Trivalent First-Row Transition Metal Ions

The interplay of extensive metal-ion hydrolysis, hgand deprotonation, parallel reaction paths, arising from combinations of these deprotonated and hydrolized species, together with metal-ligand redox reactions, which follow complex formation and which are in turn inhibited by the product, make the Fe(III)/ascorbic acid system a complicated one to study quantitatively. This challenge has been dealt with by Jordan. The initial reaction can be attributed to formation of a blue Fe(III)-ascorbate complex and a number of acid-dependent parallel paths are proposed. The rate constant for Eq. (18) (5.5 X 10 s ) is the highest observed for substitution at the hexaaquo 

Recent structural studies of squaric acid have shown it to form 1,3-bis(metal) complexes as well as 1,2-bidentate chelates with Ce(III) or with Cu(II). Stopped flow methods have been used to study the reaction between squaric acid and Fe(III). In solutions with [H ] 0.02 M, biphasic absorbance changes are observed due to operation of a number of parallel and series pathways. The initial reactions are complexations to form a 1 1 compound, [Fe(III)(H20)5(squarate)] , and a 2 1 compound, [Fe(III)2(0H)2(H20)6(squarate)] these are followed by a slower reaction leading to the reduction of Fe(III) to Fe(II) by the ligand. The reaction scheme, shown in Eq. (19)-(21), involves monomeric and dimeric squarate 

In a study which included the determination of the structure and the acidity constants of the seven-coordinate complex aqua(o-phenylenediamine-JV,AT,A, iV -tetraacetato)ferrate(III), the temperature- and pressure-dependent kinetic parameters for water exchange were determined by ONMR line broadening to be )fc(298.2 K) = 1.2 0.2x 10 s , =26 + 3 kJ mol-,  

The importance of the metalloporphyrin systems as models for biological sites is clear. The stopped flow kinetics of the reactions of various para-sab-stituted tetraphenylporphinatoiron(III) chlorides with imidazoles in acetone have been reportedReactions where the methoxy ligand in five-coordinate octaethylporphinato-iron(III) methoxide, a model for some of these biological systems, has been substituted by a range of phenols, carboxylic acids, and thiols have been reportedan increase in rate with increasing acidity of the thiol is observed. 

High-pressure stopped flow has been used to study the reactions of CN and imidazole with hemin in Under pseudo-first-order conditions, 

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The second category is the transition metal ions, all of which in Fig. 1 are six-coordinate with the exception of Pt2+ and Pd2+, which are square-planar four-coordinate (6-9). Their labilities are strongly influenced by the electronic occupancy of their d orbitals. This is illustrated by the divalent first-row transition metal ions, which should exhibit similar labilities to Zn2+ on the basis of their rM instead, however, their labilities encompass seven orders of magnitude. On a similar basis, the trivalent first-row transition metal ions might be expected to be of similar lability to Ga3+, but instead they exhibit a lability variation of 11 orders of magnitude, with Cr3 being at the... 

Rate Constants and Activation Parameters for Water Exchange on Trivalent First Row Transition Metal Ions [M(H20)6] ... 

When 8-hydroxyquinoline and derivatives of bis(8-hydroxy-quinoline) react with metal ions, coordination complexes and polymers are formed, respectively, which exhibit improved thermal stability. This paper reviews the reaction of first-row transition metal ions with such ligands and their effect on the stabilization of these organic molecules. For the polymers containing divalent Mn, Co, Ni, Cu, or Zn the decomposition temperature is related to the periodic properties of the metal as well as the composition of the ligand to which the metal is coordinated. Trivalent chromium produces a crosslinked polymer when it reacts with bis(8-hydroxy-5-quinolyl)methane, and the thermogram for this polymer is also reported. 

Ionic radii for divalent (a) and trivalent (b) first-row transition metal ions. [Data were obtained from http //www.webelements. com (accessed March 29, 2014).]... 

There are a host of potential probes for and Ca, as these ions have about the same size as many divalent first row transition metals and divalent lanthanides (Table VI). Trivalent lanthanides have the correct radii although the wrong charge as substitutes for calcium. In isomor-phous replacement in minerals, the radius is more important than the charge this may also be true in biology as Na and (radius 1.0 A) and and Ba (radius 1.3 A) often compete for sites. This means that a lanthanide may be an excellent competitor for a divalent cation site in biology and there is already some evidence to this eflFect (Table IX) (29). 

Rate Constants and Activation Parameters for Solvent Exchange on First-Row trivalent Transition-metal Ions... 

Volumes of activation for solvent exchange on high spin, first row divalent and trivalent transition metal ions are available in water and in a variety of non-aqueous solvents. The AF values indicate that the mechanism for solvent exchange is not unique, but progressively changes from an associative activation mode for the early elements to a dissociative activation mode for the later ones. 

Water exchange on first row di- and trivalent transition metal ions has been the subject of extensive experimental studies and has been widely reviewed [15, 23). Table 4.1 summarizes the experimental first order rate constants and the activation parameters. 

Many of the early principles of inorganic photochemistry have developed from studies carried out on the first-row 3d) series of transition metal ions. The majority of these ions are kinetically labile in aqueous solution, the two exceptions being complexes of the trivalent Cr " (d ) and Co " d ) ions, both of which are kinetically inert. This difference in their solution lability has resulted in complexes of these two ions receiving the majority of interest, because their slow substitution rates allows the stereochemistries of the individual photoprocesses to be determined before subsequent thermal reactions occur. 

The sulfate ion (S04 ), though a simple dianionic ligand, exhibits a rather flexible symmetry and variable mode of binding with metal ions in the crystalline state. Interest in metal sulfate interactions with the hydra-zinium cation arises from the fact that these compounds exhibit one- and three-dimensional magnetic interactions of a linear-chain antiferromag-net. Dihydrazinium sulfate forms a series of double sulfates with divalent and trivalent cation that are well crystallized and, in a few instances, are even less soluble than hydrazinium sulfate itself. The most easily prepared double sulfates are those of Cr, Mn, Fe, Co, Ni, Cu, Zn, Cd, Li, Mg, Al, La, Ce, Pr, Nd, and Sm. These salts are similar to the ammonium alums obtained with some metal sulfates. In contrast to the ammonium double salts, the hydrazinium metal sulfates of first row transition elements and lithium crystallize without water, whereas the lighter lanthanides and aluminum form hydrated salts. 

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