Effects of Non-leaving Ligands

The aquation of /ra/i -[Co(en)2(N02XMeCN)] + ion in DgO is accelerated by crown ethers such as 18-crown-6 and, to a lesser extent, 15-crown-5. A mechanism involving complexation of the methyl group protons of the co-ordinated 

Rate data for the spontaneous, ion-induced, and base hydrolyses of trans-[Co(en)aCl(RCH2C02)]+ ions (R=H, Cl, Br, I, CN, or NH2/NH3+) are collected in Table 5. Spontaneous aquation gives 80+3% m-product after a long period of time, and the more rapid Hg ion-induced aquation gives 88 4% cis-product. On the other hand, base hydrolysis via the wlcb mechanism gives 71 5 % rra/ij-product and 29 + 5 % cw-product.  

Rates of the acid, Hg ion-induced, and base hydrolyses of c/y-[Co(tmd)t-(RNH2)C1] + ions (R = H, Me, Et, Pr , Bu , Bu or Ph) are ca. 10 faster than for the analogous [Co(en)2(RNH2)Cl] + ions. This arises primarily from a reduction in the activation energy by ca. 2.5 kcal mol . A comparison of the rate data for acid, Hg -assisted, and base hydrolysis reactions is given in Table 6. cis-trans isomerization was not observed in the aquation reactions. 

Ford and K. B. Nolan, Inorg. Chim. Acta, 1979, 35, L377. 

Acid-catalysed hydrolysis of cw-[Co(en)2(L)(N02)] + ions (L=NHa, PhNHa, NCS, or HgO) and of fra/w-[Co(en)2(NCS)(N02)] ion has been investigated in perchloric acid over a wide acidity range. Protonation and loss of the co-ordinated nitro-group is involved. The effect of organic solvents on the reaction rates was also investigated. For the aquation of c -[Co(en)2(L)I] + ions (L=NHs, MeNHj, BuNHj, allylamine, PhNHa, or py) an nI reaction is proposed for all cases except L=py for which an Sn2 mechanism is proposed. Perhaps covalent hydrate formation is involved with the pyridine complex (for a discussion of this topic, see the Introduction). 

Kinetic results for aquation of cis-[Co(en)2LBr] + cations, where L = 2-aminoethanol, methylamine, butylamine, or allylamine, parallel those obtained earlier for aquation of the analogous chloro-complexes. It has been 

The kinetics of ring closure of the complex cw-[Co(en)2(OH2)-(NHaCHaCHaOH)] have been studied, and are consistent with the non-observance of this species in the aquation of cw-[Co(en)2(NH2CHaCH20H)X] + cations. An associative mechanism for the ring closure of the aquo-ethanol-amine complex is proposed. The kinetics and mechanisms of analogous ring-closure reactions of similar platinum(n) complexes are discussed in the appropriate section of Chapter 2 of this Part. Kinetic parameters for aquation of the isomeric cations cw-[Co(en)2LBr]2+ with L = 3- or 4-methylpyridine show the expected similarity.  

Rates of aquation of the related carboxylato-complexes trans-[Co(en)2(02CR)Cl]+, where R = Me or Ph, are independent of pH, and the aquation yields predominantly the cw-isomers of the products. Comparison of the stereochemical courses of aquation of these complexes with those for aquation of rra s-[Co(en)2(OAc)2]+ and of trans- Co exI)2C suggest that carboxylato-ligands encourage stereochemical change in aquation of cobalt(m) complexes.  

The ligand trien bears a strong resemblance to (en)2, so aquation of the a-and /9i-isomers of the [Co(trien)(OH2)Cl] + cation is closely related to the reactions cited in the previous paragraphs. After making allowances and corrections for subsequent racemization and isomerization, it has been deduced that aquation of these two isomers proceeds with complete retention of geometric and absolute configuration. For reaction (2) the rate constant, determined by four different methods, is between 2.15 and 2.40 x 10 s , at 25 °C, pH = 2, and ionic strength 0.013—0.020 mol From the results 

The separation of two geometric isomers of [Co(tren)(NH3)Cl] + has been reported. Preliminary kinetic results are that the red or 3-form, with the chloride trans to the tertiary nitrogen of the tren, aquates with a rate constant of 2.5 X 10 s (O.IM-HCIO4, 26 °C) and that the purple or a-form, with the 

