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Indian Journal of ChemistryVol. 18A, October 1979, pp. 287-289

NQR & Coordination Chemistry-Investigations on CertainMercury(II) Chloride _.C<?mplexest

L. RAMAKRISHNAN ,l S. SOUNDARARAJAN

Department of Inorganic and Physical Chemistry, Indian Institute ofScienceBangalore 560012

and

v. S. S. SASTRY & J. RAMAKRISHNA*

Department of Physics, Indian Institute of Science BangaIore 560012

Received 14 May 1979

The aiCI NQR frequencies in five HgCl2 complexes, viz, HgCI!.DMSO, 3HgCls' 2DMSO, HgCI •. DMF,HgCI2.THF and HgCI2• 4MePyO (4MePyO is 4-methylpyridine l-oxide) have been studied and the results dis-cussed in terms of possible structures.

MERCURY (II) halides form a number of com-plexes of the formul~ (HgX2)n~m with oxygendonor ligands. WIdely varymg structures-

have been postulated for these complexes based on.various physico-chemical studies. Brill et a12-4 havestudied a number of Hg(II) halide complexes by halo-gen NQR spectroscopy and correlated the NQRfrequencies with the structures of these complexes.They concluded that those complexes which giye asingle resonance frequen?y have a four-coordm~te'structure as in HgCl2.dlOxane5. Complexes whichshow two well-separated (at least by I MHz) reso-nances have been assigned structures with halogenbridges. Recently Wulfsberg? has pointed out t.hecomplexity of the interpretation of NQR frequenciesin mercury (II) halide complexes and has shown thatNQR data could be used for predicting the coordi-nation number of Hg(II). .

The 35ClNQR frequencies in five HgCl2 complex~shave been studied and the results are presented inthis paper and discussed in terms of the possiblestructures.

Material and MethodsThe complexes studied are HgCI2. DMSO, 3HgCl2.

2DMSO, HgCl2.DMF, HgCl2.THF and HgCI2•

4MePyO, where DMSO=dimethylsulphoxide; DMF=dimethylformamide ; THF=tetrahydrofuran ; and4-MePyO=4-methylpyridine I-oxide. The complexeswere prepared by the usual procedures which generallyinvolved the mixing of the reactants in hot methanolfollowed by stirring. The solvent was slowly evapo-rated at room temperature to get crystals, in all thecases except HgCl2.THF which was obtained byrecrystallising HgCl2 in THF.

The spectra were recorded using a super regenera-tive NQR spectrometer built in the laboratory and

tPaper presented at the symposium Structure,. Bonding &Reactivity of Inorganic Compounds held at the Indian Instituteof Science, Bangalore during October 1978

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employing a home-made lock-in-amplifier". The.complexes were finely powdered and tightly packedinto the sample tubes. For HgCl2.THF it was neces-sary to keep 0.5 ml of THF above the sample as thecomplex was unstable in the absence of the excessTHF.

Results and DiscussionThe 35ClNQR frequencies observed in the present

investigation are presented in Table 1.HgCI2·DMSO and 3HgCI2.2DMSO - Brill et al»

reported two resonances for HgCl2.DMSO at 21.184and 20.075 MHz at room temperature; these couldbe observed only for a five-month old compounds,while Maksyutin et aI.s reported the resonances at19.878 and 19.494 MHz. Ryan? could not observethe 35Cl NQR in this compound. This compoundwas, therefore, reinvestigated and four resonances

. have been observed, two in the region reported byBrill et al. and two in the region reported byMaksyutin et al.

Three resonances have been observed at roomtemperature for 3HgCl2.2DMSO for the first time.Of these resonances, two (19.678 and 19.461 MHz)have frequencies similar to those of a pair of resonan-ces observed in HgCI2.DMSO. 3HgC12.2DMSO

TABLE1 - aSCINQR FREQUENCIESOFTHECOMPLEXESOFHgCls

Compound Temp. s5CI resonances (MHz)(K)

298 20.718, 21.249, 19.680, 19.461299 19.678, 19.588, 19.46177 19.730

298 20.273, 20.033299 21.042, 19.724296 21.538, 20.11277 21.740, 20.230

HgCls·DMSO3HgCls·2DMSO

HgCI2·DMFHgCIs·THFHgCls.4MePyO

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INDIAN J. CHEM., VOL. 18A, OCTOBER 1979

exhibits only one resonance at 77K. This may bedue to a phase transition occurring between the roomtemperature and 77 K.

