Complexation of tellurium(II) with thioglycolic acid and reactivity patterns of the system

J. inorg,nucl.Chem., 1973,Vol.35, pp. 3291-3298. PergamonPress. Printedin Great Britain

COMPLEXATION OF TELLURIUM(II) WITH THIOGLYCOLIC ACID AND REACTIVITY PATTERNS OF THE SYSTEM G. A R A V A M U D A N , P. R. S E T H U R A M A N * and M. R. U D U P A Department of Chemistry, Indian Institute of Technology, Madras-36, India (Received 18 July 1972)

Abslraet- Interaction of Te(IV) with excess thioglycolic acid (TGA) in strongly acid media leads to formation of yellow diamagnetic soluble S-ligated thioglycollato-tellurium(II) neutral and anionic complexes. The neutral bis-complex is highly unstable with respect to autodecomposition to Te(O) and dithiodiglycofic acid but the anionic higher complexes are stable in absence of competing ligands. The solid H2[TeU(TGA-H)4] was isolated and charaeterised. Unfike with Te(II)-thiourea complexes, presence of halides did not stabilise but actually destabilised the T e ( I I ) - T G A system, the effect being I >> Br >> CI. Addition of thiourea also caused decompositionofthe T e ( I I ) - T G A system. The results have been rationalised on the basis of the instability of the Te(II)-(TGA-H)2 species with respect to an internal redox reaction. Addition of Hg(II) to T e ( I I ) - T G A systems caused oxidation of Te(II) to Te(IV) in absence of halide ions and interestingly to the formation of the Hg(II) chalcohalides of composition, Hg3Te2X2 (X = CI, Br, I) in presence of halides.

INTRODUCTION

known to exist in several oxidation states in its compounds, namely, (VI), (IV), (II) and (-II). Whereas the (VI), (IV) and (-II) states are thermodynamically stable in aqueous solutions in the absence of additional complexation, the Te(II) species is unstable in aqueous solutions and can be stabilised therein only on complexation with soft ligands. Thus, addition of excess thiourea and its N-substituted derivatives to Te(IV) solutions preferably in hydrohalic media gives stable Te(II)-thiourea complexes [1]. The thioureas act here in the dual role of reducing and complexing reagents. This is similar to their behaviour towards Cu(II) in which case also reduction to Cu(I) and complexation and thereby stabilisation of the latter oxidation state occurs under acid conditions. Whereas critical studies have been reported on the copper-mercapto acid interactions [2, 3], no information is available on the nature of interaction of mercapto acids with tellurium systems in aqueous media. Kirkbright and Ng[4] have merely reported the formation of an yellow colour on mixing Te(IV) and thioglycolic acid (TGA) and examined this system only from an analytical angle for the colorimetric determination of tellurium. In this paper is presented our detailed work on the formation, characterisation and reaction characteristics of tellurium(II)-thioglycolic acid complexes in solution phase. The crystalline solid H~[Te(TGA-H)4] was also T E L L U R I U M is

*Present address: Radiochemistry Division, Bhabha Atomic Research Centre, Trombay, Bombay85, India. 1. 2. 3. 4.

O. Foss, Selected Topics in Structure Chemistry, p. 149. Universitaets-forlaget, Oslo (1967). I. M. Klotz, G. H. Czerlinski and H. F. Fiess, J. Am. chem. Soc. 88, 2920 (1958). P. Krnneck, C. Naumann and P. Hemmerich, lnorg, nucl. Chem. Letters 7, 659 (1971). G. F. Kirkbright and N. K. Ng, Analytica Chim. A cta 35, 116 (1966). 3291

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G. A R A V A M U D A N , P. R. S E T H U R A M A N and M. R. U D U P A

