Thiocyanate, formation constants with

Alkali metal picrates have been used to measure formation constants for crown ethers in solution, but the selectivity of benzo crown ethers for metal picrates, relative to the analogous chlorides, nitrates, perchlorates, and thiocyanates, may vary significantly. Apparently, it—it interactions between the picrate ions and the aromatic ring(s) on the crown are responsible for the difference. The importance of the picrate effect rises as the number of benzo groups in the crown ether is increased, and it varies with their location in the macrocycle. The dependence of the picrate NMR chemical shift on the metal cation and/or macrocycle identity has been used to study picrate-crown ether 7r-stacking in large crown ether (18, 21, and 24-membered) complexes. 

L1S/ROS] and [66KEN/L1S] for the NiCF complexes, but is in reasonable agreement with other reports [67NAN/TOR], [74KUL3] in the case of NiSCN. Based on the entropies of association, the formation of mostly outer-sphere complexes (ion pairs) was suggested in the case of nickel(ll), except for thiocyanate. From the temperature dependence of the formation constant for the NiSCN species A,77 = -(8.311.0) kJ mor can be derived. This value is accepted with an increased imcertainty (A,/7 = - (8.3 2.0) kJ moT ). 

Complex formation of Ni(II), Pr(III) and Ho(III) with thiocyanate was studied in aqueous solutions (/ = 3 M (LiC104)) by IR spectroscopy. The concentration of thiocyanate was 0.36 M, the concentration of metal ions ranged from 0.36 to 0.86 M. The formation constants of the mono-complexes (MSCN" ) were calculated by three different methods, which resulted in considerably different values. The temperature of the measurements is not given in the paper. Therefore, the reported data were not considered further in this review. 

An interesting application of the electrochemical oxidation of thiocyanate ion is the preparation of alkyl and aryl thiocyanates via anodically generated thiocyanogen. Alcohols have been converted to the corresponding thiocyanates by constant current electrolysis of NaSCN in CH2CI2 containing triphenylphosphite and 2,6-lutidinium perchlorate. The yields were fair to good for the primary and secondary alcohols, but no thiocyanate formation was observed with tertiary ones. Similarly, various aromatic amines and phenols were thiocyanated in a two-step procedure, namely electrochemical preparation of (SCN)2 and subsequent reaction with the substrates k... 

Formation constants of 3d metal ions with A-m-tolyl-p-substituted benzohydroxamic acids and of rare earths with thenoylhydroxamic acid have been determined. Formation constants of proton and metal complexes of iV-phenyl-2-thenoyl- and A-p-tolyl-2-thenoyl-hydroxamic acids have also been determined. In addition, study has been made of the mixed ligand complexes involving nicotine- and isonicotino-hydroxamic acids. A method of extraction and spectrophotometric determination of vanadium with chlorophenylmethylbenzohydroxamic acid has also been published. It may be mentioned that hydroxamic acids (in particular, the A-phenylbenzohydroxamic acid) have been widely used as analytical reagents for metal ions. Solvent extraction of titanium by benzo- or salicyl-hydroxamic acid in the presence of trioctylamine in the form of coloured complexes has been reported. A-w-Tolyl-p-methoxybenzohydroxamic acid has been used for extraction and spectrophotometric determination of Mo and W from hydrochloric acid media containing thiocyanate. 

Niobium(v) forms a 1 2 complex with salicylhydroxamic acid in aqueous HCl which has a formation constant of ca. 2.5 x 10" P mol-. " The extraction of niobiumfv) with N-phenylacetylhydroxamic acid (A) in benzene and CHCI3 has been studied in the presence of pyrocatechol (B) or thiocyanate and a Nb A B complex of stoicheiometry 1 2 2 and a Nb I NCS one of stoicheio-metry 1 2 3 have been identified. " ... 

Two studies have been made of the chromium(vi) oxidation of thiocyanate, - in which the reported dependence of the ligand concentration is substantially different. In keeping with the earlier observation that a complex between Cr and SCN is sufficiently stable with respect to redox at low [H+] to allow temperature-jump studies to be made (and hence to yield a value for the formation constant), Muirhead and Haight found an immediate increase in absorbance on mixing the reactants in the stopped-flow apparatus. The spectrum of the intermediate was derived using the continuous-flow method. Assuming the oxidation reaction to proceed via this complex, the rate law may be written in the form... 

