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SHORT COMJ ~LJ XIC_=LTIOKS 325I J . HEYROVSK* -*ND D. IL Ko~I~, CoZlecLion Czech_ Chent. Co~nmzrsr., 7 (1935) 196.?- J _ J . LIPGGAXGE, C1rem. Rec. , zg (1941) I .3 D. D. DEFORD AKD D. N_ HUME, J _ Am . Chenz . SOL. 73 (1951) 5321~4 H. IRL?xG. AcEvawces izz Polaroguaphs, Vol. I. Pergamon Press. London. 1960. p 42.5 J O-CHUXXG Lru XKD HSIXO-YUAK Fu, Ko Hsueh T z cng Pno , 23 (1957) 71%6 S. S. MESARIC AXD D. N. HUME, IJ zovg. Che~x, 2 (1963) 1oG3_

Received J anuary 25th. 1965POLAROGRAPII IC STUDIES OF THE PYRIDISE COMPLESES OF ZISC _%XD CXD~IIUlrI IN AQUEOUS MEDIUM

Complexes of zinc and cadmium with p>sldine have been studied polarographically usingthe method of DEFORD AXD HUME. Two comples ions,[Zn(py)]T ar,d [Zn(py)?_=+. having over-allformation constants, fll = S-0 2 0.5 and #- = 34-o & 1.0, were detected in the zinc--p>-ridines>-stem. In the cast of the cadmium--p>-rldine system, three complex species, [Cd(py)j+, [Cd(py)z]z+and [Cd(py)s]z-. were found to be present, having constants /31 = 23 f 2.0, & = 72 & 5-o and83 = So 2 5.0. The percentage composition of the various complcsed and uncomplesed ions foboth systems are presented.

J_ Ek c l r oan a l . Chem. . g (1965) 321-325

The mechanism of charge transfer at electrodesFrom 1935 until the late sos, most treatmentsI- of the charge transfer

process have been analogous to those used in the quantum mechanics of chemicalreactions_ Stretching of ion-solvent and substrate-discharged ion bonds to giveentities of equal energy have been the essential processes govemmg the rate of chargetransfer. Such treatments allow the prediction of the effect of variablesl-7, e.g.,dielectric constant of the solvent, on the rate of charge transfer. They allow calcula-tion of the symmetry factor of electrode reactions and its variation with potentials-However, such an ion-transfer model leaves implicit the motion of the electron itself,and omits to consider the charge distribution in the activated state.

Since x958 several treatments 8-10 of charge transfer have been made in whichthe basic model is that of redox processes in solution. In these treatments attentionhas been focused on the electron, tending to rninimise the role played by ion-solventand substrate-atom interactions_ GERISCHER 11 has introduced a more general treat-ment based on the work of GURNEYS. The above treatments of charge transfer leadto incorrect interpretations of the symmetry factor 14- The significance of the crossingpoint of the potential energy surfaces, pertinent to the reaction, has not been fullyrecognized and the charge distrib-ution in the activated state has either been neglectedor given a false significance.

The potential energy profile for electron transfer, e.g., in the discharge step,HsO++Me-++MH+HeO (I)of the hydrogen evolution reaction (h-e-r.) may be represented12 by Fig. I. Theenergy of the initial state, HaOf +- Me-, with the reactants in their ground states,is represented by EL The energy of the neutralized state, M-H + Hz0 with the Hatom in the position immediately after neutralization (not necessarily the groundstate of the M-H) is represented by En_

J _ EZec t rocm aZ . Chena . . g (1965) 325-327

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32 6

I metalsurroceSHORT COM~IL~SICATIOSS

Dstonce from metal -

Fig. I. Potential energy profile for electron transfer.

The probability of electron tunnelling from the metal to the HsO+ ion for anunactivated electron is zero, if there is no acceptor state for the electron. Thus, for neu-tralization of the HsO+ ion by an electron from the Fermi level of the metal, the energyLIE (Fig. I). equal to (E II - Er). must be decreased to zero. TIzis is brozcght abozct byac t i v a t i ng t h e H+ -OHs bond by an amount of ener,7 ds. Because stretching of theH+-OHs bond activates the initial state, but stabilizes the resultant neutralizedstate, ds is equal to some fraction, p. of LZE. where /3 is, general, a function of E .When the system is arranged so that dE = o, electron tunnelling from the metal tothe proton can occur with some finite probability. The resulting H atom may theneither relax into the ground state of the system MH + HsO, or re-form a proton byelectron tunnelhng to the metal. The discharge step of the h.e.r. may then be written:

Ha Of + Me- + HzO-H+ + Me- + Hz0 + H-MHz0 + H-M + Hz0 + MH

The activated state is comprised of tie r e sonan t s t a t es , with th& same atomic, butdifferent electronic configuration (cf- a similar conclusion by MARCUS~ for redoxsystems, reached by a different approach).

