Indian Journal of ChemistryVol. 27 A, December 1988, pp. 1089-1091

Linear Free Energy Correlation inSubstitution Reactions of Octahedral

Cobalt (III) Complexes

ANADI C DASH* & AJAYA K PATNAIKt

Department of Chemistry, Utkal University,Bhubaneswar 751 004

Received 14 September 1987; revised and accepted15 January 1988

Linear free energy correlation between log kaq and log Keq forthe aquation of the carboxylatopentaamminecobalt(III) com-plexes,

k"

(NH3)5COOzCRz + '" (NHJ)5COOH32++ RCO; (Keq = kanl kaq)-;is satisfied for a variety of carboxylate ligands including the ami-no acidate betaines, -OzC(CHzlxNH;' The rate-equilibriumdata for a wide variety of ligands fit the linear plot withslope = 1.0, indicating the validity of the dissociative interchangemechanism (Id). Extension of linear free energy correlation tothe aquation of the binuclear complexes of oxalatopenta-amminecobaltffll ), (NH3)5COC204Mln + Ii+ (Mn + =Mn2+,Co2+, Nj2+, Cu2+, Zn2+, Al3+, Ga3+, In3+ and Fe3+), however,indicates that possibility of acyl cleavage mechanism for the me-tal ion cataIysed path cannot be completely ruled out.

While attempting to elucidate the mechanism ofsubstitution reactions of cobalt(III) complexes ofpentaammine class, Langford 1 noted that the plot oflog kaqversus log Keq (see Eq. 1; Ke~ = kanl kaq)waslinear with unit slope for a limited number of leavinggroups, X-;

k"

(NH3)sCoOH32+ + X"" ;=l (NH3)sCoX(3-n)+ ... (1)

k.q

The linear free energy correlation between L\ G *and 1\ G 0 (overall reaction) with unit slope was takento indicate that transition state of the reaction wasvery similar to the products" The activation volumemeasurements for the aquation of(NH3)sCoX(3-n)+(X=CI-, Br- ,NO;, SO~-, DMSO and H20) led tothe computation of the molar volume of theD-transition state.' which turned out to be independ-ent of the leaving group, X" -. From such studiesPalmer and Kelm" concluded that the transitionstate was very much like a five-coordinate interme-diate thus reinforcing the dissociative interchangemechanism (Id).

"Present address: Lecturer in Chemistry, DAY. College, Kora-put 764 020, Koraput, Orissa.

The overall reaction represented by Eq. (1), in-fact, proceeds via two stages: (i) diffusion-controlled ion-pair formation (outer sphere com-plex) (Eq. 2); and (ii) slow outer sphere-innersphere interconversion reaction (Eq. 3)

Q"

(NH3)sCoOH3; + x" ;=l (NH3)sCoOH3

2+, X2-

(2)

k"(NH

3)sCoOH3; + X'"' ;=l (NH3)sCoX(3-n)+ ... (3)

Setting

Keq= (Qoke/ kaq)we obtain

... (4)

log kaq= log (Qokexl-Iog Keq ... (5)

( where Qo is the equilibrium constant of ion-pair for-mation and «: is the outer sphere-inner sphere in-terchange rate constant of ion-pair. It is interesting

j to note that spontaneous aquation of carboxyla-toamminecobalt(III) complexes fit the linear freeenergy correlation satisfactorily">. This is an addi-tional evidence in support of the operation of disso-ciative interchange mechanism (Id) in the aquationof the carboxylatopentaamminecobalt(III) com-plexes. In other words the validity of LFER sug-gests that Co - ° bond heterolysis occurs in thespontaneous aquation of (NH3)sCo02CRn+. This isin support of the oxygen exchange experiments re-ported on the spontaneous aquation of'(NH3)5C002CCH~+, and (NH3)SC002CC02H2+(ref. 6,7). Contrastingly LFER for the aquation of

.(OH2)sCrX(3-n)+ (X=CI-, Br-, 1-, NCS-, SCN-)yielded 0.56 as the slope of the analogous plot". Themechanism for these cases has been assigned to ber;

LFERfor metal ion catalysed aquation ofoxalatopentaamminecobalt'Llh

The metal ion catalysed aquation of oxalato-pentaamminecobalt(III) and cis-oxalato(ammine)-bis(ethylenediamine)cobalt(III) have been investi-gated by us earlier"!", Recently Monk et al J J havereinvestigated the kinetics of metal ion catalysedaquation of oxalatopentaamminecobalt(III). Agree-ment between our results and those of Monk et al"is satisfactory.