isolated and characterised. This work forms part of our investigations [5] on the chemistry of selenium(II) and tellurium(II) which are being continued. EXPERIMENTAL Te(IV) stock solutions were prepared by dissolving TeOe (B.D.H. and Fluka) in slight excess of ca. 5N N a O H and dilution to desired volume with water. The tellurium strength in solution was determined by dichromate method. Thioglycolic acid (80%) (Merck) was diluted to desired volume with water and the strength of the mercapto acid was determined by iodimetry in acid medium. Potentiometric titrations were carried out with a Kaycee Research Potentiometer using platinum foil and calomel electrodes. Steady potentials in the T e ( I V ) - T G A systems were realised without any difficulty. Ion exchange experiments were made using the cationic resin Amberlite IR 120 (column 16 x I cm) and flow rates of 2 ml/min. Thermogravimetric study of the complex H2 [Te(TGA-H)~] was made on a Stanton HT-SM recording thermobalance in air in platinum crucible at a linear rate of heating of 6 ° per min using 0.5 g of solid. The u.v.-visible spectral studies were made in a Carl Zeiss PMQ II spectrophotometer using 1-cm cells. Magnetic measurements were made in a sensitive Gouy set-up. The i.r. spectra were taken in a Beckman IR 12 unit. X-ray powder patterns were taken in a Debye-Scherrer 114.6 mm camera using C u - I ~ radiation. RESULTS AND DISCUSSION

The E ° values with respect to S.H.E. of aqueous Te(IV)-Te(O) and dithiodiglycolic acid-thioglycolic acid systems are +0.53 and +0.073 V, respectively, and from this reduction of Te(IV) to Te(O) is expected on addition of T G A unless the mixed system is affected by other factors such as complexation and or kinetic aspects. Interaction of Te(IV) with excess T G A in strongly acid media gives rise to yellow complexes which are stable in presence of excess of mercapto acid. The yellow coloration is also produced initially in weakly acid media such as acetic acid but the solution is rapidly decolourised and the black tellurium gets precipitated under these conditions. In strongly alkaline media no colour is produced on mixing Te(IV) and thioglycollate solutions. However, from these colourless solutions, elemental Te precipitates very slowly over a longer period (10-24 hr). Incidentally, it is significant to note that there is no interaction between Te(IV) and hydroxyacetic acid in all media. Therefore it is clear that interaction between Te(IV) and T G A involves the mercapto group of the latter. The results obtained during potentiometric studies carried out in the Te(IV)T G A system in 1N H~SO4/HC1 media are typified in Fig. 1. In the forward titration (curve ABCDE), addition of the first few ml of Te(IV) solution (0.02 M) from the burette to the mercapto acid solution (TGA taken = 1 mM i n 100 ml of medium) in the titration vessel resulted in the formation of an yellow colour which deepened on further additions of Te(IV). A sudden increase in the potential ( - 100 mV) was realised when the Te(IV) added corresponded to 0.25 of the molar amount of T G A taken (region CD). At this stage the decolourisation of the solution occurred accompanied by the precipitation of tellurium. This shows that the yellow complex formed during interaction of Te(IV) with excess T G A is unstable in presence of excess Te(IV) and under these conditions is decomposed to Te(O) and the oxidised product of T G A , viz. dithiodiglycolic acid. In the reverse titration (curve A' B' C' D' E'), addition of T G A solution (0.05 M) from 5. P. R. Sethuraman, Ph.D. Thesis, Indian Institute of Technology, Madras, India (1972).

Complexationof tellurium(II)

3293

30C

o___~-- g-- ~-_S~ ~?,~ ....... A'

~®o -o ~~" 220260180 E o_ 140

fr .'"C, B' Co

-

/.rD' --c--- Forword o L00 o_ .~P ~ --o-- Reverse A ~'°"B J, 60 1°'1 ,,,'~sIE 1 I l I I ,~ 0-10 0.20 0 - 3 0 0.40 0'50 0'60 0,70 Mole ratio of Te(TE)/¢hioglycolic-ocid Fig. 1. Potentiometric titration of T e ( I V ) with thioglycolic acid.

the burette to Te(IV) solution (Te(IV) taken = 0.25 mM in 100 ml of medium) in the titration vessel caused in the initial stages, the transient formation of any yellow complex in the solution phase on moment of mixing and this was followed soon by decolourisation of solution and precipitation of Te since as aforesaid the yellow complex is decomposed by excess Te(IV). With further additions of T G A , more and more of Te was precipitated out. When the molar ratio of T G A added to that of Te(IV) taken reached 4 (region C' D'), there was a sudden decrease in the potential of the system by about 100 mV. Therefore, during the potentiometric experiments potential jumps were realised in both forward and reverse titrations when in the systems the amounts of T G A and Te(IV) reached 4: 1 molar proportion. Any one of the two following reaction sequences occurring separately would account for this 4:1 molar interaction and the visual observations reported above: Te(IV) + 4 H S C H 2 C O O H ~ Tew(SCHzCOOH)4 and other products ]yellow Telv(SCH2COOH)4 stable in presence of excess T G A only]

