GUEST AUTHOR I Textbook Errors, 72 G. R. H. Jones

Loughhorough University of Technolosv Substitution Reactions

~n~lond ( Octahedral Complexes

A lecture series devoted to a discussion of mechanisms of reactions of inorganic complex com- pounds, particularly of substitution and electron trans- fer reactions, is now an established part of many under- graduate courses in chemistry.' One fascination of this area of study is the uncertainty concerning the mech- anisms of some well studied reactions. For example, mechanisms of the familiar acid-hydrolysis reaction in aqueous solution,

lCo(NH3)&l'~-~+ + N20 = [CO(NH$)~ HIO]J+ + X m - .

(where Xm- is a monodentate anionic ligand), still raise considerable discussion (1). Thus, when X is S80a2-, Banerjea and Das Gupta (2) favor an SN2 mechanism, and when X is F-, C1-, Br-, and NO3-, Chan (3) suggests an SN2 mechanism, involving sol- venbassisted removal of the departing anion. (In the case of F-, the SN2 reaction is thought to occur simultaneously with an acid-catalyzed SE2 reaction.) On the other hand, when X is NOs-, Pearson and Moore (4) suggest a solvent-assisted dissociation mechanism (6), while Langford (I), when X is F-, Cl-, Br-, NOs- and H2P04-, considers the possibility of the formation of a five coordinate intermediate which reacts very rapidly with a nucleophile (i.e., a water molecule) already present in the second coordination sphere. Although a single mechanism may not be adequate to cover reactions for a wide range of anions X, mecha- nisms are still controversial in specific cases.

In view of the contentious nature of many such mechanisms, categorical statements in this field are dangerous, especially if they involve broad generaliza- tions. One such generalization which appears, either directly or by implication, in a number of texts, involves a rejection of the possibility of direct substitution, in aqueous solution, of a ligand in an octahedral complex by a nucleophile other than water or OH-. A state- ment containing this generalization may be quoted, "In water, aquation invariably precedes substitution by anions."

There is substantial evidence that, in a number of

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cases, aquation (acid-hydrolysis) does precede substitu- tion by anions (4, 6, 7). For example, the reaction (7)

[Co en2 (NO?) Cl] + + NCS- e [Co en. (NOy) (NCS)] + + C1-

appears to proceed via the steps

[Co enn (NO*) CI] + + HtO s [Co enl (NO2) (H10)l2+ + C1-

[Ca em (NOr) (HzO)IP+ + NCS- e [Co en, (NO1) (NCS)I+ + Ha0

for both cis and trans isomers. The change in optical density of the reaction mixture, a t 470 mfi, can be accounted for in terms of the stages shown. Further, with azide or nitrite ion in place of thiocyanate ion, the rate of C1- release is independent of aside or nitrite concentration, providing additional evidence for a two stage process.

In certain non-aqueous solvents, the possibility of direct anion substitution appears to be accepted (8), without the initial intervention of the solvent. This may be due to the solvent (a) being a weaker nucleo- phile than water, (b) being a less effective solvating medium for anions (Q), thereby increasing their reac- tivity, (c) favoring ion-association (9) (this being related to (b) ), in which case the anion could occupy a position in the second co-ordination sheath of the complexed metal ion ( I ) , poised for immediate attack in direct competition with the solvent, (d) being incapable of hydrogen-bonding, which, in the case of water, can hold solvent molecules in position for immediate attack on the complex (10).

In aqueous solution, where the water concentration is about 55 M, the situation for direct substitution may be less favorable. In spite of this, evidence of two types is available (11-16, 2), indicating that direct substitu- tion does occur.

(I) Evidence has been obtained for a five coordinat,e intermediate which is capable of discriminating between potential nucleophiles. In this case, the substitution reaction involves a dissociation mechanism.

(11) The nature and concentration of the substitut- ing nucleophile influences the rate of substitution in cer- tain cases, and an acceptable nucleophilic order has been compiled for a range of substituting anions and mole- cules. This suggests that a displacement mechanism is operating in these cases.

Considering (I) above, Ardon (11, I,!?) has shown that thc reaction

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where X- is CI- or Br-, can occur by direct anion substitution. Evidence of mass-law retardation (1 7) by I- indicates the formation of a five coordinate inter- mediate Cr(H20)sa+. Further, the quantities of the product Cr(HZO)SX2+ found, could not come from the reaction

[Cr (HxO)J3+ + X-- [Cr (HsO), XIS+ + HzO

due to the unfavorable equilibrium position of this reac- tion. The five coordinate intermediate must therefore be capable of reacting directly with the anion X-, as well as with the solvent.

Haim and Wilmarth (13) determined the rate of acid- catalyzed aquation of the complex ion [Co(CN),N313-. On addition of thiocyanate ion, (a) the rate of disap- pearance of [CO(CN)~N~]~- remained unaltered and (b) the product contained [CO(CN)~(SCN)]~-, as well as [CO(CN)~(H~O)]~-, even in the initial stages of reaction before anation of the aquo-complex became significant. The proportion of [CO(CN)~(SCN)]~- in the product increased with the thiocyanate ion concentration. These results are explained in terms of a mechanism in which water and thiocyanate ions compete for an inter- mediate, presumably five coordinate, whose rate of for- mation is independent of the presence of anions.