Consideration of the relative electron tunnelhng probabilities, from H to H+and H to M, leads to the cbnclusion that the resonant states are equally probable,and that the effective c ha r g e on t h e H nu c l eu s i n t h e a c t i v a t e d s t a t e i s 112 (CC_ a similarconclusion due to HUSH~, attained for redox systems by a different approach)_Apphcation of an electrical potential difference d$, changes the energy gap,&?Z, to dE(d$) _= LIE + AqW (Aq5 being negative for cathodic potentials). Theen_ergy required to activate the H+4Hz bond is then#3dE(d#) _Thus, the activationenergy-at a given d$ is #IdE(d$) and the symmetry factor is fl_ Since for the purposbf -calculating 8, we are interested only in the energy required to stretch the HzO-H-bond at-a- g-i )ren;A#. then /I nzey_&e obt a i n e d b y t h e u s ua l me t h od o f ver t i c a l s h t i f t l , 3 .6 ~ 15

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SHORT COMMU-\ ;IC_~TIONS 327an d t he symn te t r ; v f a c t o r i s i n de@wd er t t o f t h e c?aa r ge d i st r i b u t i o n i n t h e ac t i _J a t ed st a t e

The symmetry factor at a given dQ is thus the ratio of energy put into the H+-OH2 bond to that put into closing the gap dE ( d & I ) .

The present theory thus gives a tenable model for the electron movement icharge transfers-13, but it relates this movement to that of the ion-solvent and atom-substratel-7. Means other than the above described mechanism, of closing the energygap dE(d#), such as electron activation, solvent activation, or activation of modesother than the H+-OH2 stretching mode, will lead to higher energies of activationby virtue of the fact that AE = AE(Aq5) for such processes_ This suggests that, foall types of electrode processes the stretchin, u of the ion-solvent (or adion-solvent)bond constitutes the rate-determining aspect of charge transfer at electrodes.TAX E l ec t r o chem i s t r y Labo ra t o r y ,Th e U n i -J en i t _v o f Pemzsyhan i a .Ph .iZ ad eZph ia , Pa . r g r og (U .S .A . )

J _ OIM. BOCKRISD. B. MAJ THEWS*

I J_ HORIUTI ASD M. PoL_+x~I, dcla Phys icocJz i vz_ URSS. 2 (19 35 ) 50 5~2 0 . ESSIN AE?D LT. KOZHEUROV, data Pky si~o~h im . URSS, 16 (rg +-_) 16 -g.3 R. P_XRSOXS AND J. O.&I_ BOCKRIS, Tvans. Favaday Sec.. 47 (1951) 914.4 l3. E. CONWAY -*ND J_ O'&I. BOCK-CRTS.Cax J_ CJtem. , 35 (1957) 1124.5 B. E. CONWAY XND J_ O'M. BOCKRIS, EZec l r o cJz im . Ac t a , 3 (19 61 ) 340 .6 A. R. DESPIC AND J. O'M. BOCKRIS,~. Chem. PJcys . , 32 (1960) 389.7 J_ O'N BOCKRIS. J_ Ch im . P la ys . , 49 (19 52 ) 41.8 Ti. A. BIARCUS, /_ Chem. Pkys . , 3 s (r g6 3 ) 155 8 ; 39 (196 3 ) 173 4 ; / _ Pkys . CJzem. . 67 (19 63 ) 853

Can . J _ Chem. . 37 (rg5 g ) 155; D isczcss i o , r s Fa r aday So t . , 2 9 ( 1 9 6 0 ) 21; Tuaxs. Sy~np. EZec l r odeP rocesses . Ph iZad e l ph i a . Pa . . r g5 9 , edited by E. YB_XGER, \ Viley, Xew York, 1961. ch. 13, p_ 239g N. S. HUSH, J_ CJrem_ Phys . . 28 (1959) 962.IO R. R. DOGON_+DZE AXD Yu. _I. HIZX~DZHEV, Dok l . A k a d . Xa i t k SSSR. 144 (1962) 1077; 14(1962) 849; 150 (1963) 333.II H. GERISCHER, Z. Pk ys i k . Chem_ Fran k f r c r l , 26 (I gGo ) 223 , 325 ; 27 (1961) 48.12 R. GURKEY. Proc_ Roy. Sot . (Lon don ). Ser_ A, 134 (1931) 137~13 J_ A. V_ BUTLER, Pvoc_ Roy_ Sot_ (Lu nd ox) .Ser_ A. 157 (1936) 423_14 D. B. &TAITHEWS, Ph.D. Thesis, Univ. of Pennsylvania. 1965.15 J_ O'M. BOCKRIS. A lode r n Aspec t s o f EZeci r o cJ t em i s l r y , Vol. I, Butterworth, London, 1954,ch. 4_

J_ EZecZroan a i . Chem. . g (igG5) 325-327

* NowattheDepartmentof Matkials Science, University of Virginia.-Chtilottesville, a. (U.S.A.)