Krishna, Vijai

Nigam, H. L.

Prasad, Jagdish

English

NOPR

indian Journal of ChemistryVol. 15A, September 1971, pp. 768-770K-Absorption Edges of Some Cu(II) Complexes InvolvingMetal-Metal Exchange InteractionJAGDISH PRASAD. VIJAI KRISHNA 8;.H. L. KIGAM*Department of Chemistry. University of Allahabad. Allahabad 211002Received 25 June 1976; revised and accepted 17 February 1977K-Absorption edges and the extended fine structures of some copper(U) complexes Involvingmetat-rnetat exchange interaction formed with dithiols and dtthiones as li~ands have beenrecorded ernptoying a 40-CIll curved crystal spectrogr-aph of transrnlssfon type. Edge-widthsof thiol complexes have been found to agree with their coordination stoichiornetrtes, but in the caseof dithione cornplexes, computation of their coordination stolchtornetrtes appears to suggest thepossibility of molecular association. Average metal-Ilgand bond distances for these complexeshave also been calculated from the maxtma-rninima separations in the observed fine structures.THE use of X-ray absorption edge spectro-metry (XAES) as a conventional tool forinvestigating many vital problems in thefield of coordination chemistry has been empha-sized!". However, very little X-ray spectroscopicwork is known? to have appeared on compoundsinvolving metal-metal exchange interaction. Thepresent paper, therefore, is an attempt to applythis technique in the case of some especially chosenset of Cu(II) complexes showing metal-metal ex-change interaction, the nature of which continuesto evade an adequate explanation8•9. The studiescomprise of measurements of edge-width, obser-vations on edge structure and exter.de d fine struc-ture.Materials and MethodsCopper(II) complexes with the following ligandswith 1-phenyl-tetrazoline-5-thiu;e (C,H6N4S), 1-P-bromo-phenyl tetrazoline-S-thione (C7Hf>N4SBr),2-mercaptopyrimidine-4-ol (C4H4N20S), and 3,4,5-pvridazine trithiol (C4H4N2S3) were chosen. Themethods of preparation and characterization ofthese complexes, viz. Cu(C7H£N4S)2' Cu(C7H4N4SBr)2'Cu(C4H2N20S)·2H20 and Cu;(Th(TH)2'lOH20; T =C HN2S3, have been described earlier10-I2.4X-raV absorption studies were carried out ona Machlett scaled X-ray tube having a tungstentarget which was operated at 20 k.V and 10 mA.A 40-cm radius Cauchois type curved cryst al spectro-graph using (100) reflection planes of mica as analyserwas employed to record the spectra c'n Kr dak Xvrayfilms. The dispersion of the spectrographic set upwas found to be ,.....,12XU/mm. Tungsten LOtI,LOt2,L/ll and LIl4emission lines were used as referencestandards and their wavelengths were taken frc mstandard tables of Beardcnt". The absorbing screenswere prepared by evenly spreading the substancein the fine powder form on Scotch tape. Afterseveral trials. screens of suitable thickness wereobtained and were placed between the X-ray tube.Present address: Chemistry Department. Indore Uni-versity. Indore.768window and the analysing crystal. The exposuretime varied from 15 to 20 hr. Several spectrogramswere taken cn a Kipp and Zonen 1\1011 rnicrophoto-meter with magnification 8X for each "ample.Results and DiscussionThe measured (,clge-"idth~, i.e , the energy diffe-rence bet ween the principal absorption maximum,A and the main eclge, K are prcsentr d in Table 1.A correlation betwr en edge width ar.d coordinationstoichiometry as also the overall metal-nearestneighbour electrenegativity difference LIXM-XLidetermined frem the latt er are given in Table 2.The edge-width of CU5(T)2(TH)2'10H20 andCu(C4H2N20S)·2H20 reasonably satisfy the empiricalrelationship (1) proposed by Nigam et al.14•I5.[EW'L!XM-XLIJ! = constant for a given metalin a given region ... (1)provided XJyJ=X t., where XJyJand XL are Paulingelectronegat.ivit ies of the metal and its nearestneighbours respectively. The complex Cu5(T)2(THk10H20 has earlierI2 becn shown to contain fivecopper ail ms having three different types of co-ordination stoichiometries (Table 2). The measurededge-width. 