On the EPR spectrum of the trivalent nickel EDTA complex in aqueous solutions

VOlll_mZ

33, il&TibGi2

Chemistry Department,

CHEMICAL. PHYSICS LETTERS

Berr Grriion Urtiveisigy of the Negev, Beer-SherlJ,

1 June

197.5

13rGel

9. KQRESH

and D. MEYERSTEIN

The EPR spectnm of the Ni(II!)EDTA complex is reported. It is concluded ttnt the complex distorted octahedral structure with a dz,, dzz dJz d:?_,.z electronic confiuration.

1. Introduction

In a recent study [I :I we have reported that a trivalent nickel EDTA comples (EDTA = ethylendiaminetetraacetic acid) is formed in the reaction Ni(I1)EDT.k

+ OH + Ni(II1)EDT.k

i OH-.

This trivalent nickel complex was found to be stab!e in deoxygenated aqueous solutions for several days. However oxygen was found to react wi’ch this comglex with a specific rate of k = 6.5 X IO2 Wi s-l [lj Though the tietic data identifying the prcduct as an

Ni(III) cm-&x were convhc~~ng, hthei experimefitai data coAntkning the xsignment seemed desiiable. We wish to report here tIze EPR spectrum of this product which conkins thz eiuliei assignment.

Nitious~.tide saturai:ed solutions containirrg 1 X 1CY3-2 X iO-! M Ni(II)EDTA at $$I = 4.3 f 0.1 were irradiated b a 6*Ca “y souxe. The dose rzte was 2 X

has 2 flattened

tetragonzl

IO” rad/min. Irradiation times were between 2 to 8 minutes. The solutions thus obtained were tlmnsfer-red into a flat cell for room tempsi2ture measurements or a quartz tube for measurements at 77 K avoiding contact with air. The ERR measurements were czried out uskg an X-band E-12 Vcrian EPR spectrome:er.

3. Results and discussion An F,PR ebsorption band was observed Fn all irrzdieted solutions. In 6g. la a typical room temperature spectlTm is shown. Ihe band has a ha!f width of about 40 G with g = 2.204,. The signal inte,nsity was found to increase linearly with tii-ne of irradiation thus pcoving that it is due to a product of the irradiation. The addition of o:5ygen to the irradiated sc!ution?s caused tie dirzpp_oezance of the absorption bvld (fig. Ib). T&s absorption band is attributed to IG(IIi)EDTA formed by the following reactions [l] :

Volume 33, numb-_r 2

CYEWCAL

1 June 1975

PHYSLCS LETTERS

ingimikites

th2t the electron exchange reaction

*NiCIIE)EDTA + Ni(IEI)EDTA 2 W~(IE)EDT_4 i Ni(lll)C,DTA

I

Magnetic

field _

I

Fig. 1. EPR zpectia. _U sa-nples contzinned 1 X lo-” hi NiSOs + 1 X lo-’ hl Na,H,EDT_A atpH = 4.3 i 0.1 aid were N20 saturated. Irradiak~ time was 300 s. Spzctrz A and B at room temperature md 9.459 GHz, 25 nW; spectrum Cat 75 K md 9.209 GI-Iz, 2.5 mW; spectrum B is for a sample LOwhich oxygen mas added after irradiation.

ei

+ N,O + H,O + +

OH + N, + H,O,

OH + Ni(II)EDTA + Ni(II!)EDTA

f OH-,

is quite slow in agreement with expectations for nickel EDTA complexes [ 1O] . Wren the solutions were frozen after irradiation to 77 K the spectrum changed 2s seen in fig. Ic. This spectrum clearly indicates th2t the Ni(III)EDTA complex is not octahedrzliy symmetric 2s both g,, = 2.330, 2nd gL = 2.139 1 are observed. The observatidn that gll > ,g, seemed 2t fkst quite surprising 2s it indicates th2t the&paired electron is not located in the dZz orbitd [I I] . For most other Ni(II1) complexes studied g, > g,, was reported [7-91. F or 2ll the exceptions to this rule a planar configuration of the complex witi no ligands out of p!ane was suggested [8,9] . It is difficult to erksage 2 planar con5guration for the Ni(iII)EGTA complex. This compiex c2n be considered 2s 211 octnhedml bJA,I3, complex with 2 cis configuration (where A = -O,C- 2nd B = N(-CH,-), . The level order in this case is expected to be inverted relative to that in a trnlzs configuration [ 121 :

H, 0 + H + Ni(III)EDTA -+ Ni(Il)EDTA + H,O’, 2H,O + 2Ni(III)EDTA

+ H,O, _ _ -+ ZNi(II)EDTA + Q2 + 2H3 0’)

the yield of Ni(III)EDTA being G(Ni(III)EDTA)

= 32

[II.

‘The observation of the EPR absorption band clearly indicates that the d7 Ni(III)EDTA complex is a low-spin comp!ex. This obsenlarion is somewhat surprising 2s EDTA is known to be 2 we& field lig2nd. Thus the Mn(EII)EDTA and Fe(IlI)EDTA have 3 higbspin configuration. it seems therefore that she fact that the Co(III)EDTA complex 112s2 low-spin co.nfiguration is due more to the decrease in +Yne ionic Lmdii than to the stabilization of the d6 configurztion [2] . This conclusion is in agreement with the observztion that trklent nickel ions have 2 low-spin configuration both in different oxide lattices (e.g., in MgO [3] , C20 [4] and Al303 [5] ), 2s NiFz- [6] 2nd in ccmpkes wi’J macrocyclic lig2nds [7-9] . Tne line width of the absorption band was found to be nearly independent on the Ni(il)EDTA co~centration in the 1 X lo-’ -2 X lW1 M rulge. This &id-

d,,

d,.,

-d..

=

A>’

t2,.-_

dX)’-

.----d,,

ti,.;

tetqoilel

octahedral

tetragonz!

c!istorted c!s

configuration

distorted tmms configuration;

configuration The observation that predicted

that q

> g, is irk full agreement

from this configuration

with

2s the unpaired

eIectron is located in the CQ _;?z orbitd. In other words the distortion of the octahedron causes here fiattetig 2nd not elongation of the complex. No hy-

perfme

splitting

due to the nitrogens was observed.

Acknowledgement We wish to eXpiesS our thanks to the

israe! Natlor?rJ 287

Yolumc 33, number 2

CZEXICAL

L June 1975

PHYSICS LEnERS [6] C.K. J$igengn,

Inorgalic

complexes

(Andernic

Press,

NEW York, !9633 p. 31. [7] A. Wolberg 2365; References

! 11 J. I_z.:i 2nd D. Xeyerstcin, Intern. J. PZdh:. Phys. Chem., to be ?ublkixd. L.E. Oqel. An introdwtion to tranG-:ion metal chemistry (Mehen, Lor,don, 19G9) ?. 49. J.W. Orion, P. Auzins, J.H.E. Griffiths and J.E. Wertz, Proc. Phys. Sot. (Lond,zn) 7S (1961) 554. IV. Low and J.T. Suss, l’hys. Letters 7 (1963) 3i0. !?. LaCiOiu, LI. Hochli and K.A. MiilIer, I-Ielv. Phys. ACi3 37 (1963) 627; D.S. BkClure, J. Chem. Phys. 36 (1962) 2755.

2nd j.hi~~s_cdn,Inorg.C‘hem.

9 (1970)

J.B.FZyItOr, N~tUXPhyS.SCi. 230 (1971)179. [a] N. Tn!