Valency Three and Higher

Variable-pressure and variable-temperature NMR studies have been reported of solvent exchange on various lanthanide ions in DMF, while the exchange of 1,1,3,3-tetramethylurea (tmu) on [Lu(tmu)6] in CD3CN solution is apparently dissociatively activated. The kinetics of DMF exchange and anation by SCN and NJ on [U02(DMF)5] have been measured by the NMR and stopped-flow techniques, respectively. 

Complex Formation Involving Unsubstituted Metal Ions Multidentate Ligands 

2 Complex Formation involving Unsubstituted Metal Ions Multid tate Ligands 

The formation of complexes with cyclic polyethers and cryptands is considered in Part III. 

Metals of Valency Three and Higher.—A comprehensive list of the rate parameters for ligand-replacement processes in octahedral complexes of metals in oxidation state three has appeared.  

Of all the first-row transition-metal ions, Fe has been one of the most difficult to 

Williams, S. Petrucci, B. Sesta, and M. Battistini, Inorg. Chem, 1974,13, 1968. 

The kinetics of complexation of silver(in) by periodate and tellurate ions have been measured by the stopped-flow method. Both reactions result in a rate law attributable to stepwise formation of the bis-complex  

The results of three ultrasonic investigations on lanthanide salts have been reported. The studies on erbium(iii) perchlorate in aqueous methanol suggest that inner-sphere perchlorate complexes occur at water mole fractions of less than 0.9. On that basis, the rate constant for the formation of the inner-sphere complex from the outer-sphere complex at 25 °C is 1.2 x 10 s. The case of erbium(m) nitrate in aqueous methanol is more complicated and it is suggested that the mechanism involves the existence of two forms of the solvated lanthanide ion, differing in coordination number, in equilibrium with the outer- and inner-sphere complexes. The results for aqueous yttrium nitrate, on the other hand, represent a simplification over those of previous ultrasonic studies on the lanthanides. The authors reject the normal multistep mechanism in favour of a single diffusion-controlled process. Unfortunately, the computed value for the formation rate constant kt of 1.0 x 10 1 mol s is at least two orders of magnitude lower than the value calculated on the Debye-Smoluchowski approach, but the discrepancy is attributed to steric effects. 

Metals of Valency Three and Higher.— There have been several reports of the measurement by n.m.r. of the kinetic characteristics of complexes of Group IIIA tervalent metal ions. Delpuech et al. have used the spectra associated with the A1 and nuclei to identify the species present in solutions of the metal and a number of phosphorus-containing ligands, and to measure their exchange characteristics. It appears that the major solvates of AP+ in trimethyl (TMPA) and triethyl phosphates (TEPA), dimethyl methyl- (DMMP) and diethyl ethyl-phosphonates (DEEP), and dimethyl hydrogen phosphite (DMHP) are octahedral. In each case the rate law for the exchange process, 

Richardson and Alger have measured the solvation number and exchange rate for a number of AF and Ga salts in ethanol and methanol, using the OH proton signal from the free and bound alcohol. They obtain solvation numbers ranging from 4 to 7 but point out that the situation is sometimes complicated by the intrusion of the anion (chloride or nitrate) into the inner co-ordination sphere of the metal. They do conclude, however, that in all of the cases they studied the solvent-exchange process is 5n2. 

In solutions of indium(in) and thallium(m) chloride in concentrated hydrochloric acid it appears that the major species are, respectively, [InCl4(H20)2] and [TlCle] (or possibly [TlCls ). Lincoln et al. have used the C1 resonance to study the 

Measurements of the ultrasonic absorption of aqueous samarium(m) sulphate have been repeated and the data analysed in terms of the usual two-step mechanism (diffusion, followed by water loss at the metal ion). 

In a preliminary note, the kinetics of alcohol exchange with trialkyl arsenates and vanadates are reported,  

Metals of Valency Three and Higher.—Work on the mechanism of ligand substitution in weak complexes has been extended to AISO4 and GaS04+, and to Al[Co(CN)e]. The activation energies for the forward and reverse reactions 

The synthesis of hexakis(dimethyl sulphoxide)vanadium(m) perchlorate and the kinetics of its reaction with SCN , bipy, and sulphosalicylic acid have been reported. The complex-formation rates show some dependence on the nature of the ligand but a definite mechanistic assignment cannot yet be made. 

The reaction of monothenoyltrifluoroacetone (7) with iron(m) has been studied but the situation is even more obscure than in the nickel(n), cobalt(n), and copper(n) systems because there are so few criteria for 

In the reaction of La with 4-(2 -pyridylazo)-resorcinol (8) the ratedetermining step is the rupture of the internal hydrogen bond. A comparison of the rates of complex formation between Nd and S04 in H2O and DgO suggests that solvent-loss at the metal ion is not the 

The acid-independent term in the rate expression for complex formation of hexa-aquoiron(m) has been attributed to reactions (1) and (2), 

The influence of lithium, potassium, and caesium nitrates on the kinetics of the reverse reaction (i.e. the aquation of Fe ) have been reported in the case of L = Cl. Although no specific cationic effects are predicted by the theory of salt effects, it is found that the acid-independent and the acid-dependent pathways are affected in opposite directions. The latter pathway is thought to involve [FeOHCl]+ and the observed salt effects are tentatively ascribed to the influence of water structure. 