Following earlier kinetic studies of the isomerization and acid and base-catalysed hydrolysis of the [Co(RR- and 55 -Hdtu)NCS] + ions an analogous study has now appeared for the [Co(/ / -Hdtu)N3] + ion (Hdtu = tetren = 1,11-diamino-3,6,9-triazaundecane). The reaction is complicated by protonation of the azido-ligand and by conjugate base formation with loss of a proton from the amine ligand  

As in previous volumes, the remainder of this section will be subdivided into parts dealing first with the [Co(en)aLX] + ions, then the [CoL4XJ + complexes, and finally dioximato-complexes. 

The complex ions cA-[Co(en)2LCl] + with L = aniline or m-toluidine have both been shown readily to lose a proton to form their respective conjugate bases (L = PhNH or /M-MeCsHjNH-), and rate data for chloride-ion aquation are reported for all four species (Table 3). Above pH 3 significant concentrations of these coryugate bases are present since the equilibrium formation constants are ca. 10 . The enhanced reactivity of the conjugate bases is in line with the now well established S Nlcb mechanism for base hydrolysis. 

A study of the aquation of the cw-[Co(en)2LBr] + ion (L = py or PhNH2 is reported to involve the slow loss of L following the more rapid aquation of the bromide ion.  

Rate data associated with the two rate constants and are collected in Table 4. 

Qualitative deductions of trans effects of various ligands have been made from observations on reactions of the complexes rra775-[PtClL(NH3)2] with a range of nucleophiles, and of the complexes ci5 -[PtMe2L2] with phosphines. Further information on the trans effect of ligands L comes from preparative observations on 

Effects of non-leaving ligands, and of chelating ligands in particular, on reactivities have been reviewed. The cis and trans influences, i.e. initial-state effects as far as kinetic phenomena are concerned, of some groups in platinum(ii) complexes have been assessed by CNDO-MO calculations. The cis influences were found to be comparable in magnitude to trans influences, but the ligand orders in the two series of influences were not identieal.  

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On Main Group Metal Ions by Al3+ Effect of Non-leaving Ligands on the Rate Constants and Activation Parameters for Solvent Exchange on Al3+... 

Effect of Non-leaving Ligands on the Solvent Exchange Rate Constants on Second and Third Row Divalent Transition-metal Ions... 

Effects of Non-leaving Ligands.—[Co(en)aXY] + Complexes. A number of different groups have reported rate data for the aquation of cobalt(iii) complexes of the type cis- and /ra/7 -[Co(en)aXY] + (X and Y are unidentate ligands with X the leaving group). The results are collected in Table 5. When X = Cl, the ratio of the rate... 

Studies of the loss of unidentate ligands from complexes which also contain multi-dentate ligands e.g. en, ox , tren, or edta) will be found in a later section dealing with the effects of non-leaving ligands. 

Effects of Non-leaving Ligands.—[Co(en)2XY]"+ Complexes. A brief review in this area has appeared. ... 

Effects of Non-leaving Ligands.— The usual way of assessing the effect of a unidentate non-leaving ligand L on reactivity of cobalt(m>-amine-halide complexes is to investigate aquation kinetics of compounds [Co(en)3-LC1] +. Recently described examples include complexes of cij-geometry,... 

One other type of complex wherein the effects of non-leaving ligands have been studied is that of the dioximato (6)-cobalt(in) series [Co(LLH)2-(S03H)C1] (Table 4), The complex with LLH2 = methylglyoxime (rngHj)... 

The relative reactivity of pairs of chloride and bromide complexes is often reported and discussed. Recent studies in this category which refer to such pairs of cobalt(in)-amine-halide complexes include those of aquation of cw-[Co(en)g(cyclohexylamine)X] +, of tra 5-[Co(cyclam)(N02)X]+, and of c/j-[Co(phen)2(N02)X]+, in all cases with X = Cl or Br. Similar reactivity comparisons have been made for cobalt(in)-oxime-halide complexes, including the series /ra 5-[Co(dmgH)2LX], with L = semicarbazide or thiosemi-carbazide, and traw-[Co(dmgH)2(tu)X] (in 20% ethanol). In these latter cases X = Cl, Br, or I. Several of these systems will be mentioned again later, in the section on effects of non-leaving ligands. 

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