Replacement Unidentate by Multidentate

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

The kinetics of reaction of nickel(n) aminocarboxylates with cyanide, and of the reverse reaction, can all be accommodated by the following reaction 

This scheme, which applies to a variety of monoaminocarboxylate ligands L, including inter a//a, edta, edda, nta, or ida, is different from the reaction scheme for reaction of [NifOHa) ] or of [Ni(trien)] + with cyanide, where four cyanides per nickel are required in the transition state for the rate- 

The cations [Fe(LL)a] +, where LL = phen or substituted phen, or a Schiff base of type (28), react with cyanide to form initially compounds 

The ky term is assigned to rate-determining dissociation of the iron(u) complex the k term is assigned to bimolecular nucleophilic attack of the cyanide at the iron. For both the phenanthroline and the Schiff base complexes the rate of reaction with cyanide increases greatly as the mole fraction of alcohol increases in methanol- or ethanol-water mixed solvents. 

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  

Activation energies are 11.3 0.5 kcal mol for the kx reaction and 14.1 0.4 kcalmol for the reaction. Both the kinetic pattern and the activation energies are markedly different from similar previously studied nickel(ii) and cobalt(n) systems. The reaction of [Ni(edds)] with cyanide ion follows fourth-order kinetics, first-order in nickel complex and third-order in cyanide. This behaviour parallels that reported previously for reactions of other aminocarboxylatonickel(n) chelates with cyanide ion. In these reactions, and in the reactions of INi(ttha)] with cyanide, there is strong kinetic 

Ligands such as phen and bipy appear to withdraw enough electron density from the vicinity of the iron atom in compounds of the type [Fe(LL)a] + to permit bimolecular nucleophilic attack. Such attack by cyanide and by hydroxide at complexes where LL = bipy, phen, or one of their substituted derivatives is fairly well established by now. Similar nucleophilic attack by these two ions has now been demonstrated for the complex with LL = ppsa (37). Activation parameters were determined for cyanide attack at this complex they have also been obtained for the attack of cyanide at [Fe(bipy)3] +. Activation parameters for cyanide attack at these and at 

Other iron(n) complexes are collected together in Table 23. A little information is available on the kinetics of the reaction between [Fe(4,7-diPh-phen)3] + and cyanide in methanol solution.  

Contradictory reports have appeared on the stereochemical course of the reaction between the [Fe(phen)3] + cation and cyanide ion. On the one hand optical inversion is reported - the first report of inversion at octahedral iron(n) - while on the other hand a total loss of optical activity is claimed. The earliest reference to the kinetics of the reaction of [Fe(phen)s] + with Qranide reported the rate law to be 

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

Ligand Replacement Unidentate by Multidentate.—Some kinetic information is available on the reactions of [Ni(CN)4] with the multidentate amino-carboxylate ligands edds and ttha. In both cases the displacement of cyanide was of secondary interest to the reverse reactions, which have already been mentioned in the previous section on the replacement of multidentate by unidentate ligands. 

Ligand Replacement Unidentate by Multidentate.—The [Co(COs)3] ion exists in aqueous solution together with the [Co(C03)2(H20)(HC03)3 and [ 0(003)2-(H20)2] ions. Replacement of the unidentate and bidentate carbonato-ligands by chelating amines was considered previously. ... 

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). 

Ligand Replacement Unidentate by Multidentate.— Reactions of the cobaltinitrite... 

Ligand Replacement Unidentate by Multidentate.—1,2-Cyclohexanedione dioxime replaces the two chlorides of /rfl/w-[Co(en)2Cl2]+, in aqueous solution, by a complicated mechanism whose essential features are an 8- 2. reaction of the incoming ligand with the complex and a later intramolecular 5 n2 reaction in which chelate ring formation is synchronous with chloride expulsion. The evidence in favour of the bimolecularity of the first step is the activation entropy of — 11 cal deg mol the evidence in favour of the latter 5 2 step is that its rate is 100 times faster than normal... 

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