14·1 eV for this complex satisfies thecror dinat ir n st oichir me t rics (a) and (b) (Table 2).However, the edge-widths for the complexes,Cu(C,H,N4S)2 ar d Cu(C7H4N4SBr)2 show deviationfrcm this relatic nship. These complexes have earlierbee-n shown'? to be square planar consisting ofCuN2S2 chrr mophcrc. Acurdirgly, the value ob-tained fcr ~iXM-XLI would be 3·4. However,the value ccmputcd frem the X -EW relationshipus ing the measured edge-widths is found to be....•..4·9. It can be seen that this value very nearlycorre- pc r ds to the coordination stoichiometry,Cu:N :S::1:3:3; LIXM-XLI= 5·1. That the metalatem might have one nitrogen and one sulphuratcm as its nearest neighbours in addition to theCuN2S2 chrornophore, appears to be conceivablein view of the following facts. On the basis ofdiamagnetic and semiconducting properties of thel'HASAD el al.: K-A.13S0RPTtON EDGES OF SOME Cu(II) CO:.\IPLEXESTABLE 1 - EDGE-WIDTHSOF THE ABSORPT[ONEDGESCompound ),A(XU) (v/R)A ),g(XU) (v/R)l{ !l(v/R) W(v/R)(±0'05) (±0'05) eV (±0·6)Cu(C,H5N,S)2 1375·20 662·64 1377-53 661·53 1·11 15·1Cu (C,l[,N4SI $r\, 1375·26 662-62 1377 48 661·55 1·07 14·5Cu(C.H2N,OS).2H,O 1375·03 662·73 1376·81 661'87 0·86 11'7Cu5(T)2(TH)..10B2O 1374·55 662·96 1376·70 661·92 1-04 14-1Compound Coordination stoichiometryTABLE 2 - CORRELATIONlJETWEEN EDGE-WIDTHSAND COORDINATIONSTOICHIOMETRYEdge-width [EW ~IXM-XLI]!(eV)11111111S N2 2 3·4 15·1 7-23 3* 4·8 852 2 3·4 1+5 7·03 3* 5·() 8·51 2 6·0 11·7 8·44 1 5·1 14·1 8'43 5'() 8·41 1 6·5 9'5*Computed from edge width.Cu 0Cu(C.H,N 205).21-120Cu5(T)2(TH)2·1()HP (a)(b)(c)2123complexes, Cu(C7H5N4S)2 and Cu(C7H4N4SBr)2' ithas been suggested10 earlier that the molecule shouldadopt a polymeric configuration in which diamagne-tism arises from the direct intramolecular exchangebetween the two copper atoms, supported virtuallyin contact with one another by four bridging tetra-zoline-S-thione groups and the semi-conductivityof these complexes has been explained by throughconjugation, the metal atoms using their valenceshell 'd' orbitals to form d7t-P7t bonds to theadjacent ligands. Raising of coordination stoichio-metry on the basis of computation from edge-widthvalue has been proposed= recently in the Co(II)complex with l-phcnyltotruzoline-Svthiono. .K-absorption spectra of the complexes studiedhere are shown in Fig. 1. The energy scale is basedon the first inflection point of the metallic copperedge as zero in the usual manner. The sho~lderappearing on all the curves (marked as 5 for Ill~s-tration in Fig. 1) at ,.....,10eV lower energy WIthrespect to the main peak, may be assigned, .assuggested by Glen and Dodd>, to a normally dis-allowed transition ls-;..4s* antibonding. It maybe noted that in Zn2+ which provides a close approxi-mation to the K-excited Cu2+ ion, the configuration3d!J4S and 3dg4P are about 10 eV apart!", In Cu2+,therefore, the ls-;..4s* transition may well be expectedto occur at ,.....,10eV below ls-;..4p* as observed.This transition may, become allowed due eitherto mixing of states (s-d or s-P) or to symmetrychanges produced by vibrational excitations-". Fur-ther, comparing the edges of CU(C7HSN4S)2 andCu(C7H4N4SBr)2' one observes that this transitionis more prominent in the latter case. Though eitherof the complexes involve square planar configuration,the substituent bromine in the aryl radical mayprobably be responsible for additional perturbationand consequently more distortion due to sterichindrance in the complex, Cll(C7H4N4SBr)z whichhas been reflected in the observed spectra. Broad>t:'"~I-Zo...