Referring back to (11) above, Banerjea and Gupta (2) studied the reaction

[CO (NHa)c (S*Oa)I+ + Rn-+ [Co (NH8)6 R](Sa'+ + SIOsP-

in aqueous solution, with R being OH-, Cl-, NH3, HzO, and conclude that the reaction rate can be expressed by a general equation,

rate = k' [Complex] + k" [Complex] [RI (1)

Here [Complex] represents the concentration of the thiosulphate complex. This expression is interpreted as follows. k' is the pseudo f is t order rate constant for aquation, and k" the second order rate constant for direct anion or ammonia substitution. When water is the only nucleophile present, the term involving k" disappears, as does that involving k' when OH- is the nucleophile. The first order dependence of the term involving k", on the concentration of the nucleophile, coupled with the fact that the values of k" increase with the nucleophilic character of R, in the order

H20 < NHs < CI- < OH-

is assumed to indicate an SN2 mechanism for substitu- tion. Although this interpretation may be questioned on the grounds that (a) OH- probably reacts by an SN1 CB mechanism (IS), and (b) Pearson and Moore (4) have shown that substitution of nitrate by thio- cyanate ion, in the complex [CO(NH~)~NO~]~+, pro- ceeds almost entirely via the aquo complex, in aqueous solution, the possibility of direct substitution by an SN2 mechanism still remains.

Margerum and Morgenthaler (15) studied the rate of dissociation of Fe(phen),=+ in aqueous solution, in the presence of OH-, CN- and N3-. These anions in- creased the rate of disappearance of F e ( ~ h e n ) ~ ~ + , giving a term in the rate expression dependent on the first power of the anion concentration, e.g., k = kl + kZ [CN-1. This expression can be explained in terms of the reactions

phen

where kz = (KG,-) (kc,-). For a given anion concen- tration, the observed rate constant k increases with change of anionin the order N3- < OH- < CN-. Here, direct anion substitution appears to have occurred.

The preceding examples all involve transition metal complexes. That direct substitution can also occur in an octahedral complex of a nontransition element in aqueous solution, by nucleophiles other than water, is shown by the following example.

Pearson, et al. (16) studied the rate of the substitution reaction

and observed a marked dependence of rate of disap- pearance of [Si (acac)J+ on the nature of any nucleo- philes present. A range of anionic and molecular nucleophiles were studied, and the order of nucleophilic activity was explained in terms of changes in basicity, polarinability and alpha effect (19). This indicates an initial bimolecular nucleophilic attack at the octahedral Sir' atom, resulting in the displacement of one end of an acetylacetonate chelating ion. Rapid loss of the displacing nucleophile and the acetylacetonate groups follows, yielding Si(OH),. Again, a direct substitution by the nucleophiles studied, e.g., OH-, H02-, RTH2NH1, HPOe2-, F-, NO2-, S20?, is proposed.

This brief discussion is not meant to be exhaustive. It is simply intended to help to prevent the dissemina- tion of the idea that, for octahedral complexes in aqueous solution, all nucleophilic substitution reactions might go via an aquo complex.

Literature Cited

(1) LANGFORD, C. H., Inorg. Chem., 4,265 (1965). (2) BANERJEA, D. AND DAS GUPTI, T. P., J . Inorg. Nucl.

Chem., 27,2617 (1965). (3) CHAN, S. C., J. Chem. Soc., 2375 (1964). (4) PEARSON, R. G., AND MOORE, J. H., I ~ O T R . Chem., 3, 1334

(1964). (5) JONES, T. P., HARRIS, W. E., G i D \V.~LLACE, W. J., Can. J .

Chem.. 39. 2371 (1961). (6) ETTLE, 6. W. AND JOHNSON, C. H., J. Chem. Soc., 1490

(19.19). , - - ~ ~ ,~ (7) BASOLO, F., STONE, B. D., BERGXINN, J. G., AND PEARGON,

R. G., J. Am. Chem. Soe., 76, 3079 (1954). (8) BASOLO, F., AND PEARSON, R. G., Advances in Inorg. and

Radiochem., 3, 43 (1961). (9) PARKER, A. J., Quad. Rev., 16, 163 (1962).

(10) ADAMSON. A. W.. AND BMOLO. F.. Acta. C h m . Seand.. 9. . . . . . . 1261 (1955). '

(11) ARDON, M., Pwc. Chem. Soc., 333 (1964). (12) ARDON, M., Inorg. Chem., 4, 373 (1965). (13) HAM, A,, AND WILMARTH, W. K., Inwg. C h m . 1,583 (1962). (14) HAIM, A,, AND TAUBE, H., Inorg. Chem. 2,1199 (1963). (15) MARQERUM, D. W., AND MORGENTHALER, L. P., "Advances

in the Chemistly of the Co-ordination Compounds," The MacMillan Co., New York, 1961, p. 481.

(16) PEARSON, R. G., EDGINGTON, D. ?I., AND BASOLO, F., J. .4m. Chem. Soc. 84, 3233 (1962).

658 / Journal of Chemicol Education

(17) INGOLD, C. K., "Structure and Mechanism of Organic (18) BASOLO, F., SZL?V~Y O~PTOQT~SS 1% Chemistry, 2, 21 (1964). Chemistry," Cornell University Prem, Ithaca, N. Y., (19) EDWARDB, J. O., AND PEARSON, R. G., J. Am. Chem. Soc., 84, 1953, p. 362. 16 (1962).

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