The Changing Chemistry of Mercury A. Renuka
Central Electrochemical Research Insititute, Madras Unit, CSlR Complex,Madras,600 113, INDIA Mercury-mercury covalent bonds in mercurous compounds were the fust and for a long time the only known metal-metal bonds. The presence of the diamagnetic Hgz, unit in mercurous compounds has been proved unequivocally by various investigations including X-ray crystallography, Raman spectra of solutions, magnetic susceptihilities, solution equilibria, and electrical conductivities. Chemistry and physics textbooks have been highlighting the significance of the application of these techniques in the structural elucidation of chemical compounds and mercurous mercury has been a good example. Thus, pupils'as well as teachers'-knowledge of mercury chemistry advocates that mercurous mercury exists only in the dimeric form, which has resulted in an opinion that mercury(1) cannot exist in the monomeric form. This, however, is not true. Mercury can and does exist in the monomeric state and in a few other unusual oxidation states. Monomeric Mercurous Mercury
The first report of the existence of monomeric mercurous mercury, Hg' apppeared in 1940. This revolutionary report made by Kolthoff and Barnum (1)is based upon their investigation of the electrochemical oxidation of mercury in the presence of the amino acid, cysteiue. According to them, below a concentration of 10-% in solution all mercurous ions are present only as Hg+and not as Hgz? In 1951, Higgison (2) observed a deviation of the W absorbance of mercurous perchlorate solutions of mncentration below 10dM and proposed a significant degree of dissociation of the dimer into individual ions. Other evidence for the existence of IIg- was obtained by Y-irradiationof solid inowanic compounds doped with mer& ~ ( I I )or radiolysis andphotolysis of solutions of mercury compounds. Symons et al. (3)detected the Hg+ion in y-irradiated glasses, samples of cadmium carbonate containing H e impurity, cadmium acetate doped with mercury(I1) and i n pure mercuric acetate by t h e EPR technique. They indicated that Hg+is produced in a mva-
Wave Length,nm Figure 2. Diffuse reflectances spectrum of Hg-hiscompound lent environment. Dalal et al. ( 4 , s )showed that Hg. can he substituted for K' and NHd7inKHSO. and NHaH~POI.respectively,Faraggi and &zig(6) characterized &e &aviolet spedrum of Hg+ with a hmax at 272 nm. They produced Hg+ ions by the reaction of hydrogen atoms with H e or HgzZf. Gupta and Kaur (7)who investigated the polarography of o-mercaptobenzoic acid showed that the anodic wave in 60% methanol is due to the formation of the complex, RSHg. A one-electron oxidation of mercury also has been noticed in the presence of KN08 (81, tryptophan (9,101 and histidine (11); paramagnetic Hg-trp and Hg-his derivatives also were synthesized electrochemically. The W spectra of the Hg-trp and Hg-his compounds are shown in Figures 1and 2, respectively. Normally, the absorption between 230 and 235 nm is attributed to Hgzz+We(2,ll-13); whereas, a h a x of 272 nm is due to Hgt (6). The hmax values of the compounds are in the range 253-273 n m (table) and support the existence of Hgt. Infrared spectroscopy showed the presence of tryptophan and histidine, respectively, in the compounds and a vibration characteristic of an Hg-Hg bond expected to occur in the vicinity of 180 cm-' (14) is absent. Magnetic Moment and hmaxValues of Hg+ Compounds
Compound
Magnetic Moment
Laxnm
BM
W a v e n u m b e r X lo3, crn-I Figure 1. UV spectrum of Hg-trp compound. Volume 70 Number 11 November 1993
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tafluoride or antimony pentafluoride in liquid SO2(19,20). A golden metallic-looking crystalline material having the formula Hgzss AsFs (21, 22) also was synthesized wherein each Hg atom carries a formal charge of +0.35. Another product obtained in subsequent work (22) i s a red-black compound, Hg4(ksFs)z containing Hg in the formal +0.5 oxidation state.
