Ligand, unidentate, multidentate

Ligand Replacemoit Multidentate by Unidentate.—Cyanide ion is often used to displace multidentate ligands from nickel(n), and recently kinetic studies of such reactions have appeared ... 

Ligand Replacement Multidentate by Unidentate.— The cyanide-ion-induced dissociations of [Fe(LL)a] + and [FeCterpy) " ions [LL = phen, bipy, or iV-(2-pyridylmethylene)aniline] were described in Section 4.21. 216 Cyanide ion has a marked stabilizing effect upon the rate of dissociation of [Ni(gga)X] and [Ni(ggg)] ions (gga = NH2CH2CONCH2CONH, X = CN or HjO ggg = triglycinate ion with one proton ionized from each peptide nitrogen atom). The observed rate law is ... 

Ligand Replacement Multidentate by Unidentate and Unidentate by Multidentate.—... 

Ligand Replacement Multidentate by Unidentate.— The kinetic study of the reaction of [Co(mnt)2(LL)]" with tri-n-butylphosphine, in which the... 

Ligand Replacement Multidentate by Unidentate.—Kinetic results on the reaction of the cydta complex of manganese(iii), which in alkaline solution is [Mn(cydta)(OH)] , with cyanide ion indicate the following reaction sequence ... 

Placing a proton on the X group presumably weakens the Cr-X bond. The enhanced lability is due to a reduced enthalpy of activation, compared with that associated with the step. Normally, the removal of unidentate ammonia or amine ligands from metal complexes is not accelerated by acid, since the nitrogen is coordinately saturated. This situation changes when we consider multidentates (Sec. 4.4.2). 

It has been tacitally assumed in this discussion that the second-order formation rate constants measure the simple water substitution process. Although this must apply when unidentate ligands replace coordinated water, a composite process could describe the replacement by multidentate ligands. However, consideration of rate constants for successive formation and dissociation processes suggests that the overall rate of complex formation with flexible bidentate (and probably multidentate) ligands such as diamines, dipyridyl, glycine is probably determined by the rate of expulsion of the first water molecule from the metal aqua ion (56, 80, cf. 3 and 84). 

Describe three general methods for performing EDTA titrations. What are the advantages of each 17-3. Why are multidentate ligands preferable to unidentate ligands for complexometric titrations 17-4. Write chemical equations and equilibrium-constant expressions for the stepwise formation of (a) Ni(CN)5-. 

Multidentate ligands usually form more stable complexes than unidentate ligands. They often form only a single complex with the cation, simplifying their titration curves and making end-point detection easier. 

N-3 Acyclic multidentate amine ligands yield polymeric solid complexes with Ag(I). Thus, diethylenetetramine and tris(2-aminoethyl)amine give 128 and 129, respectively. In the presence of unidentate ligands such as PPhs, PMe3 and /-BuNC polymer formation may be hampered, as in 130215. 

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