Blue species equilibria

A solution of the isolated platinum blue compound usually contains several chemical species described in the previous section. Such complicated behaviors had long been unexplored, but were gradually unveiled as a result of the detailed equilibrium and kinetic studies in recent years. The basic reactions can be classified into four categories (l)HH-HT isomerization (2) redox disproportionation reactions (3) ligand substitution reactions, especially at the axial coordination sites of both Pt(3.0+)2 and Pt(2.5+)4 and (4) redox reactions with coexisting solvents and atmosphere, such as water and 02. In this chapter, reactions 1-4 are summarized. 

The absorption spectra of the hydroxyphenylbenzo-triazole derivatives in various solvents and polymer films indicate that two ground-state forms of the molecules exist. These species are proposed to be a planar and non-planar form of the stabilizers. The position of the equilibrium between these two forms is affected by both the polarity and the hydrogen-bonding strength of the medium. The blue fluorescence (A.max = 400 nm) observed for these stabilizers originates from an excited-state species in which intramolecular proton transfer is disrupted. 

Some macrocyclic Ni(II) complexes exist as equilibrium mixtures of the yellow, diamagnetic, square-planar [Ni(L)]2+ and blue (or violet), paramagnetic, octahedral [Ni(L)(S)2]2+ species in coordinating solvents... 

The reduction of PhsB is known to result in a radical anion PhsB- that is in equilibrium with the dimeric species [PhsB BPhs] ". However, if bulky substituents as in MessB are present and thus prevent attack of nucleophiles or dimerization, a blue-colored stable radical anion can readily be identified. While arylboranes are also known to form purple-colored dianions, the second reduction of simple triarylboranes is typically irreversible. In a recent study, Okada and Oda reported the formation of purple-colored solutions of dimesifylphenylborane dianions [Mes2BAr] (146 Ar = Ph, d-MesSiCeUi, biphenyl) upon extended reaction of Mes2BAr with Na K alloy in THE (Scheme 21). Formation of dianions was confirmed by multinuclear NMR specfroscopy. While the bulky mesityl groups provide chemical sfabihty to the dianions and thus prevent further chemical reactions, the presence of the unsubstituted phenyl group is beheved to allow for effective tt-interactions with the / -orbital on boron. 

Jorgensen indicated that the absorption spectra of aqueous ethanolic solutions of cupric ion containing bipyridyl or phenanthroline (ratio 1 2) were compatible with the presence either of a cis-diaquobischelate cation or with a trigonal bipyramidal monoaquo species 409). The weak electrolyte behavior in nitrobenzene or nitromethane of deep blue Cu(bipy)2(C104)2 has been attributed to the following equilibrium ... 

FIGURE 14.2 Sketch of the change with time of the concentrations of products and reactants in the spontaneous reactions illustrated in Figure 14.1. For ease of display, concentrations are expressed as percent of the total Co(ll) present in each species, (a) Partial conversion of pink hexaaqua complex A into blue tetrachloro complex C. (b) Partial conversion of blue tetrachloro complex C into pink hexaaqua complex A. After changes in the slope of each species concentration become imperceptibly small, we say the reaction has arrived at chemical equilibrium. 

In 1998, Baker and Kirby conducted a 31P NMR investigation of electron exchange in the two-electron reduced heteropoly blue complex of [(P2Wi706i)2Th]18 (which contains an equilibrium mixture of oxidized, two-electron- and four-electron-reduced species) as a function of alkali metal counterion, concentration, and temperature. They interpreted their data in terms of Equation (8) in which the more strongly pairing alkali metals (M in Equation (8) = K+, Rb+, and Cs+ but not Li+) form an ion bridge between the two defect HPA units in the syn isomer. This interaction stabilizes the syn isomer and drives an apparent syn-anti equilibrium, Equation (8), to the left.118 The change in chemical shifts and other features of the 31P NMR spectra of these Th sandwich POM complexes as a function of the counterion defined a qualitative method to estimate the association of monocations with POM polyanions ... 

MV+ case.(24) Optical spectral changes as a function of the concentration of the MV+ (or the one-electron reduced form of III)(25) are consistent with the reversible equilibrium represented by equation (6). The [(PQ2+/+)n]8urf. system is violet in color(25), not blue, consistent with aggregation of the PQ centers due to the high effective concentration. Since the E data for (MV2+/+) in H2O solution are for low concentrations, the E is not directly comparable to that for the surface-confined analogue. These properties (optical spectra and E ) associated with aggregation of the redox center represent one of the ways that the surface-confined species may depart from... 

Qualitative detection of excess protein in urine is largely based on use of dipstick tests. The reactive portion of the stick is coated with a buffered indicator that develops color in the presence of protein. A typical example is Albustix (Bayer Corporation, Diagnostics Division, Tarrytown, NY), in which bromphenoi blue, buffered to pH 3 with citrate, is present mostly in the protonated, yellow form. When protein is added, the affinity of the anionic form of the indicator dye for protein causes a shift of the equilibrium between anionic and protonated forms of the indicator toward formation of the blue anionic species. The intensity of the shade of blue produced is then proportional to the concentration of protein in the specimen. Combur 8 strips (Roche Diagnostics, Inc., Indianapolis, IN) are said to be less subject to drug interferences. Their detection hmit is 7mg/dL. 

We begin with a purple equilibrium mixture of the pink and blue complexes at room temperature (not shown). In hot water the forward reaction (endothermic) is favored and is higher, so the solution is blue right). At 0°C, the reverse reaction (exothermic) is favored and is lower, so the solution is pink left). Each insert shows the structure of the cobalt complex species present in highest concentration other ions and solvent molecules are not shown. 

Sulfur is weakly soluble in H2O (10 M at 298 K) [33, 34], but Na2S is very soluble [35]. In deaerated aqueous solutions, the alkali-metal polysulfide system contains, in addition to H2O and alkali-metal cations, OH , H+, H2S, HS , S-, 82 , 83 , 84 , and 85 . It is usually considered that 85 is the least reduced polysulfide in water. However, it has been reported, in several papers [26, 36], that in basic aqueous solutions, at high temperatures, a blue color is observed, suggesting the stability of 83 . The polysulfide equilibrium constants, interrelating polysulfide speciation, A a, K i, and Kq, have been well established [36, 37]. The species in solution are related by the equilibria ... 

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