•l-I-s'"zc(a:I-o 40 80 120 160ENERGY «»»Fig. 1 - X-ray absorption spectra of some Cn(H) com-plexes [A and 13 refer to absorption maxima and '" and ~to absorption minima. Structures in the region A", andB~ are labelled as ",'A' and WB' respectively]. S refers toshoulderbands at 90011111and 550 nm have been obscrvedll,lZin the visible spectra of the distorted octahedralcomplexes Cu(C4H2NPS)'2HzO and Cu5(T)2(TH)z'10H20. It is interesting to note that broad natureof the band has been shown to be indicative ofJohn-Teller distortion1s. A comparatively intensels-;..4s* absorption in the complex, Cn(C~H2N20S)·2H20 may probably be due to unsymmetrical natuorof the Land appearing at 900 nm which sel111Stcbe resolved into two bands.The positions of various maxima and minimain the extended fine structure spectra of thesecomplexes are given in Table 3. The spectra of769INDIAN J. CHEM., VOL. 15A, SEP1'EMBER 1977TAHLE 3 - POSITIONS IN ;VIAXIMA AND MINIMA IN THEEXTENDED FINE STRUCTURE SPECTRA OFSOME COPPER(Il) COMPLEXES'Compound A a' A' a B (3Cu(C7H sN,S), 16·3 37·0 73-5 108·9Cu (C7H.N.SBrh 16·0 21·2 25·0 39·0 72·0 105·8Cu(C.H2N ,OS) .2HzO 17·5 23·6 29·0 37·8 68·4 102·5CuslTh(THh.l OH2O 10·6 24·0 31·0 46·0 71-0 103·0=Capitals A, B, etc. refer to absorption maxima andGreek letters Ct, (3, etc. to absorption minima. Structuresin the region A a and B(3 are labelled as «', A' and (3', B'rospecti vel y.TABLE 4 - METAL-LIGAND AVERAGE BOND LENGTHSCompound I!t.E((3-B)(oV)35·433·834·132·0Calculated(A)2·062·112·102·17Cu(C7HsN.S).Cu(C,H.K.SBrj,Cu(C.H2N20S).2H,OCU5(T),CIH)..10H20Cu(C7HsN4S)z and Cu(C7Hr,N4SDr)z are more orless the same (Fig. 1). both from energy positionand transmitted intensity considerations. It maybe recalled that both the complexes involve similarsurroundings. However, a small kink rt.'A' appear-ing in the case of Cu(C7Hr,N4SBr)z' may probablybe due to the effect of the substituent bromine asdiscussed in preceding pages. In the fine structurespectra of the complexes Cu(C4HzNzOS)·2HzO andCus(T)z(TH)2'lOHP, the fluctuations at A and rt.'are very small and a sharp peak, A' of higher inten-sity than the principal absorption maximum, Ahas appeared on its higher energy side.The difference in energy, t-.E from the secondmaximum, B to the next minimum, (3gives a measureof the radius of the first coordinat ion sphore-?around the central metal atom through the Bragg770relation (2)rl=(151/t-.E)t A ... (2)The estimated bond distances in these complexesarc given in Table 4 and are found to lie withinthe expected range of reported metal-ligand bondlengths.AcknowledgementThe authors arc thankful to Prof. B. K. Agarwaland Dr A. N. Vishnoi of the Physics Department,University of Allahabad, for providing Iaboratoryfacilities.References1. COTTON, F. A. & HANSON, H. P., J. chem. Phys., 25(1956), 619.2. GLEN, G. L. & DODD, C. G., J. appt. Phys., 39 (1968), 5372.3 BHIDE, V. G. & BHAT, N. V., J. chcm, Phys., 48 (1968),3103.4. SRIVASTAVA, U. C. & NIGAM, H. L., Coord. chem, ta«,9 (1973). 275.5. lVIANDE, C. & PENDHARKAR, A. V., Chern. Phys., 7(1975), 2+4.6. l'ADALIA, 13. D. & KRISHNAN, V., Indian ]. pure apt)t.Phys., 9 (1971), 813.7. AGARWAL, B. K. & VERMA, L. P., J. pltys. Chem., 3(1970), 535.8. KOKOSZKA, G. F. & GOlmON, G., Tvunsition metalchemistry, Vol. 5, edited by R. L. Carlin (MarcelDekker, I-OW York). 1<)6<), 181.9. Km'EHT, D. L. & VUlEZE, K., Comprchcusioe inorganicchemistry, Vol. IV, edited by J. C. Bailer et at. (l'er-galllon Press, Oxford), 1973, 201.10. DIKSHlT, S. K., Ph.D. Thesis, lIT, Kanpur (1968).11. KHULLAR, 1. P., ru.o. Thesis, Ill', Kanpur (1974).12. DWIVEDl, J. S. & AGARWALA, U., J. inorg. nucl. Chem.,35 (1973), 2229.13. BEARDEN, J. A., X-ray wavelengths, Tennessee, USAtomic Energy Commission (1964).14. SI<lVASTAVA, U. C. & NIGAM, H. L., cu»: Comniun,(1971), 761.15. hHISJINA, VilAI, PRASAD, JAGDISH & NIGAM, H. L.,luorg. clnm. Acta, 20 (19/6), 193.16. DED!AN, W. \V., FORSS, J. & HUm'HHEY, J. N., Pltys.us«, 67 (1945), 212.17. KAUER, E., Z. pltys. Chetn., (, (1956). 105.18. FIGGIS, B. N. & LEWIS, J., Modcr« coordination che-unstry (Intcrscicuco, New York), 1960.19. LEVY, R M.,J. chcni. l'hys., 43 (1965) 1846.

K-Absorption Edges of Some Cu(II) Complexes Involving Metal-Metal Exchange Interaction

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K-Absorption Edges of Some Cu(II) Complexes Involving Metal-Metal Exchange Interaction

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