Cation transition metal ions

This structural variation notwithstanding, only a few cationic transition-metal ions react efficiently with molecular oxygen under gas-phase conditions (see below). In contrast, many anionic metal complexes and clusters are readily oxidized by O2 to afford various metal-oxide anions [19]. From a conceptual point of view, however, anionic species appear to be inadequate reagents for the activation of hydrocarbons, because they generally require electrophilic attack. At present, only a few oxidations by transition-metal oxide anions have been reported to occur in the gas phase, and they are mostly limited to relatively polar substrates, such as the CH3OH CH2O conversion [20]. Because of the lower reactivity of hydrocarbons, their C-H bond activation by metal-oxide anions is likely to be limited to radical pathways driven purely thermodynamically, i.e., when Z)(0-H) exceeds Z)(C-H) of the substrate [21]. As radical-type pathways are prone to create selectivity problems, and over-oxidation is particularly difficult to control, the anionic route appears less attractive as far as partial oxidation of alkanes is concerned. 

Metrohm C3 Polyvinyl alcohol with carboxyl groups High efRciency, for mono- and divalent cations, transition metal ions... 

Simplest examples are prepared by the cyclic oligomerization of ethylene oxide. They act as complexing agents which solubilize alkali metal ions in non-polar solvents, complex alkaline earth cations, transition metal cations and ammonium cations, e.g. 12—crown —4 is specific for the lithium cation. Used in phase-transfer chemistry. ... 

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,... 

Color from Transition-Metal Compounds and Impurities. The energy levels of the excited states of the unpaked electrons of transition-metal ions in crystals are controlled by the field of the surrounding cations or cationic groups. Erom a purely ionic point of view, this is explained by the electrostatic interactions of crystal field theory ligand field theory is a more advanced approach also incorporating molecular orbital concepts. 

Essentially all transition metal ions behave like Zn2+, forming a weakly acidic solution. Among the main-group cations, Al3+ and, to a lesser extent, Mg2+, act as weak acids. In contrast the cations in Group 1 show little or no tendency to react with water. 

Some cations with an octahedral-site preference (such as Ni2+, Co3+, and Cr3+) are expected to occupy the 16d sites of the spinel with Mn, whereas cations with a strong tetrahedral-site preference (such as Zn2+) are expected to occupy the 8a sites and to dislodge corresponding lithium ions into the 16d sites. In cases where Mn is substituted by transition metal ions (such as Co, Ni, and Cr) that can partake in the electrochemical reaction, voltage plateaus between 4 and 5V have been observed [135, 136],... 

Organic Molecules It can be seen from our earlier discussion that the presence of a transition metal ion is not always required for an electrochromic effect. Indeed, many organic molecules can yield colored products as a result of reversible reduction or oxidation. 4,4 -Bipyridinium salts are the best known example of such compounds. These compounds can be prepared, stored, and purchased in colorless dicationic form (bipm +). One electron reduction of the dication leads to the intensely colored radical cation (bipm+ ). Such radical cations exist in equilibrium with their dimers (bipm ). In the case of methyl viologen, the radical cation is blue and the dimer is red. By varying the substient group in the molecule, different colors can be obtained. 

Cation-selective ionophores are the most successful in polymeric ISEs and selectivi-ties exceeding ten orders of magnitude became quite common. The cation-ionophore binding occurs dominantly due to Lewis interactions and could be understood in terms of hard and soft acid and bases theory (HSAB). While hard base oxygen atoms originate from ester, ether or carbonyl functionalities, and interact with hard acid alkaline cations, the softer sulfur or nitrogen atoms better bind with transition metal ions. Cation... 

In an ionic compound, the partial covalence of a bond formed between a transition metal ion and its ligand modifies the magnetic properties of the cation. It can be seen, for example, that if electrons were... 

Peroxyl radicals with a strong oxidative effect along with ROOH are continuously generated in oxidized organic compounds. They rapidly react with ion-reducing agents such as transition metal cations. Hydroxyl radicals react with transition metal ions in an aqueous solution extremely rapidly. Alkyl radicals are oxidized by transition metal ions in the higher valence state. The rate constants of these reactions are collected in Table 10.5. 

Group 1 superoxides are coloured compounds, which is unusual for group 1 compounds not containing a transition metal ion. The trend in oxide formation is the result of the increasing size of the metal cations as the group is descended. 

The most significant class of inorganic supports, which is used for the direct ion exchange of positively charged transition-metal complexes, are smectite clays. Pin-navaia has introduced the use of these swelling, layered silicate clays for catalysis. Other clays include montmorillonite, bentonite, and laponite. As shown by Pinna-vaia, cationic transition-metal complexes can be readily exchanged (intercalated) into the solvated interlayers of these silicates (Eq. (1)) [117] ... 

The primary, secondary, and tertiary aliphatic amines do not form simple addition complex ions with bare transition metal ions. Only Ag+ reacts with MeNH2 to form a simple addition product [AgMeNH2]+ (107). The Pb+ ion also forms addition products, [PbMeNH2]+ and [Pb(MeNH2)2]+, with methylamine (143). Other bare transition metal ions (144) react with amines via removal of one hydrogen to form the metal hydride and the amine cation with one hydrogen removed [RR N]+. 

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