Metal ions transition

These ions have a higher surface charge density than the alkali and alkaline earth ions and, as a result, the coordinated solvent molecules are held more firmly. Mechanistically, this implies that step (2) to (3) is slower than step (1) to (2) (p. 250). That is, a slow stage is preceded by a rapid pre-equilibrium and the overall formation rate coefficient is given by 

This result is compatible with relaxation measurements (see Chap. 2, Vol. 1). On the Eigen mechanism, in principle, two relaxation times (tj, and Tj) should be observed, given by [6, 7] 

More elaborately, complex formation can be viewed as in Fig. 2 (the detailed kinetic implications of this refinement to the basic Eigen mechanism are covered by Benton and Moore [71]). It is reasonable to relate the rate coefficient for the dissociative slow stage with that for solvent exchange, since breaking of a metal ion—solvent bond is involved in both processes. 

Agreement between the value of fejs (derived from the observed value of kf and an observed or theoretical value of, 2) and the rate coefficient for solvent exchange (sometimes corrected by various statistical factors) is, on the whole, quite good and suggests that the concerted route is a valid model for the majority of substitutions at labile metal ions. 

A rough order of reactivity of +2 metal ions follows from the rates of complex formation in aqueous solution, and is 

Transition metal (TM) ions are frequently used as optically active dopants in commercial phosphors and in tunable sohd state lasers. TM ions are formed from atoms in the 

The 3d orbitals in TM ions have a relatively large radius and are unshielded by outer shells, so that strong ion-lattice coupling tend to occur in TM ions. As a result, the spectra of TM ions present both broad (S 0) and sharp (S 0) bands, opposite to the spectra of (RE) + ions, discussed in section 6.2.1, which only showed sharp bands (S 0). 

The ions of the first members of the second and third transition series (Me + = Y, La and Me + = Zr, Hf) are so large, as to achieve higher C.N.s than 6 already in some compounds AMe +F4 (page 33) and AgMe +Fj (47, 42) as well as in their binary fluorides (pages 37, 30). To deal with these structures which correspond closely to those of the lanthanide and actinide fluorides, is beyond the scope of this review. As for the stereochemistry of 8-coordination the reader is referred to papers by Clark et al. 68) and Kepert 189). 

Probably also connected with the size of Me-ions is the collinear array of linked octahedra e. g. in the pentafluorides of Nb, Ta and Mo (page 27), whereas angles occur in those of the RuFs-type (page 27). Similar conditions may be expected in tetrafluorides, but only the linear case of the NbF4-type is known so far (page 31). 

In terms of electron transfer reactions, transition metal ions can be the one- or two-electron type. The two-electron ions transform into unstable states on unit change of the metal oxidation number. In the outer-sphere mechanism, two-electron transfer is a combination of two one-electron steps. 

In all other cases, inner-sphere mechanisms are at work. These mechanisms include addition and subsequent dissociation. Eor C—H dissociation, the so-called metallocomplex activation 

One example of outer-sphere electron transfer is the reaction between the dipotassium cycloocta-tetraene (KjCgHg) and the cobalt complex of bis(salicylidenediamine) (Co Salen) (Levitin et al. 1971). 

Titration according to this scheme showed that the treatment of Co Salen with excess amounts of sodium resulted in nonquantitative formation of [(Co°Salen) 2Na ]. Thus, catalytic and, especially, kinetic investigations of such complexes have to take into account the presence of Co Salen or (Co Salen) in the samples studied. The described convenient method of quantitative electron transfer in solutions is good at determining low-valence metallocomplexes. 

In connection to the data described earlier, one reasonable question arises Why does the stabilizing role of a nonaqueous solvent become significant for Cu salts and remain insignificant for Cu salts It was shown (Myagchenko et al. 1989) that Cu salts (and salts of Hg and Pd as well) exist in nonaqueous solutions as polynuclear compounds. Thus, CUCI2 forms lamellar lattices with chlorine chains as bridges between copper atoms. In contrast, CuCl does not form such chain structures. 

Our overview of hydration so far is based upon ions which are simple, mostly those from groups lA and llA. We haven t yet said anything about the ions with more complex electronic structures, for example, the transition-metal ions, two-valent and three-valent entities. There are many who wouid expect such ions to have vaience-force interactions (orbital bond formation) with water moiecuies, but in fact they don t. It is possible to interpret their hydration heats in terms of electrostatic interactions with water, but one has to be more sophisticated and no longer regard the ions as simple spheres but take into account the shape and direction of their moiecuiar orbitals and how these affect the electrostatics of the interactions with water molecules. 