The molecular structure (I) of 3HgCI2.2DMSO isknown!". The structure (I) of 3HgCI2.2DMSOconsists of two types of HgCl2 units. Hg(l), situatedon a centre of symmetry, shows a two coordinated

. " II

linear structure [Hg(1)-CI(1)=2.306 A, CI-Hg-G= 180°]. Two pairs of chlorine atoms belonging toadjacent dimeric groups (at (""oJ 3.0 A) complete thecoordination sphere to form an oblate octahedron.This kind of coordination with four long and twoshort contacts corresponding to covalent bonds istypical of Hg(II). Hg(2) is covalently coordinatedby two chlorine atoms (Hg-CI=2.309 A, 2.320 A,

II

CI-Hg-CI=166.00) and weakly coordinated bytwo oxygen atoms (Hg-O=2.52 A and 2.56 A) ofthe sulphoxide group. These ligands are arrangedin a highly distorted tetrahedral configuration withangles ranging from 75.3° to 166.0°. The configu-ration may also be considered as a highly distortedoctahedron, if two chlorine atoms from two adjacentHgCl2 molecules are regarded as a part of the Hg(2)environment. The values of these Hg .... CI dis-tances (3.302 and 3.372 A) are very close to thesum of the Van der Waal radii (3.30 A). In simplerterms, the structure can. be regarded as consistingof HgCl2 molecules and dimetric (HgCI2.DMSO)2units (with oxygen bridges) bridged by chlorines.Thus, there are two types of HgCl2 units.

From the similarity of the resonances observed forHgCl2DMSO and two of those observed for 3HgC12.2DMSO; it may be suggested that these resonancesmay be due to chlorines in the (HgCI2.DMSOJ2dimer. The separation of these frequencies (220 KHz)may be attributed to the two different bond lengthsexhibited by two chlorines covalently bound to Hg~2).One may attribute the resonance at 19.460 MHz tothe chlorine bound to Hg(2) at a distance of 2.320 A.The higher frequency resonance at 19.680 MHz isthen attributed to the other chlorine at a distance of2.309 A from Hg~2). Similar assignments based onthe metal halogen distance have been made in a fewcomplexes of CuCl2 (ref. 11). The third resonanceof 3HgCI2.2DMSO occurring at 19.588 MHz isassigned to the chlorines attached to Hg(l) ri.e,belonging to HgCI2). The appearance of only oneresonance for these chlorines is consistent with thestructure of the complex, as these two chlorines areat the same distance from Hg(l) and have similarinteractions with the neighbouring mercury as well aschlorine atoms. The shifts in the resonance fre-quencies of 3HgCI2.2DMSO from those of HgCl2 a etowards the lower frequency region. This is dueto the coordination of the ligand to the metal. Theinteraction between the HgCI2(1) molecule and the(HgCI2.DMSO)2 dimer seems to be relatively strongso as to bring down the resonance frequency ofHgCl2 to 19.588 MHz.

Based on the assignment of frequencies for 3HgC12.2DMSO, the resonances of HgCl2.DMSO have beenassigned. As already pointed out, the resonances at

288

I

• MERCURY

o CHLORINE

o OXYGEN

~ SULPHUR

o CARBON

19.680 and 19.460 MHz in HgCI2.DMSO indicatethe presence of chlorines with interactions similarto those observed in 3HgCI2.2DMSO. Thus, thesetwo resonances may be attributed to those chlorineswhich have interactions with the neighbouring Hg(II)ions (so as to satisfy the coordination requirement ofa hexa-coordinated mercury), apart from covalentlybonding to a Hg(II). The appearance of resonancesat higher frequencies (20.718 and 21.249 MHz) andtheir separation indicate that the structure consistsof at least two more chlorine atoms which differfrom the above mentioned chlorines in their inter-action with the neighbouring Hg(II) and that thesetwo chlorines differ in their Hg-CI bond lengths.Keeping the above considerations in view, structure(Il) based on (HgCI2.DMSO)2 dimers (oxygen-bridged)bridged by halogens may be suggested.