(1)

or Te(IV) + 4 H S C H 2 C O O H ~ Ten(SCH2COOH)2 + H O O C C H 2 S S C H 2 C O O H and other products

(2)

[ yellow TeII(SCHzCOOH)2 stable in presence of excess T G A only]. In reaction sequence (1), the oxidation state of Te is retained at IV whereas in (2), the tellurium is reduced from (IV) to (II) by T G A and this involves loss of two T G A moles per mole of Te(IV) taken leading to the formation of dithiodigiycolic acid. The Te(II) can form complexes with T G A giving rise to [TeH(TGA H)=] x-2 where x/> 2. Since one of the electrodes used (namely platinum foil) in potentiometry is a redox indicator electrode the potential jumps observed strongly favour the reaction sequence (2) which features a redox process. In the forward

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G. A R A V A M U D A N , P. R. S E T H U R A M A N and M. R. U D U P A

titration (ABCDE in Fig. 1), when small amounts of Te(IV) are being added to TGA solutions (region ABC) the species present in solution phase would be excess TGA, dithiodiglycolic acid and the yellow [Ten(TGA-H)x]x-2 species; whenthe Te(IV) is added in molar proportion exceeding 0.25 times TGA taken (region DE), the species present in solution phase would be only the excess Te(IV) and dithiodiglycolic acid, since the yellow species wouM have then decomposed to Te(O) and dithiodiglycolic acid. Thus, the platinum electrode is responsive to the redox couples, (i) TGA and its oxidised product dithiodiglycolic acid, and (ii) Te(IV) and its reduced products which could be the soluble [Te n(SCH2-COOH)~] x-2 species or the insoluble Te(O) depending on the conditions. In the reverse titration (A'B'C'D'E'), during the initial stages (,4'B') the potential readings are due to the reduction of Te(IV) to Te(O) by the small amounts of TGA added and the formation of Te(O) is preceded by the formation Ten(TGAH)z species which as aforesaid is unstable under these conditions. Instability of TeU(TGA-H)2 species in presence of excess Te(IV) is attributed to competetion between Te(II) and Te(IV) for the (TGA-H) group leading to release of Te(II) and its disportionation to Te(O) and Te(IV) in the aqueous medium. The Te(IV) will interact with the (TGA-H) group to give Te via the Te(II) stage and the cycle wouM repeat till all the small amount of TGA is oxidised. In the region D' E', the potential readings are due to the TGA- dithiodiglycolic acid system as no tellurium will be present in solution phase. The +2 oxidation state of Te in the yellow solutions formed in the T e - T G A system was confirmed by chemical methods as follows: Known amounts Of Te(IV) were treated with known amounts of excess TGA in 1N sulphuric/ hydrochloric acid media. The resulting solution containing the yellow complex and excess unreacted TGA was treated with H2S gas which reduced all the tellurium in the complex within a minute to elemental tellurium. After filtering off the tellurium and removal of unreacted hydrogen sulphide by passing nitrogen through the solution phase, the TGA in the solution was determined iodometrically. Had the Te(IV)-TGA interaction proceeded as reaction sequence(l) all the TGA taken originally would have been recovered after the H~S treatment. However, if the interaction had proceeded as reaction sequence (2), then 2 moles of TGA per every mole of Te(IV) taken cannot be recovered since they would have been utilised for reduction of Te(IV) to Te(II) stage. A representative set of results obtained is given in Table 1. It is evident that each mole of Te(IV) has oxidised 2 moles of TGA and is itself reduced to Te(II). The Te(II) forms complexes with TGA of composition [Ten(TGA-H)x]~-2 with x I> 2. The lowest complex with x = 2 is very unstable and autodecomposes to Te(0) and dithiodiglycolic acid (regions CD and C'D' in Fig. 1). The higher complexes are however stable in strong acid media. The observed precipitation, isolation and characterisation of the brick red solid bisdiethyldithiocarbamatotellurium(II) on addition of sodium diethyldithiocarbamate solution to the yellow solutions of telluriumthioglycolic acid complexes prepared in strongly acid media confirmed by yet another chemical method the reduction of Te(IV) to Te(II) by TGA since diethyldithiocarbamate species does not itself reduce Te(IV). Strongly acid medium and presence of excess TGA were essential for the stability of the Te(II) species in aqueous solutions. Solutions containing Te(II)