Herein an attempt has been made to elucidate theintimate mechanism of aquation of binuclear spe-

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INDIAN J. CHEM., VOL. 27 A, DECEMBER 1988

cies, (NH3)sCOC204M(n+ 1)+through relevant linearfree energy relationship. The rate-equilibrium rela-tionship for the reactions stated in Eq. (6) may begiven as in Eq. (7)

... (6)

MOX I K'!. k an + og eq ... (7)

; uc overall equilibrium constant,«; = [ROH~+ j[C204M(n-2)+ ]I[RC204M(n+ 1)+1

... (8)

(R = (NH3)sC03 +) may be redefined as

Keq= KMox/(KMQ) ... (9)where KMOX,KM and Q are forward equilibriumconstants for the reactions (10-12), respectively

KMOX

C 02- + Mn+ ~ CO M(n-2)+2 4 2 4 ... (10)

K ••RC20; + Mn+ ~ RC204

M(n+l)+ ... (11)

Qox

ROH3; + C202; ~ RC20: ... (12)

Combining Eqs (7) and (9) we obtain the linear freeenergy relationship, given by Eq. (13)

log k~ = log (kM~xI Qox) + log (KMOXI KM)... (13)

The values of k Mat 25°C for different metal ionsaq M Twere calculated by extrapolation (log kaq versus 1Iplot) of our earlier data", The stability constants ofmonoxalato complexes of metal ions (KMOX)at zeroionic strength at 25°C are available for Mn(II),Co(II), Ni(II), Cu(II), Zn(II) and Fe(ill)12,13.The va-lues of log K MOXat ] = 1 mol dm - 3 and 25°C areavailable for A13+, In3+; the same for Ga3+ is avail-able at l= 1.0 mol dm "? and 20°C (ref. 12). Thetemperature effect was disregarded and the logK values of these metal ions were corrected toMOX] = 0 using the relationship (14)

log KMOX(1= 0) = logKMox (I = 1 mol dm -3) + 1.8." (14)

as it is noted'v" that log KFeOX'at ]=0 is 1.8 unithigher than the same at ] = 1.0 mol dm - 3 (25°C).

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Table 1-Rate and Equilibrium Constant Data forAquation of Binuclear Complexes,

(NH3)sCOC204M(n+I)+Mn+ log (KM)' log (KMOX)b log(k~)d log (KMoxIKM)

(dm! mol" ') (dm? mol-I) (S-I)

1.2 3.97 -7.141.7 4.79 -6.722.1 5.16 -6~82.7 6.2 - 5.281.8 4.89 - 6.893.2 9.4 - 4.621.7 7.9 - 5.102.7 8.25c - 5.032.4 7.1 -5.75

Mn2+C02 +

Nj2+Cu2+

Zn2"+Fe3+AP+Ga3+In3+

2.773.093.063.503.096.26.25.554.7

(a) 1- 0.3 mol dm", 30°C ref(14)(b) 1= 0, 25°C, ref (12,13) and see discussion part(c) I~0,20°Cref(12)(d) 1= 0.3 mol dm " ', 25°Cref(9)

-5,..."i'1I'l

U:I: 0-6..-01o- -7

2.f..2+ CoNI .•

.. Zn2+Iotn~+

2+0Cu

-8,L-~~2--L-~3~L-~4~~5~L-~6~~7log(KMoxjKM)

Fig. I-log (ka~/s-l) versus log (KMoxl KM) plot for aquation of(NH3)5CoC204Mln + I)+ at 25°C.