Studies of the complex-ion chemistry of Es have been made in conjunction with measurements of the stability constants of other trivalent actinides. A summary of the known stability constants for einsteinium complexes is shown in Table 12.5. With the possible exception of the two lower thiocyanate complexes, chloride is the only outer-sphere complex in which water of hydration lies between the ligand and einsteinium ioa The remaining complexes are believed to be inner-sphere as inferred from the increase of a given stability constant with increase in atomic number and from the enthalpy and entropy of formation of the complex. 

The dependence of rate constants for approach to equilibrium for reaction of the mixed oxide-sulfide complex [Mo3((i3-S)((i-0)3(H20)9] 1+ with thiocyanate has been analyzed into formation and aquation contributions. These reactions involve positions trans to p-oxo groups, mechanisms are dissociative (391). Kinetic and thermodynamic studies on reaction of [Mo3MS4(H20)io]4+ (M = Ni, Pd) with CO have yielded rate constants for reaction with CO. These were put into context with substitution by halide and thiocyanate for the nickel-containing cluster (392). A review of the chemistry of [Mo3S4(H20)9]4+ and related clusters contains some information on substitution in mixed metal derivatives [Mo3MS4(H20)re]4+ (M = Cr, Fe, Ni, Cu, Pd) (393). There are a few asides of mechanistic relevance in a review of synthetic Mo-Fe-S clusters and their relevance to nitrogenase (394). 

An important property of the S-nitroso thiourea derivatives is the ability to effect electrophilic nitrosation of any of the conventional nucleophilic centres. This is manifest kinet-ically by the catalysis of nitrous acid nitrosation effected by added thiourea (equation 29). The situation is completely analogous to the catalysis of the same reactions by added halide ion or thiocyanate ion. The catalytic efficiency of thiourea depends on both the equilibrium constant Xxno for the formation of the intermediate and also its rate constant k with typically a secondary amine65. Since Xxno is known (5000 dm6 mol-2), it is easy to obtain... 

When thiocyanate ions are added to nitrous acid in water, a pink colouration develops which is believed to be due to the formation of nitrosyl thiocyanate (equation 34), which is too unstable to be isolated but which can be used as a nitrosating agent in aqueous solution. Because the equilibrium constant for ONSCN formation81 is quite large (30 dm6mol 2) at 25 °C, thiocyanate ion is an excellent catalyst for aqueous electrophilic nitrosation. The well established82 series is Cl- < Br < SCN < (NH2)2CS. Thiocyanate ion is also a sufficiently powerful nucleophile to react in acid solution with nitrosamines in a denitrosation process (equation 35), which can only be driven to the right if the nitrosyl thiocyanate is removed by, e.g., reaction with a nitrite trap such as hydrazoic acid. 

Re—O—Re linkage.The complexing of rhenium(iv) with thiourea (L) has been studied in HCl solutions in the presence of tin(ii) by a cation-exchange method, and equilibrium stability constants Pj—P4 for [ReOL4] -t- nCl" [ReOCl L4 -I- nh determined.The complexing of rhenium(iv) with thiocyanate ions in HCl solutions in the presence of SnClj has also been investigated and the formation of a 1 1 Re -SCN" complex confirmed. ° ... 

Thiocyanate. — On the basis of /-orbital hybridization Diamond [351] predicted the formation of stronger actinide complexes with thiocyanate ion than for the rare earths. Subls and Chopfin [352] have studied the ion exchange behaviour of many actinide and rare earth thiocyanate complexes and have shown that europium is eluted much sooner than americium from Dowex-1 with ammonium thiocyanate. The stability constants for the formation of MSCN2+ and M(SCN)2 complexes for Nd3+, Eus+, Pu3+, Am3+, Cm3+, and Cf34 have been measured [353] and are tabulated in Table 25. It is apparent from the table that the formation... 

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