Complexation of tellurium(II)

3295

Table 1. Interaction of Te(IV) with excess T G A in strongly acid medium

Te(IV) taken (m-moles) a

T G A taken (m-moles) b

T G A found* (m-moles)

T G A oxidised (m-moles) b-c

Molar ratio of Te(IV) taken to T G A oxidised b-c/a

0-4895 0.4895 0.4832 0.2416

2.808 3-370 2"790 1.685

1.824 2.395 1.814 1.202

0.984 0.975 0.976 0.483

2-01 1.99 2-02 2.00

*After H2S treatment; see text. Medium: 1-2 N HzSO4 or 0-5 N HC1; overall volume approx. 100 ml.

and T G A in molar ratios greater than six were stable. On addition of excess alkali, the T e ( I I ) - T G A complex disproportionated to yield Te(O) precipitate and Te(IV) species. Polar oxygen-donor solvents such as ethyl acetate, n-butanol and tributylphosphate were useful in extracting the T e ( I I ) - T G A complex from aqueous phase; these solvents extracted thioglycolic acid also. The solutions of the T e ( I I ) - T G A complex in the organic solvents were stable for months at 30 °. The solid complex, H2[TeH(TGA-H)4] was isolated as follows: Te(IV) and T G A were mixed in 1:8 molar ratio in 1N sulphuric acid medium and the Te(II) complex formed was extracted into ethylacetate along with the excess free TGA. The organic phase was shaken repeatedly with aqueous solutions of cadmium chloride to remove the free T G A . The ethylacetate phase was dried over anhydrous sodium sulphate and the organic solvent evaporated at low pressure at 0 ° when beautiful yellow prismatic crystals separated. (Found: C = 20.2, H = 2-93, S = 26.0, Te = 25.7 per cent; required for Hz[TeH(TGA-H)4] C = 19.5, H = 2.84, S = 25.9, Te = 25.9 per cent). The crystals were stable indefinitely at 0 ° in absence of moisture but decomposed gradually within 24-28 hr at 30 ° to Te and dithiodiglycolic acid. The i.r. spectrum of the solid complex taken in Nujol (decomposition of the complex occurred within an hour when pressed into KBr pellets) showed the absence of absorption around 2500 cm -1 indicating the deprotonation of the SH group in the T G A on ligation to Te(II). This is to be expected since mercapto sulphur is a soft ligand and Te(II) has been established as a soft acid by Foss and his school[I,6]. Co-ordination of Te(II) with S(-II) and Se(-II) sites in various ligands is widely known and there has been hitherto no report of oxygen ligation in stable Te(II) complexes. Therefore even though the C O O H group is a stronger acid than the SH group in the normal aqueous chemistry of TGA, complexation with the soft acid Te(II) to sulphur forces deprotonation of the SH group and not of the C O O H group in the complex. The same trend has been noticed by us (unpublished results) in the T G A complexes of other soft metal ions such as Ag(I) and Hg(II). The X-ray powder pattern of the yellow crystals gave the following d spacings (in A): 1.60(m), 1.70(m), 1-85(w), 1-93(w), 2.12(w); 2.67(s), 2-88(s), 3.31(m), 4.00(s) and 5.02(m). (m = medium, w = weak, s = strong). It is intended to undertake the determination of the crystal 6. O. Foss, Pure appl. Chem. 24, 31 (1970).

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G. ARAVAMUDAN, P. R. SETHURAMAN and M. R. UDUPA

