Thiocyanates unidentate ligands

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. 

For many reactions it has been found that there is a relationship between AH and AS the more negative the value of AH, the less positive the value of AS (Choppin and Strazik, 1965). For 1 1 complexes of unidentate ligands it has been further suggested that for outer sphere complex formation AH should favor complexation and AS should oppose it. This circumstance would result from the essential retention of the primary hydration sphere of the rare earth ion. This is the behavior found for dilute solutions containing the nitrate or thiocyanate ion and should correspond to the formation of outer sphere complexes for these ligands. If the converse relationship exists for AH and AS, such as with the fluoride ion or carboxylate ions, then it is assumed that the primary hydration sphere has been ruptured and the resulting complex is of the inner-sphere type (Choppin, 1971). 

Among the triatomic ligands (incl. neutral molecules such as HjO, SO2, and CO2) the anions N02, N3 , NCO", NCS, NCSe , and NCTe are well-known as ambidentate ligands. These anions have two or three donor atoms and can coordinate with a metal atom at one or the other position. Furthermore, they can serve not only as unidentate ligands but also as bridging ligands, connecting two or more metal atoms in various ways. This chapter focusses on the thiocyanate anion the various coordination modes will be shown. 

The difference in the charge densities on the N and S atoms is not very large. This may be the reason why the NCS anion exhibits a variety of coordination modes as shown in Fig. 33. There are a large number of metal complexes containing an N-bonded or S-bonded thiocyanate ion as a unidentate ligand. Various factors influencing the relative stabilities of these two bonding modes will be discussed in Section 4.3. 

Cyanide and Thiocyanate Complexes Among other unidentate anionic ligands commonly encountered, cyanide forms stable complexes with both Cu(II) and Cu(I) however, the CuCN salt is so insoluble (p gp = 19.5)[169] that only the reduced complex has been characterized, that is, the addition of cyanide to an aqueous solution initially containing Cu(II) results in autoreduction to Cu(I). The overall equilibrium constant ( 34) for the reaction of Cu(I) with four cyanide ions was determined as early as 1904 by Kunschert [170] and subsequent measurements have yielded virtually identical values ... 

The mechanism for the displacement of oxalate from [Pt(ox)3] by thiocyanate is complicated. An intermediate anion [Pt(ox)2(SCN)2] , presumably containing unidentate oxalate ligands, has been isolated and characterized as the trihydrate of its potassium salt. Other transient intermediates containing unidentate oxalate (LL), (2)—(4), are proposed, and many rate constants relating to various steps in the postulated reaction sequence have been estimated. ... 

Ligand Replacement Unidentate by Unidentate.—dissociative mechanism is proposed for the replacement of nitrito- or of thiocyanato-ligands in trans-[Co(dmgH)2(N02)2] and in rm/w-[Co(dmgH)2(NCS)2] respectively by thiourea. Thiocyanate is replaced much more rapidly than nitrite from these cobalt(m) centres. Substitution at bisdimethylglyoximatocobalt(m) complexes can be catalysed by cobalt(n) complexes. This has been demonstrated by the elucidation of the rate law for the cobalt(n)-catalysed reaction of trans-[Co(dmgH)2(PPhs)(NOa)] with pyridine ... 

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