Metals divalent, complexes

Metal ion complexation rates have been studied by the T-jump method. ° Divalent nickel and cobalt have coordination numbers of 6, so they can form complexes ML with monodentate ligands L with n = 1—6 or with bidentate ligands, n = 1-3. The ligands are Bronsted bases, and only the conjugate base form undergoes coordination with the metal ion. The complex formation reaction is then... 

Divalent Metal Ion Complexes of Imidazoles and Pyridines Having... 

AT-heterocyclic carbenes show a pure donor nature. Comparing them to other monodentate ligands such as phosphines and amines on several metal-carbonyl complexes showed the significantly increased donor capacity relative to phosphines, even to trialkylphosphines, while the 7r-acceptor capability of the NHCs is in the order of those of nitriles and pyridine [29]. This was used to synthesize the metathesis catalysts discussed in the next section. Experimental evidence comes from the fact that it has been shown for several metals that an exchange of phosphines versus NHCs proceeds rapidly and without the need of an excess quantity of the NHC. X-ray structures of the NHC complexes show exceptionally long metal-carbon bonds indicating a different type of bond compared to the Schrock-type carbene double bond. As a result, the reactivity of these NHC complexes is also unique. They are relatively resistant towards an attack by nucleophiles and electrophiles at the divalent carbon atom. 

Soil pH is the most important factor controlling solution speciation of trace elements in soil solution. The hydrolysis process of trace elements is an essential reaction in aqueous solution (Table 3.6). As a function of pH, trace metals undergo a series of protonation reactions to form metal hydroxide complexes. For a divalent metal cation, Me(OH)+, Me(OH)2° and Me(OH)3 are the most common species in arid soil solution with high pH. Increasing pH increases the proportion of metal hydroxide ions. Table 3.6 lists the first hydrolysis reaction constant (Kl). Metals with lower pKl may form the metal hydroxide species (Me(OH)+) at lower pH. pK serves as an indicator for examining the tendency to form metal hydroxide ions. 

The ligands H2L19, H2L23-H2L31 are effective dinucleating ligands towards various divalent metal ions. Complexes of composition [Mn2(LR)(p-L )]+ (M — Mn, Fe, Co, Ni, Zn, Cd)... 

In summary, complexation kinetics for most 1 1 metal ligand complexes are generally rapid, with water-loss rate constants, k, ranging from 104 to 109 s-1 [164], In many cases involving the formation of simple divalent metal complexes, the Eigen Wilkens mechanism gives a reasonable estimate of a maximum value of k[. Nonetheless, it must be kept in mind that in complicated... 

For some metals the transfer of metallic ions involves a reaction intermediate of an adsorbed metallic ion complex which is coordinated with anionic ligands hence, the overall reaction occurs in a series of two elementaiy steps rather than one. Such a multistep transfer of ions can result, in the course of metallic ion transfer, from the reduction of the activation energy for ion transfer due to the formation of adsorbed intermediates. We examine a transfer reaction of divalent metallic ions via an adsorbed complex ion according to the steps in Eqn. 9-13 ... 

This Group IIA (or Group 2) element (atomic symbol, Ca atomic number, 20 atomic weight, 40.078 electronic configuration = ls 2s 2p 3s 3p 4s ) loses both As electrons to form a divalent cation of 0.99A ionic radius. Ionic calcium combines readily with oxygen ligands (chiefly water, phosphates, polyphosphates, and carbox-ylates) to form stable metal ion complexes. Ca under-... 

Chromatography cyclophosphazenes, 21 46, 59 technetium, 11 48-49 Chromites, as spinel structures, 2 30 Chromium, see Tetranuclear d-block metal complexes, chromium acetylene complexes of, 4 104 alkoxides, 26 276-283 bimetallics, 26 328 dimeric cyclopentdienyl, 26 282-283 divalent complexes, 26 282 nitrosyls, 26 280-281 trivalent complexes, 26 276-280 adamantoxides, 26 320 di(/ >rt-butyl)methoxides, 26 321-325 electronic spectra, 26 277-279 isocyanate insertion, 26 280 substitution reactions, 26 278-279 [9]aneS, complexes, 35 11 atom... 

The use of a fatty acid monolayer on a subphase of divalent metal ions compromises the archetypal preparation of LB films containing metal ions. Complexation of subphase metal ions to the fatty acid monolayer occurs through fatty acid proton/metal ion exchange and is pH dependent. For C,d2+ ions, at 2 X 10 4 M. for example, the onset of the exchange is at approximately pH 5 and stoichiometric exchange occurs at pH 6. This stoichiometric reaction is depicted for the general case for divalent metal ions and fatty acids in Eq. (2) ... 

The hydrolysis of certain Schiff bases is catalyzed by divalent metal ions such as cupric or nickelous ion. Spectrophotometric evidence indicates the formation of a metal ion complex of the substrate with subsequent facile cleavage of the complex. 

Ammonia can form many "coordinated complexes with the above mentioned metals and these complexes, called "ammines or "ammoniates" are among the most important. Usually divalent metals form complexes with four NH, groups, while trivalent metals usually coordinate with six NH, groups. There are, however, many exceptions... 

X-Ray diffraction shows that the [Pt(CN)6]2 ion is regular octahedral with linear Pt—C—N bonds.3"2 The IR and Raman spectra of K2[Pt(CN)6] have C—N stretching vibrations at higher frequencies and C—N force constants which are larger than the corresponding divalent complexes.303,304 This correlates with less n back-donation from the metal to the empty n orbitals of the cyanide ligand in the PtIV complexes. 

A variety of chromium(III) and cobalt(III) oligomers can be prepared by oxidation of an aqueous solution of the appropriate divalent metal-ion complex with oxidants such as 02 and Tl3 +. The... 

As discussed earlier, the enzymic reaction catalyzed by glutamine synthetase requires the presence of divalent metal ions. Extensive work has been conducted on the binding of Mn2+ to the enzyme isolated from E. coli (82, 109-112). Three types of sites, each with different affinities for Mn2+, exist per dodecamer n, (12 sites, 1 per subunit) of high affinity, responsible for inducing a change from a relaxed metal ion free protein to a conformationally tightened catalytically active protein n2 (12 sites) of moderate affinity, involved in active site activation via a metal-ATP complex and n3 (48 sites) of low affinity unnecessary for catalysis, but perhaps involved in overall enzyme stability. The state of adenylylation and pH value alter the metal ion specificity and affinities. 

The divalent metal squarate complexes, M(squarate)(H20)2, decompose in a vacuum in two stages. First there is a reversible loss of two water molecules, followed at higher temperatures by decomposition to the metal. The nature of the anhydrous complexes is not fully understood but the metal atoms appear to be four-coordinate.174... 

As mentioned above, the electrophilic metal carbene complexes are stabilised by the presence of heteroatoms or phenyl rings at the divalent carbon atom, while hydrogen or alkyl groups stabilise the nucleophilic complexes. Therefore, there is a distinction between carbenoids and alkylidenes when designing carbene ligands corresponding to the former or the latter class. 

Table II. Thermal Stabilities of Divalent First-Row Transition Metal Coordination Complexes and Polymers...

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