structure by collecting single crystal data at low temperatures. Attempts to prepare salts of [Ten(TGA-H)4] 2- with bulky cations such as Ba2*, [Et4N] + and [enH2] 2+ were unsuccessful. During thermogravimetric study (rate of heating 6°/min.) it was seen that the complex Hz[TeH(TGA-H)4] decomposed without weight loss before 80 ° to tellurium and dithiodiglycolic acid. On further heating gradual weight loss occurred from this mixture up to 240 °, followed by a rapid decomposition up to 520 °. A small gain in weight was noticed in the range 520-600 °. The final product was found to be tellurium dioxide by both chemical analysis and X-ray examination. The gain in weight at 520-600 ° is attributed to the oxidation of tellurium via unstable TeO to TeO~. The electronic spectrum of H2[Te(TGA-H)4] dissolved in ethylacetate to which a few drops of thioglycolic acid were added was measured in the range 200-800 m/x. Only one absorption maximum was found. This was at 246 m/x (em~x = 1"7 × 106) and is ascribed to the sulphur to tellurium charge transfer transition. The yellow colour of the solid complex and its solutions is due to the tailing offthe u.v. band into the visible region. On passing the T e ( I I ) - T G A system containing excess T G A in 1N sulphuric acid medium through anionic resin column it was observed that the yellow complex was completely retained on the resin column and the colourless eluate contained no trace of tellurium. In contrast, there was no retention of the Te(II)T G A complex when the solution was passed through cationic resin columns. The results indicate that in the solution phase also the species [TeI~(TGA-H)4] ~exists. The T e ( I I ) - T G A complexes were diamagnetic both in the solid state and in solution. Apparently in [ T e ( T G A - H ) j 2- Te(II) exhibits a square planar coordination with ligation to 4S of the ( T G A - H ) groups analogous to the numerous other Te(II) S-ligated complexes whose structures have been extensively studied by Foss and coworkers [ 1,6]. The reactivity patterns of the T e ( I I ) - T G A complexes in solution phase were extremely interesting. Unlike in the case of Te(II)-thiourea systems, presence of other ligands which had coordinating tendencies towards Te(II) caused decomposition of the T e ( I I ) - T G A systems to tellurium even inpresence of excess T G A , at a rate dependent on the concentration of the added ligand and on its coordinating ability towards Te(II). Qualitative observations made on effect of halide i o n concentration o n rate of decomposition of T e ( I I ) - T G A systems is summarised in Table 2. Less concentration of bromide and still less of iodide in comparison to chloride was required to bring about the decomposition of the complex to tellurium and dithiodiglycolic acid. The observed effects could be rationalised on the following basis: [TeH(TGA-H)4] 2- + 2 X - + 2H + ~ [Te(TGA-H)2X2] ~- + 2TGA. (Longer Te-S bond; quite stable towards autodecomposition)

(Shorter Te-S bond; rapid autodecomposition)

Partial replacement of ligated T G A from the higher T G A complexes of Te(II) by the halide ions (X-) occurs and in the resulting species the T e - S bond and interaction is stronger than in the higher T G A complexes leading to virtual

Complexation of tellurium(II)

3297

Table 2. Effect of halide ion on stability of T e ( I I ) - T G A system in 1-4N H2SO4 medium Halide concentration

Nil 0.5N chloride 1.0N chloride 2-0N chloride 0-5N bromide l .ON bromide 0. l N iodide

Stability of system at 30 °*

Stable for days. Stable for more than 24 hr. Only slow decomposition. Stable for more than 12 hr. Later slow decomposition. Stable only up to 30 min. Moderately fast decomposition. Decomposition initiated within I hr. Decomposition initiated within a few minutes. Complete decomposition within a few minutes.

*Initiation of decomposition is indicated by precipitation of black tellurium from the homogeneous yellow solution phase. Amount of T e ( I I ) = 1 m-mole; Amount of T G A = 10m-moles; Overall volume: Approx. 100 ml.

oxidation-reduction between the Te(II) and the two ligated T G A groups and thus causing formation of tellurium and dithiodiglycolic acid. The suggestion of variations in the Te(II)-S bond lengths with changes in coordination around Te(II) is compatible with the trans effect discussed by Foss[1,6] with regard to Te(II) complexes. Thus, the TeH(TGA-H)2 species though highly stable with respect to ligand displacement by halides is nevertheless intrinsically highly unstable with respect to autoredox decomposition. Addition of even small amounts of thiourea sufficed to decompose the Te(II)T G A complexes rapidly and caused formation of not only tellurium but sulphur also. This observation and the confirmation of the quantitative retention of thiourea in solution phase by chemical tests* after decomposition of the Te(II)T G A complex had occurred indicated that the sulphur accompanying tellurium in the solid decomposition product should have originated from the ligated T G A i n the Te(II)-TGA complex. It is thus a peculiar observation that whereas excess T G A and thiourea independently stabilize the Te(II) oxidation state their presence together causes rapid change over of Te(II) to elemental tellurium: This is explained as follows: Similar to halide ions, thiourea also replaces some of T G A from the stable higher TGA complexes of Te(II) to give mixed ligand species such as Te(TGA-H)z(thiourea)2. Again because of trans effect, there is such an extensive shortening of the Te-S bond relating to the ligated [TGA-H]group in the mixed ligand complex that this results in the rupture of the S-C bond in the ligated [TGA-H]- group causing formation of TeS which being unstable[7] disproportionates to give a mixture of elemental tellurium and sulphur. Presence of metal ions which are on the borderline between class a and class *The decomposed solution after filtration of the precipitated tellurium and sulphur was treated with cadmium chloride and a little excess of 0.5N sodium hydroxide. On heating the mixture yellow cadmium sulphide was precipitated in amount (as determined iodometrically) corresponding to thiourea taken. Dehydrosulphurisation of only thiourea and not of thioglycolic acid or dithiodiglycolic acid was found to take place under these conditions. 7. W. O. Sneiling, J . A m . chem. Soc. 34, 802 (1912).