The values of KMat ] = 0.3 mol dm - 3 (30°C) as re-ported earlier from equilibrium measurements'"were used as such. Relevant data are collected inTable 1.The plot of log ka~versus log (K MOXI K M) is

. " C 2+ d .shown in Fig. 1. Only the data pornt ror uaq eVI-ates significantly from the linear plot. The higherreactivity of the binuclear complex of Cu2+ relativeto that predicted by the lFER is a stereochemicalconsequence; CU;q+being strongly tetragonal, exhi-bits much higher reactivity than other octahedralbivalent aquo actions M(OH2)~+ of the first trans-ition series. The effect of the tetragonal distortion onlog (Kcuoxl Kcu) is presumably annuled to a greatextent while it is not so on log k;qu.

It is to be noted that the slope of lFER (Fig. 1) is0.7. This is indicative of the fact that the mechanismof aquation of binuclear species is predominantly Idinvolving a transition state in which C204M(n-2)+species, at the most, is weakly bound to ~ecobalt(III) centre (i.e. heterolysis of Co - °bond IS

(NH,~ce(NH.>sCO-o\ H4--",JoHrl==-<\("~ /C<,,'79

H.. ~~~~I \-J'-ai' /(n+l)+

• ••t~)+(NHa)Scoot+ C.04

SCHEME' I

rate-determining), The intercept of the plot yieldsk~oX = 2.1 X 10-6 dm! mol-I S-1 at 25QC [based onQox = 2138 dm'' mol-I, see Eq. (12)] which agreessatisfactorily with the anation rate constant for(NH3)sCoOH~+ with a wide variety of anating li-gands">, further supporting the Id mechanism ofaquation of the binuclear complexes. The alterna-tive path of reaction involving metal ion promotedhydration of the acyl carbonyl (see Scheme 1) of thecarboxyl group bridging cobaltflll) and M"" fol-lowed by C - ° bond cleavage (ester cleavage)though appears to be less important cannot be com-pletely ruled out as the slope of lFER is less than 1.The oxygen exchange. experiments have demon.-strated that the acyl cleavage mechanism operates i"the H+ -catalysed path of aquation "I

(NH3)SCOC204H2+ and (NH3)SC002CCfj+, thecarboxylatocobaltflll) complexes possessing acti-vated carboxyl groups 7• It is not unreasonable to ex-pect that the acyl cleavage (C - 0) mechanism mightbe operative not as a substitute for Id mechanism but

NOTES

as an additional one, in the aquation of binuclearcompexes of trivalent metal ions since the bridgingcarboxyl group is likely to be prone to hydration un-der the activating influence of the M3+ ions.

Further work is necessary to elucidate this aspectof the metal ion catalysed aquation of the oxalatocomplexes.

References1 Langford C H, Inorg Chern, 4 (1965) 265.2 Pross A in Ad" phys org Chern, edited by V Gold & D Be-

thell (Academic Press) Vol 14 (1977) 71; Hammond G S,J Arn chem Soc, 77 (1955) 334; Leffler J E & GrunwaldE, Rates and equilibria of organic reactions (Wiley, NewYork) 1963, pp 163.

3 Palmer D A& KeIrn H, Coord chem Rev, 36(1981) 89.4 Haim A, Inorg Chern, 9 (1970) 426.5 House D A, Coord chem Rev, 23 (1977) 223; Dash A C &

Nanda R K, Inorg Chern, 12 (1973) 2024; Dash A C &Nanda R K, J inorg nucl chem; 38 (1976) 119; Dash A C& Dash M S, J coord Chern, 6 (1976) 1; ibid, 10 (1980)79; Dash A C & Mohanty B, Indian J Chern, 17A (1979)296; Dash A C & Ray N,lndianJ Chern, 14A (1976) 78.

6 Bunton C A & Uewellyn D R, J chem Soc, (1953)1692.7 Andrade C, Jordan R B & Taube H, Inorg Chern, 9 (1970)

711.8 Swaddle T W, Coord chem Rev, 14 (1974) 217.9 DashAC&NandaRK,lnorgChern, 13(1974)655.

1<. lash A C & Nanda R K, J Indian chem Soc, 52 (1975) 289.r: Abdullah P B & Monk C B, J chem Soc Dalton Trans, (1983)

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(Plenum, New York) 1977, pp 94.13 Stability constants: Part I, Special Publication No 6, (The

Chemical Society, London) 1957.14 Dash AC & Nanda RK,J inorg nucl Chem; 36 (1974) 1595.

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