Thiocyanate complexes solvent effects

Thiocyanates are rather stable to air, oxidation, and dilute nitric acid. Of considerable practical importance are the reactions of thiocyanate with metal cations. Silver, mercury, lead, and cuprous thiocyanates precipitate. Many metals form complexes. The deep red complex of ferric iron with thiocyanate, [Fe(SCN)g] , is an effective iadicator for either ion. Various metal thiocyanate complexes with transition metals can be extracted iato organic solvents. 

Solvent effects on quantum yields have been studied to some extent. Yields for substitution fall precipitously and the nature of the reaction may change if the medium consists of a noncoordinating solvent. Thus tra s-[Cr(NCS)4(NH3)2] shows a 0 of about 0.3 in aqueous media (for thiocyanate aquation), but is photoinert in nitromethane. In a mixed solvent study, it was concluded that the photochemical behavior of this complex depended on the solvent composition of the solvation shell rather than on the stoichiometric composition.41 42... 

Taking into account the fact that the solvation of ambident anions in the activated complex may differ considerably from that of the free anion, another explanation for the solvent effect on orientation, based on the concept of hard and soft acids and bases (HSAB) [275] (see also Section 3.3.2), seems preferable [366]. In ambident anions, the less electronegative and more polarizable donor atom is usually the softer base, whereas the more electronegative atom is a hard Lewis base. Thus, in enolate ions, the oxygen atom is hard and the carbon atom is soft, in the thiocyanate ion the nitrogen atom is hard and the sulfur atom is soft, etc. The mode of reaction can be predicted from the hardness or softness of the electrophile. In protic solvents, the two nucleophilic sites in the ambident anion must interact with two electrophiles, the protic solvent and the substrate RX, of which the protic solvent is a hard and RX a soft acid. Therefore, in protic solvents it is to be expected that the softer of the two nucleophilic atoms (C versus O, N versus O, S versus N) should react with the softer acid RX. 

Gutmann et al. (355-357) have investigated the effect of solvents on the ease of formation of thiocyanate complexes. In the series [VO(NCS)J (n = 1-4) complexes with low values of n are formed more readily in solvents with low donor numbers and vice versa (355, 356). The NCS is a stronger donor toward V0(acac)2 than either the neutral molecules investigated or the halide ions (367). 

M — N bonds in solvents of low dielectric constant,as shown in Figure 9-19(a). There are also compounds with both M-SCN (thiocyanate) and M-NCS (isothio-cyanato) [isothiocyanatothiocyanato(l-diphenylphosphino-3-dimethylaininopropane) palladium(II) Figure 9-19(b)]. M-NCS combinations are linear and M-SCN combinations are bent at the S atom in all thiocyanate complexes. This bend means that the M-SCN isomer has a larger steric effect, particularly if it can rotate about the M — S bond. 

Complex Formation Labile Cations. Solvent effects on reactivity in the formation of complexes of metal(n) cations with unidentate ligands have been reviewed, with special reference to magnesium(n) and to the solvents methanol, acetonitrile, DMF, and DMSO. There has been controversy over the mechanism of reaction of thiocyanate with nickel(n) in DMSO, with supporters of the usual Eigen-Wilkins la mechanism and of a D mechanism. The most recent investigators of this reaction report rate constants and activation parameters and favour the la mechanism. There has been further discussion of the mechanism of the reaction between nickel(n) and bipy in DMSO an earlier suggestion that the rate-determining step is ring closure is not supported by recent observations. Rate constants for the reaction of acetate, of other carboxylates, and of pada with nickel(ii) in several non-aqueous solvents have been determined. 

The kinetics of complex formation between Ni(II) and ortho-phosphate, ribose monophosphate and cytidine monophosphate (CMP) in water have been reported.The results are consistent with an/ mechanism involving protonated (HL ) and unprotonated (L ) ligands, and rate constants for reaction with L and HL" are about 1.5 x 10 dm mol s and 2.3 x 10" dm mol" s respectively, for all three systems. The cytidine ring appears to exert no effect on the binding of Ni(II) to CMP. The rate parameters have also been reported for the reaction of nickel(II) with 4-phenylpyridine and isoquinoline in water-r-butanol mixtures. Rate parameters for the dissociation of the isoquinoline and thiocyanate complexes of Ni(II) in 1-propanol, and of the former in ethanol and in water, are accommodated within an Id mechanism, and Tanaka has commented further on the relationships between the activation enthalpies for the dissociation of the same two complexes and the Gutmann donor number of the solvent. 

The clue for these deviations possibly lies in the sample preparation. Whereas Kidd and Spinney prepared the complexes and dissolved them in dry MeCN, Tarasov and Buslaev used mixtures of NbClj/NbBrs and NbCl5/NH4SCN, and their MeCN may have contained water and amines which (i) give solvent effects (which may account for variations around 20-30 ppm), (ii) initiate hydrolysis, which may give anionic species, and (iii) may steer the coordination of the thiocyanate in such a manner that only complexes containing N-bonded SCN are formed. In dry MeCN, resonances for the neutral complexes [NbCl5 Br (MeCN)] are observed. ... 

However, the energy difference between N- and S-bonded thiocyanate is very small and is influenced by an interplay of several factors steric effects, solvent and the counter-ion in ionic complexes. To illustrate the last point, in complexes [Pd[Et2N(CH2)2NH(CH2)2NH2]NCS]+, the PFg salt is N-bonded, as it is in the unsolvated BPhg salt. However, though the acetone solvate of the tetraphenylborate is N-bonded, the methanol solvate is S-bonded [126],... 

Most palladium (II) selenocyanate complexes have a Pd—Se bond, but the compound [Pd(Et4dien)(SeCN)]BPh4, isolated at low temperatures, isomerizes in a number of different solvents via a dissociative process, whereas [Pd(dien)(SeCN)]BPh4 shows no signs of such isomerization. Further, if [Pd(Et4dien)NCSe]BPh4 is isolated it reisomerizes to the Se-bonded form at room temperature in the solid state 153, 156). This behavior parallels that of the thiocyanate group under similar circumstances and provides evidence for a steric effect modified by the nature of the anion. 

One early and successful route to better leaving groups involved the use of metal ions that are external to the complex cation under study to promote the loss of a halide or thiocyanate ion from the coordination sphere (158). This occurs by complexation of the added metal ion with the coordinated ligand, effectively generating a cationic or neutral leaving group that departs more rapidly (see Section III,B). Conducted in potentially coordinating solvents, this chemistry permits the syn-... 

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