DPPH
-, E
0
C--
P
--
Tripositive Mercury The study of the electrochemical oxidation of the complex, [Hg~yclaml(BF~)~ in acetonitrile medium by Allred et al. (23) provides evidence for Hg?'. Hg?' has a half life of about 5s and was characterized by cyclic voltammetrv. .. EPR and electronic absorption spectroscopy. The estimated potential for [ ~ g e y c l a m+l ~ e = [Hgcyclam12+
Figure 3. EPR spectra of Hg-trp and Hg-hiscompounds. The paramagnetic nature of these compounds is evidenced bv the EPR spectra (Fin. 3) and the mametic moment measurements ?table).~ h & ecompounds o ? ~were ~ + found to be unstable as they decomposed upon storage and also upon contact with air and moisture. during decampsition, they expelled fine droplets of mercury accompanied by a loss of paramagnetism. The decomposed sample absorbed a t 235 nm indicating the conversion of Hg+ into Hgz2+andlor H e . The following equilibria (11)can be proposed for the decomposition,taking Hg-trp as an example. 2Hg (trp)t,2Hgt + 2trp2Hg+ tt Hg2" tt Hg Mercury in Unusual Oxidation States Remarkable developments have taken place in mercury chemistry particularly the identification of unusual oxidation states and synthesis of their compounds. Possible intermediate oxidation states between HgzZI(+l)and the element appear to have been first noticed in the phase studies of the HgClz/Hg2Clz/Hgsystem (15).Following this, an electrochemical study of the reduction, H$
+ ~~p + H~
in the molten salt eutectic, AIC13/NaCVKC1using polarography (16, 17) provided unequivocal proof of Hgz2+The standard potential and equilibrium constant values also have been reported (18) for Hg3'+ and the interesting compound Hg3(AlC14hhas been prepared (17) as yellow needles by prolonged heating of stoichiometric amounts of HgC12, Hg, and AlCL followed by slow cooling. The HgS2+ ion was discovered independently in another nonaqueous system by the oxidation of mercury with arsenic pen-
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Journal of Chemical Education
is En = 1.6 to 1.8V. From the foregoing, it is evident that we have come a long way from the popular Hga and Hgz2+oxidation states for mercury. It is interesting to study the progressive develooments that have taken lace in this direction. The usef;lness of the electrochemkal methods in unravelling the unusual oxidation states of Hgis praiseworthy. The inclusion of these newer findings in chemistry curricula becomes essential as it would enable the students to understand and appreciate the new scene in mercury chemistry. Literature Cited 1. KolthoB I. M.; Bamum, 0. J Am. Chem. Soe. 1840,62.306L3063. 2. Hi@nsan,W. C. E. J. Cham. Soe. 1951,143MMO. 3. Symons, M . C.R.;Eaehus, R. S.;Yandell, J. K. J Chrm. Soc. Chem Comm 1970,70: Can. J C k m . 1971,49.28&&2871.
4. Dalal, N. S.; Hebden, J. A.; McDowell, C. A. J Mog Reon. 1974,16,289. 5. 0alal.N. S.; Nettar, D.:Gmdinetti,P J.Am. Chem. Soc. 1988,104,2054-2056. 6. Fara@, M.:Amozig.A.lnf.J. Rudht. Phys. Chem. 1978,4,353358. 7. Guota. K C.: Kaur. T J. Ekclmcham Soc 1985.12990. . . 8. ~ e n u k aA, , :~hakuhtela, K 8.EkLmchPm 1990,6,546548. 9. Shakuntsla. K; Renub, A.;Negarajan,V.B. Elecfmham. 1987,3,181-189. lo. Shankuntala, K: Renuka. A.Pme Internotional Seminor. E k t m n d y t v o l TPehnWues. Nou 1987. India, 1989, pp 121-123. 11. Shahmntala, K;RenuLa,A. Tolonlo 1991.38, 100%1014. 12. Yamane,T.;Davldsm,N. J.Am. Cham. Sm. 1900,82,212b2126. 13. Onat, E. J Inorg.Nucl. Chem. 1974,36,2029-2031. 14. Breit-, K; Bmdersen,K: L i m e r , J. Cham. Bsr. 1970,103,2388--2394. 15. Yosirn. S. J.: Maver. S. W. J Phv* Cham. 1980.64.9W913. . 16. Gut, R. If& Ado. 1960;43,83M33. 17. Tomi. G.: Mamantoy G l n o r g Nucl Cham. lerL 1960.6.833, 18. Tavlor M. J Mefol to Meld Bonded Sfoas d ! h a Main Gmuo Elements: Academic &a;: New York. 1975; p 41. 19. Davies, C. G.; Dean, P A. W.; Gillespie, R. J.;Umrnat, P. K. J. Chem. S a Cham. on".- --.-, 1-67 7x1-"119 20. Cutforth. B. D.:Davles, C. G.;Dean,P. A . W, Gillespie, R. J.; Ireland, P. R.; Ummat, P. K Inorg. Chem. 1973,IZ,343347. 21. Gillespie, R.J.;Pasamore,J. Cham. in Britain 1982,8,475478. 22. Cutforth, B. D.;GiUerpie,R. J.;Ireland, P R. J. Cham. Soc Chem. Common. 1973,
him:
.
-"
79.2
23. A1lred.A. L.;Deming,R.L.; Dahl,A.R.;Herlhger,A.L.;Kestner, M.0. J A m . Cham. Soe. lW6,98,4132A135.