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Electron Spin Resonance Spectroscopy. Several ESR studies have been reported for adsorption systems [85-90]. ESR signals are strong enough to allow the detection of quite small amounts of unpaired electrons, and the shape of the signal can, in the case of adsorbed transition metal ions, give an indication of the geometry of the adsorption site. Ref. 91 provides a contemporary example of the use of ESR and of electron spin echo modulation (ESEM) to locate the environment of Cu(II) relative to in a microporous aluminophosphate molecular sieve. 

An atom or a molecule with the total spin of the electrons S = 1 is said to be in a triplet state. The multiplicity of such a state is (2.S +1)=3. Triplet systems occur in both excited and ground state molecules, in some compounds containing transition metal ions, in radical pair systems, and in some defects in solids. 

J. S. Griffith, The theory of transition metal ions, Cambridge University Press, 1961. 

In an aquo-complex, loss of protons from the coordinated water molecules can occur, as with hydrated non-transition metal ions (p. 45). To prevent proton loss by aquo complexes, therefore, acid must usually be added. It is for these conditions that redox potentials in Chapter 4 are usually quoted. Thus, in acid solutions, we have... 

First, the use of water limits the choice of Lewis-acid catalysts. The most active Lewis acids such as BFj, TiQ4 and AlClj react violently with water and cannot be used However, bivalent transition metal ions and trivalent lanthanide ions have proven to be active catalysts in aqueous solution for other organic reactions and are anticipated to be good candidates for the catalysis of aqueous Diels-Alder reactions. 

Inspired by the many hydrolytically-active metallo enzymes encountered in nature, extensive studies have been performed on so-called metallo micelles. These investigations usually focus on mixed micelles of a common surfactant together with a special chelating surfactant that exhibits a high affinity for transition-metal ions. These aggregates can have remarkable catalytic effects on the hydrolysis of activated carboxylic acid esters, phosphate esters and amides. In these reactions the exact role of the metal ion is not clear and may vary from one system to another. However, there are strong indications that the major function of the metal ion is the coordination of hydroxide anion in the Stem region of the micelle where it is in the proximity of the micelle-bound substrate. The first report of catalysis of a hydrolysis reaction by me tall omi cell es stems from 1978. In the years that... 

The aromatic shifts that are induced by 5.1c, 5.If and S.lg on the H-NMR spectrum of SDS, CTAB and Zn(DS)2 have been determined. Zn(DS)2 is used as a model system for Cu(DS)2, which is paramagnetic. The cjkcs and counterion binding for Cu(DS)2 and Zn(DS)2 are similar and it has been demonstrated in Chapter 2 that Zn(II) ions are also capable of coordinating to 5.1, albeit somewhat less efficiently than copper ions. Figure 5.7 shows the results of the shift measurements. For comparison purposes also the data for chalcone (5.4) have been added. This compound has almost no tendency to coordinate to transition-metal ions in aqueous solutions. From Figure 5.7 a number of conclusions can be drawn. (1) The shifts induced by 5.1c on the NMR signals of SDS and CTAB... 

Shannon and Prewitt base their effective ionic radii on the assumption that the ionic radius of (CN 6) is 140 pm and that of (CN 6) is 133 pm. Also taken into consideration is the coordination number (CN) and electronic spin state (HS and LS, high spin and low spin) of first-row transition metal ions. These radii are empirical and include effects of covalence in specific metal-oxygen or metal-fiuorine bonds. Older crystal ionic radii were based on the radius of (CN 6) equal to 119 pm these radii are 14-18 percent larger than the effective ionic radii. 

The site preference of several transition-metal ions is discussed in References 4 and 24. The occupation of the sites is usually denoted by placing the cations on B-sites in stmcture formulas between brackets. There are three types of spinels normal spinels where the A-sites have all divalent cations and the B-sites all trivalent cations, eg, Zn-ferrite, [Fe ]04j inverse spinels where all the divalent cations are in B-sites and trivalent ions are distributed over A- and B-sites, eg, Ni-ferrite, Fe Fe ]04 and mixed spinels where both divalent and trivalent cations are distributed over both types of sites,... 

Viable glass fibers for optical communication are made from glass of an extremely high purity as well as a precise refractive index stmcture. The first fibers produced for this purpose in the 1960s attempted to improve on the quahty of traditional optical glasses, which at that time exhibited losses on the order of 1000 dB/km. To achieve optical transmission over sufficient distance to be competitive with existing systems, the optical losses had to be reduced to below 20 dB/km. It was realized that impurities such as transition-metal ion contamination in this glass must be reduced to unprecedented levels (see Fig. 

Transition-metal ions also react with the generated radicals to convert the radicals to ions ... 

