such conditions. Although earlier they had been proposed to participate (10), side reactions with dehydroascorbic acid could be excluded. 

A metal-nucleotide complex that exhibits low rates of ligand exchange as a result of substituting higher oxidation state metal ions with ionic radii nearly equal to the naturally bound metal ion. Such compounds can be prepared with chromium(III), cobalt(III), and rhodi-um(III) in place of magnesium or calcium ion. Because these exchange-inert complexes can be resolved into their various optically active isomers, they have proven to be powerful mechanistic probes, particularly for kinases, NTPases, and nucleotidyl transferases. In the case of Cr(III) coordination complexes with the two phosphates of ATP or ADP, the second phosphate becomes chiral, and the screw sense must be specified to describe the three-dimensional configuration of atoms. 

Nitrogen protonation or incorporation of high oxidation state metal ions facilitates nucleophilic substitution (especially at Craeso) whereas N-deprotonation or coordination to low valent metal ions enhances the electrophilic reaction (especially at Cme50). The effects of electron-donating substituents (alkyl) are less than those of electron-withdrawing groups (especially n acceptors, N02, COR). 

ATRP is the most versatile CRP technique and has already attracted considerable interest from industry.(13) It relies on establishing an equilibrium between an alkyl halide initiator (RX, X = Br, Cl) and a low oxidation state metal complex, generating a radical and a higher oxidation state complex with a coordinated halide ion. The atom transfer equilibrium is presented schematically on the left hand side of Figure 1.(14) The ATRP equilibrium constant, which is the ratio of the rate constants of RX activation and of radical deactivation, can be formally presented as a product of several contributing equilibrium constants shown in Figure 1 and eq. 1. 

Low oxidation state metals and electron-rich moieties interact with the C atom, whereas electron-poor species (metal ions, protons) interact at the O terminal. As a consequence of such ability, CO2 can give rise to divers bonds with metal centres depending on the nature of the latter. 

Another means of in situ metal-carbene complex formation in an ionic liquid is the direct oxidative addition of the imidazolium cation to a metal center in a low oxidation state (see Scheme 5.2-2, route b)). Cavell and co-workers have observed oxidative addition on heating 1,3-dimethylimidazolium tetrafluoroborate with Pt(PPli3)4 in refluxing THF [32]. The Pt-carbene complex formed can decompose by reductive elimination. Winterton et al. have also described the formation of a Pt-car-bene complex by oxidative addition of the [EMIM] cation to PtCl2 in a basic [EMIM]C1/A1C13 system (free CP ions present) under ethylene pressure [33]. The formation of a Pt-carbene complex by oxidative addition of the imidazolium cation is displayed in Scheme 5.2-4. 

Two possible reasons may be noted by which just the coordinatively insufficient ions of the low oxidation state are necessary to provide the catalytic activity in olefin polymerization. First, the formation of the transition metal-carbon bond in the case of one-component catalysts seems to be realized through the oxidative addition of olefin to the transition metal ion that should possess the ability for a concurrent increase of degree of oxidation and coordination number (177). Second, a strong enough interaction of the monomer with the propagation center resulting in monomer activation is possible by 7r-back-donation of electrons into the antibonding orbitals of olefin that may take place only with the participation of low-valency ions of the transition metal in the formation of intermediate 71-complexes. 

A combination of P- and N- donors is another useful approach to potentially reactive (and cataly-tically active) Ni species. Similar to O- donors, N is a hard donor capable of stabilizing metal ions in higher oxidation states, whereas the soft donor P is best suited to stabilize medium or low oxidation states. A neutral bidentate P,N ligand combining a hard dimethylamino and a soft phosphine donor in /V,/V -dimethyl-2-(diphenylphosphino)aniline (241) affords the neutral trigonal bipyramidal and the... 

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