HgC/2.DMF - The two resonances at 20.273 and20.033 Hz with a separation of 240 KHz for HgCI2.DMF indicate that the structure of this complex atroom temperature is different from that of HgC12.DMSO. Among the complexes which exhibit 35CINQR in this region, the structure of only HgCI2.Dioxane is known". The structure of HgCl2.Dioxaneis shown to contain infinite chains of alternatingHgCl2 and dioxane molecules. Each mercury atomis surrounded by two chlorines and two other oxy-gens such that all five atoms are in the same plane.HgCI2.DMF may also have such a structure withtwo differing Hg-CI distances.

HgC/2.THF and HgC/2.4MePyO - Two reso-nances are observed in each case, at about 19.5 and21.00 MHz. Considering the structures of the com-plexes which exhibit resonances in these regions itseems possible that a dimer structure(III) withhalogen bridges as suggested by Brill et al. adequate-ly describes these complexes. Brill et al. haveassigned the low frequency signal to the bridgingchlorine and the high frequency one to the terminalone. This assignment is consistent with the assign-ments made for similar compounds of Pt(II) andPd(II)12. Photoelectron spectroscopic studies'> on

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NJ:~<J 20.2

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RAMAKRISHNAN et of. : "CI NQR STUDY OF HgC1a COMfLEXES

20.4..-----------------,21.8

• HIGH FREQUENCY llN.E

o lOW FREQUENCY LINE

21.7

21.6u

onMrl

21.5

T OK

Fig. 1 - Temperature dependence of 15Cl NQR frequenciesof HgClj.4-MePyO

10.3

20.1

20.0L---:l~OO,---..L--'--::!20:;:-O--L-.--:;-30~O---.J 21. 4

some of the above cited Pt(l!) complexes, also indicatethat this assignment may be correct. A temperaturedependence study of these resonance frequencies mayalso be helpful in assigning the frequencies. Thebridging chlorine frequency is expected to show alower (ay/8np than that by the terminal chlorine.Hence a study of the temperature dependence of 3sClfrequencies in HgCl2.3MePyO was undertaken andthe results are shown in Fig. 1. The low frequencyresonance as expected, exhibits a low (8v/8T)p com-pared to that of the high frequency resonance.

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Further, the low values of (Oy/onp for both the reso-nances in HgC12.4MePyO compared to the value of(oy/8np for HgC12 indicate a damping of the internalmotions on complex formation (A report on thetemperature dependence of NQR in HgC12.4MePyOhas already appearedj=.

Thus NQR can provide an inexpensive analyticaltool for studying structural aspects of molecules andalso detect phase transitions if present.

AcknowledgementL. Ramakrishnan is grateful to the Indian Institute

of Science, Bangalore for financial support whileV. S. S. Sastry acknowledges with thanks the financialassistance from the Indian Institute of Science and theCSIR, New Delhi.

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33 (1971), 371.3. BRILL, T. B., J. inorg. nucl. Chem., 32 (1970), 1869.4. BRILL, T. B. & HUGUS(Jr), Z. Z., Inorg, Chem., 9 (970),

984.5. HASSEL, O. & HVOSLEF,J., Acta chem. scand., 8 (1954),

1953.6. WULFSBERG, G., Inorg, Chem., 15 (1976), 1791.7. SASTRY,V. V. S., Ph.D. Thesis, Indian Institute of

Science, Bangalore (1976).8. MAKSYUTKIN,Yu. K., GURYANOVA,E. N. & SEMlN,G. K.,

Russ. Chem. Rev., 39 (1970), 334.9. RYAN, J. A., Ph. D. thesis, Michigan State University,

Michigan, USA, (1967).10. BISCARINI,P., NIVELLlNI, G. D., MANGIA, M. & PELIZZI,

G., J. chem. Soc., Dalton Trans., (1974), 1846.11. FAUSTODA SILVA,J. J. R., VILAS,BoAS, L. F. & WOOTON,

J. inorg. nucl. Chem., 33 (1971),. 2029.12. FRYER,C. W. & SMITH,J. A. S., J. chem. Soc. (A), (1979),

1029.13. CLARK, D. T., BRIGGS, D. & ADAMS, D. B., J. chem.

Soc. Dalton Trans., (1973). 169.14. RAMAKRISHNAN,L., SOUNDARARAJAN,S., SASTRY,V. S. S.

& RAMAKRISHNA,J., Aust. J. Chem., (1978), in press.

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