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b series such as cadmium and zinc did not influence the stability of the Te(II)TGA systems in strongly acidic media. However, Hg(II), a typical class b ion, decomposed the systems to give products which depended on the experimental conditions. Thus, addition of Hg(NO3)2 (1.0mM) to a Te(II)-TGA system obtained by mixing sodium tellurite (0.125 mM) and TGA (1.0mM) in 100 ml of IN HzSO4 resulted in the disappearance of the yellow colour and the precipitation of the white Hg(TGA-H)2 complex; all the tellurium was found in the solution phase as Te(IV). I n absence of Hg(NOa)~, the interaction of Te(IV) and TGA initially leads to formation of Te(II)-TGA complex and dithiodiglycolic acid. The reoxidation of Te(II) to Te(IV) by dithiodiglycolic acid in presence of Hg(NOa)z proceeds as [TeIt(TGA-H)4]2- + HOOCCH2SSCH~COOH + 3 Hg 2+ ---> Te(IV) + 3 Hg(TGA-H)~. The driving force for the above reaction sequence is the pronounced preference of Hg-S ligation in relation to Te-S ligation o f the TGA molecule. However, in presence of other notable Hg(II) coordinating ligands such as halides, interestingly an altogether different reaction sequence was observed. Thus addition of HgCI~ (1 mM) to a Te(II)-TGA system obtained by mixing sodium tellurite (0.125 mM) and TGA (1.0 mM) in 100 ml of 1N HCI gave a yellow precipitate along with small amounts of the white precipitate Hg(TGA-H)2. No tellurium was found in the solution phase. After leaching out the Hg(TGA-H)2 by repeated treatment with 1N HC1, the pure yellow precipitate was found on analysis to be mercuric telluride chloride of composition, HgoTe2C12 (found: Hg = 63.78, Te = 28.04, CI = 7.51 per cent: required Hg = 64.86, Te = 27.50, C1 = 7.65 per cent). The X-ray pattern of the yellow solid also agreed with that reported previously for HgzTe~C12[8]. HgaTe~Clz was also obtained by treating Hg(TGAH)2 with excess Te(IV) in 1N HC1 medium. (As expected there is no interaction between Hg(TGA-H)z and Te(IV) in 1N sulphuric acid medium.) The formation of HgaTe2CI~ is explained as follows: There is initially partial decomposition of [TeII(TGA-H)4] 2- species on addition of Hg(II) in presence of chloride ions to Te~(TGA-H)2 which being unstable forms Te(O) and dithiodiglycolic acid. The Te(O) further disproportionates in presence of Hg(II) and chloride ions to Te(-II) and Te(IV), the driving force for this being the ready formation and precipitation of the highly stable HgaTe~Cl2. The Te(IV) formed would again react with TGA to give Te(II)-TGA complexes and this process continues till all tellurium taken is fixed as Hg3Te2C12. New methods (via rapid solution phase reactions instead of hitherto employed slow diffusion controlled solid state reactions) have been developed by us for the preparation of a wide range of mercuric chalcohalides of composition HgaX2Y~ (where X = Se, Te; Y = CI, Br, I, NCS) based o n reaction sequences similar to the one described above. These will be reported shortly. Acknowledgement-One of us (P.R.S.) likes to thank the Council of Scientific and Industrial Research, Government of India for grant of a fellowship. 8. H. Puffand J. Kuester,

Naturwissenschaften 49, 299 (1962).