Tetrahedral-octahedral equilibrium

In contrast, the addition of deionized water to NiBr2(PEG)ie shifts the Ni(II) coordination from that characteristic of the tetrahedral-octahedral equilibrium just described to a spectrum typical of [Ni(H20)e] [46] (Figure 4.9), which shows that the position of the [Ni(H20)] transition band is shifted to shorter wavelengths than that arising from [NiOe] ", a result consistent with the logical assumption that the ether oxygens of the polymer chains are weaker electron donors than H2O. 

Ammonia complexes.. EflFect of complex formation on solubility. Cyanide complexes. The cyanide process of treating gold and silver ores. Complex halides and other complexes. Sodium thiosulfate as photographic fixer. Hydroxide complexes. Amphoteric hydroxides. Sulfide complexes. Equilibrium expressions for complex formation. Structural chemistry—tetrahedral, octahedral, square complexes. Existence of isomers. 

The most obvious distinction between the octahedral and tetrahedral compounds is that in general the former arc pink to violet in colour whereas the latter are blue, as exemplified by the well-known equilibrium ... 

Combining volumes, law of, 26, 236 Combustion, heat of hydrogen, 40 Complex ions, 392 amphoteric, 396 bonding in, 395 formation, 413 geometry of. 393 in nature, 396 isomers, 394 linear, 395 octahedral, 393 significance of, 395 square planar, 395 tetrahedral, 394 weak acids, 396 Compound, 28 bonding in, 306 Concentration and equilibrium, 148 and E zero s, 213 and Le Chatelier s Principle, 149 effect on reaction rate, 126, 128 molar, 72... 

Even when forward reactions proceed rapidly at laboratory conditions, as is observed with Se(IV) and Cr(VI) reduction, evidence exists that chemical and isotopic equilibrium are not approached rapidly. Altman and King (1961) studied the kinetics of equilibration between Cr(III) and Cr(Vt) at pH = 2.0 to 2.5 and 94.8°C. Radioactive Cr was used to determine exchange rates, and Cr concentrations were greater than 1 mmol/L. Time scales for equilibration were found to be days to weeks. The mechanism of the reaction was inferred to involve unstable, ephemeral Cr(V) and Cr(IV) intermediates. Altman and King (1961) stated that the slowness of the equilibration was expected because the overall Cr(VI)-Cr(III) transformation involves transfer of three electrons and a change in cooordination (tetrahedral to octahedral). Se redox reactions also involve multiple electron transfers and changes in coordination. 

A variety of geometries have been established with Co(II). The interconversion of tetrahedral and octahedral species has been studied in nonaqueous solution (Sec. 7.2.4). The low spin, high spin equilibrium observed in a small number of cobalt(Il) complexes is rapidly attained (relaxation times < ns) (Sec. 7.3). The six-coordinated solvated cobalt(ll) species has been established in a number of solvents and kinetic parameters for solvent(S) exchange with Co(S)6 indicate an mechanism (Tables 4.1-4.4). The volumes of activation for Co " complexing with a variety of neutral ligands in aqueous solution are in the range h-4 to + 1 cm mol, reemphasizing an mechanism. 

Spectral evidence" indicates an equilibrium between tetrahedral and octahedral Co" in iViV-dimethylacetamide and the equilibrium constant for [Co (tet)]/[Co (oct)] is reported at various temperatures. The complexes of acetylhydrazine (A), [CoA3]X2 (X = Cl or Br) and [Co(NCS)2A2]H20 and the tri-N-deuterio-analogue[Co(NCS)2(Ad3)2]D20 have been isolated and examined by i.r. Cationic complexes of JV-acyl hydrazines have been isolated with ligands in their keto-form, RCO-NH-NH2 however, the ligands also react in their enol form, RC(OH) = NNH2, forming neutral complexes (R = Me, Pr", Pr , or Ph). ... 

Cobalt(II) forms more tetrahedral complexes than any other transition metal ion. Also, because of small energy differences between the tetrahedral and octahedral complexes, often the same ligand forms both types of Co(II) complexes in equilibrium in solutions. 

To measure the equilibrium in low-dielectric medium requires quite a strong base. Unless this condition is met, measurable fractions of the octahedral form are not seen. Thus, for example, CoCl2 in quinoline shows only the spectrum of the tetrahedral configuration, although pyridine solvent shows both octahedral and tetrahedral CoCl2 spectra (6). Primarily for this reason, the bases most used have been pyridine and substituted pyridines. The equilibrium has been studied in pyridine as a function of temperature for CoCl2 (8, 39). In pure pyridine the octahedral form predominates at room temperature, and only at temperatures approximating 60°C are the two forms equally abundant (8). 

Much work has been devoted to the halide complexation of these elements in non-aqueous media. Equilibrium and calorimetric measurements for the formation of the [MX ](n-2) (M = Zn or Cd X = Cl, Br, I or SCN n = 1-4) anions in dimethyl sulfoxide (DMSO) have shown that stability constants follow the same order, but are much larger than those found for aqueous solution zinc exhibits an enhanced hardness as an acceptor in DMSO as compared to cadmium. Calorimetric measurements indicate a change from octahedral to tetrahedral coordination with increasing halide concentrations.1002-1006... 

Four-coordinate d8 complexes can display a closely related electronic and geometric equilibrium between paramagnetic tetrahedral and diamagnetic planar isomers. Numerous examples are known in nickel(II) chemistry (80). In this case, as well as with the octahedral complexes described above, there is no change in the coordination number of the metal ion. 

The phenomenon of spin equilibrium in octahedral complexes was first reported by Cambi and co-workers in a series of papers between 1931 and 1933 describing magnetic properties of tris(iV,iV-dialkyldithio-carbamato)iron(III) complexes. By 1968 the concept of a thermal equilibrium between different spin states was sufficiently well established that the definitive review by Martin and White described the phenomenon in terms which have not been substantially altered subsequently (112). During the 1960s the planar-tetrahedral equilibria of nickel(II) complexes were thoroughly explored and the results were summarized in comprehensive reviews published by Holm and coworkers in 1966 and 1973 ( 79, 80). Also, in 1968, Busch and co-workers... 

The difference in the composition of these two size fractions is similar to the difference between the two types of clay vermiculite described by Barshad and Kishk. The two sets of data confirm the idea that clay vermiculites developed by mild leaching action of pre-existing sheet structures tend to inherit much of their octahedral and tetrahedral character. Clay vermiculites formed by relatively intense weathering and by diagenetic alterations and in approximate equilibrium with their soil environment will have little if any tetrahedral Al the octahedral sheet can be quite variable in composition and depends on the availability of Al, Fe, and Mg. 

---