Complex formation between iron(in) and ten phenols has been studied and it has been confirmed that the reaction rate is acid-independent in the pH range 0—2. A mechanism for the aquation of the iron(ni)-phenol complex is proposed in which a proton transfers intramolecularly from a co-ordinated water molecule to a phenolate ion  

The kinetics and mechanism of the reaction of iron(m) with thiocyanate have received further attention.  

Further ultrasonic absorption data have been reported on aqueous solutions of the nitrates and sulphates of four of the lanthanides, but the nature of the overall complexation mechanism remains uncertain. It appears that the deuterium isotope effect observed when DgO is substituted for HgO as the solvent is not a function of the solvated cation alone, but rather of the cation-ligand complex. 

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Metals of Valency Three and Higher.—Stopped-flow studies have been reported for iron(iii) reacting with o-cresolsulphonphthalein and with a series of substituted catechols, including adrenaline and L-dopa. With the catechols, the value of kt was more or less independent of the substituent groups present on the ligand. 

Metals of Valency Three and Higher.— The kinetics of complex formation of iron(m) with various aminopolycarboxylates and catechol have been reported (see Table 8). After considering the pH dependence of the observed rate constants,... 

Metals of Valency Three and Higher.—Stopped-flow Fourier-transform n.m.r. experiments reveaP two distinct stages in the reaction between [Al(DMSO)6l + and bipy in nitromethane - those represented by equations (1) and (2) of Scheme 1. The results are compared in Table 5 with those from a previous study" with the same... 

Metals of Valency Three and Higher.— The kinetics of the formation of complexes of gallium(iii), indium(iii), and thallium(iii) with semi-xylenol orange (5 shown in the form H3SXO ) in acid solution have been reported. The... 

In order to establish the model of intergranular impedance for doped barium titanate, it is important to notice that miorostructure properties of BaTiOj based materials, expressed in their grain boundary contacts, are of basic importance for electric properties of these materials. The barrier character of the grain boundaries is especially pronounced for doped BaTiOs materials which are used as PTC resistors. Basically two types of dopants can be introduced into BaTiOs large ions of valence 3+ and higher, can be incorporated into Ba positions, while the small ions of valence 5+ and higher, can be incorporated into the Ti sublattice [9-11], Usually, the extent of the solid solution of a dopant ion in a host structure depends on the site where the dopant ion is incorporated into the host structure, the compensation mechanism and the solid solubility limit [12], For the rare-earth-ion incorporation into the BaTiOs lattice, the BaTiOs defect chemistry mainly depends on the lattice site where the ion is incorporated [13], It has been shown that the three-valent ions incorporated at the Ba -sites act as donors, which extra donor charge is compensated by ionized Ti vacancies (V -), the three-valent ions... 

Three-body and higher terms are sometimes incorporated into solid-state potentials. The Axilrod-Teller term is the most obvious way to achieve this. For systems such as the alkali halides this makes a small contribution to the total energy. Other approaches involve the use of terms equivalent to the harmonic angle-bending terms in valence force fields these have the advantage of simplicity but, as we have already discussed, are only really appropriate for small deviations from the equilibrium bond angle. Nevertheless, it can make a significant difference to the quality of the results in some cases. 

Most of the known borides are compounds of the rare-earth metals. In these metals magnetic criteria are used to decide how many electrons from each rare-earth atom contribute to the bonding (usually three), and this metallic valence is also reflected in the value of the metallic radius, r, (metallic radii for 12 coordination). Similar behavior appears in the borides of the rare-earth metals and r, becomes a useful indicator for the properties and the relative stabilities of these compounds (Fig. 1). The use of r, as a correlation parameter in discussing the higher borides of other metals is consistent with the observed distribution of these compounds among the five structural types pointed out above the borides of the actinides metals, U, Pu and Am lead to complications that require special comment. 

The concept of an octet of electrons is one of the foundations of chemical bonding. In fact, C, N, and O, the three elements that occur most frequently in organic and biological molecules, rarely stray from the pattern of octets. Nevertheless, an octet of electrons does not guarantee that an inner atom is in its most stable configuration. In particular, elements that occupy the third and higher rows of the periodic table and have more than four valence electrons may be most stable with more than an octet of electrons. Atoms of these elements have valence d orbitals, which allow them to accommodate more than eight electrons. In the third row, phosphoms, with five valence electrons, can form as many as five bonds. Sulfur, with six valence electrons, can form six bonds, and chlorine, with seven valence electrons, can form as many as seven bonds. 

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