This reaction is one example of several possible radical transition-metal ion interactions. The significance of this and similar reactions is that radicals are destroyed and are no longer available for initiation of useful radical reactions. Consequentiy, the optimum use levels of transition metals are very low. Although the hydroperoxide decomposes quickly when excess transition metal is employed, the efficiency of radical generation is poor. 

The red tetrathiomolybdate ion appears to be a principal participant in the biological Cu—Mo antagonism and is reactive toward other transition-metal ions to produce a wide variety of heteronuclear transition-metal sulfide complexes and clusters (13,14). For example, tetrathiomolybdate serves as a bidentate ligand for Co, forming Co(MoSTetrathiomolybdates and their mixed metal complexes are of interest as catalyst precursors for the hydrotreating of petroleum (qv) (15) and the hydroHquefaction of coal (see Coal conversion processes) (16). The intermediate forms MoOS Mo02S 2> MoO S have also been prepared (17). 

Reactions involving the peroxodisulfate ion are usually slow at ca 20°C. The peroxodisulfate ion decomposes into free radicals, which are initiators for numerous chain reactions. These radicals act either thermally or by electron transfer with transition-metal ions or reducing agents (79). 

An expanding development is the use of peroxodisulfates as oxidants in organic chemistry (80,81). These reactions are initiated by heat, light, gamma rays, or transition-metal ions. The primary oxidising species is usually the sulfate ion radical, P hskip -3pt peroxodisulfate anion... 

With most transition metals, eg, Cu, Co, and Mn, both valence states react with hydroperoxides via one electron transfer (eqs. 11 andl2). Thus, a small amount of transition-metal ion can decompose a large amount of hydroperoxide and, consequendy, inadvertent contamination of hydroperoxides with traces of transition-metal impurities should be avoided. 

The reactions of alkyl hydroperoxides with ferrous ion (eq. 11) generate alkoxy radicals. These free-radical initiator systems are used industrially for the emulsion polymerization and copolymerization of vinyl monomers, eg, butadiene—styrene. The use of hydroperoxides in the presence of transition-metal ions to synthesize a large variety of products has been reviewed (48,51). 

Alkyl hydroperoxides are among the most thermally stable organic peroxides. However, hydroperoxides are sensitive to chain decomposition reactions initiated by radicals and/or transition-metal ions. Such decompositions, if not controlled, can be auto accelerating and sometimes can lead to violent decompositions when neat hydroperoxides or concentrated solutions of hydroperoxides are involved. 

Transition-metal ions also interact with hydroperoxide-generated radicals by converting them into ions, eg ... 

The radicals are destroyed and are not available to take part in the desired radical reactions, eg, polymerizations. Thus, transition-metal ion concentrations of metal—hydroperoxide initiating systems are optimized to maximize radical generation. 

As with other hydroperoxides, hydroxyaLkyl hydroperoxides are decomposed by transition-metal ions in an electron-transfer process. This is tme even for those hydroxyaLkyl hydroperoxides that only exist in equiUbrium. For example, those hydroperoxides from cycHc ketones (R, R = alkylene) form an oxygen-centered radical initially which then undergoes ring-opening -scission forming an intermediate carboxyalkyl radical (124) ... 

Magnesium reacts slowly at lower temperatures to give the amide, as do all active metals this reaction is catalyzed by transition metal ions. Aluminum nitride [24304-00-5] AIN, barium nitride [12047-79-9] Ba2N2, calcium nitride [12013-82-0] Ca2N2, strontium nitride [12033-82-8], Sr2N2, and titanium nitride [25583-20-4], TiN, may be formed by heating the corresponding amides. 

Metals. Transition-metal ions, such as iron, copper, manganese, and cobalt, when present even in small amounts, cataly2e mbber oxidative reactions by affecting the breakdown of peroxides in such a way as to accelerate further attack by oxygen (36). Natural mbber vulcani2ates are especially affected. Therefore, these metals and their salts, such as oleates and stearates, soluble in mbber should be avoided. 

Multilayers of Diphosphates. One way to find surface reactions that may lead to the formation of SAMs is to look for reactions that result in an insoluble salt. This is the case for phosphate monolayers, based on their highly insoluble salts with tetravalent transition metal ions. In these salts, the phosphates form layer stmctures, one OH group sticking to either side. Thus, replacing the OH with an alkyl chain to form the alkyl phosphonic acid was expected to result in a bilayer stmcture with alkyl chains extending from both sides of the metal phosphate sheet (335). When zirconium (TV) is used the distance between next neighbor alkyl chains is - 0.53 nm, which forces either chain disorder or chain tilt so that VDW attractive interactions can be reestablished. 

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