Solvation of Hg(CN)2, Hg(NO3)2 and Hg(CNS)2 by liquid ammonia; a raman spectroscopic study

Journal ofMoZeculur Structure, 49 (1978) 301-307

@Ekevier Scientific Publishing Company, Amsterdam - Printed in The Nether&&a

SOLVATION OF Hg(CN)2s Hg(NO& AND Mg(CNS)z BY LIQUID AMMQNIA; A RlhMAN SP~ROSCOPrC STUDY

DEREK J. GAILDINER Department of Chemishy, NE1 SST (Gt. B~t~i~)

Newcastle upon Tyne Poiytechnic, Newcastle upon Tyne,

AL1 H. HAJI and BR‘TAN P. STRAUGHAN* Department of Inorganic Chemistry, NE1 7RU (Gt. Britain)

University of Newcastle, Newcastle upon T”yne,

(Received 13 March 1978)

ABSTRACT Ramau spectra have been obtained for Hg(CN),, Hg(NO,), and Hg(CNS), in both liquid NH, and NI), at ambient temperature.The spectra indicate the presence of the species Hg[(CN)~(NH~)*~ and ~Hg(NH~~~~z+ in the mercury(II) cyanide and nitrate solutions respectively. The mercury(II) thiocyanate solutions, however, appear to contain s4vated Hg(CNS), and [Hg(NH,), 3’” cations.

The studies of the structures of inorganic salts dissolved in liquid ammonia are being continued. Liquid ammonia is expected to be a better Zigand than

water because the single lone pair of electrons on the ammonia molecule is much more polar&able than the two lone pairs on water. Its strong ligand properties are confirmed by the large number of stable xnetaf--ammine complexes known. The ability of the solvent to interact strongly with metal ions leads to the formation of some interesting species in solution and we are studying the nature of these species using Raman spectroscopy. Our previous spectroscopic studies have been concerned with thallium(I) nitrate, mean halides and methyl mercury(I1) halides dissolved in liquid NE& and ND3 [l-3]. We now report the I&man spectra for ammonia and ND3 solutions of Hg(NO&, Hg(CN)2 and Hg(CNS)2 and the results have been satisfactorily interpreted in terms of the formation of solvated species. EXPERIMENTAL

AII the compounds were thoroughly dried under vacuum before use and the samples were handled on a vacuum line or in a nitrogen-filled glove bag to preserve their anhydrous condition. Anhydrous mercury(II) thiocyanate was recrystallized several times from pure anbydrous ammonia.

Mercury@) nitrate and cyanide were both recrystallised from the pure solvent several times and then pumped under vacuum for several hours before the solutions were prepared. For all three mercury compounds the formation of the amldo species was suppressed by the addition of a trace amount of ammonium chloride after the first filtration. The final solutions were filtered through a glass sinter (porosity 4). Rarnan spectra were recorded on a Gary 81 instrument after it had been converted to 90’ operation by Anaspec Ltd. The 530.8 nm line from a Coherent Radiation model 52 krypton ion laser (ea. 100 mW at sample) was used for excitation. The spectra of the solutions contained in 3 mm diameter tubes were recorded at ambient temperature and they were celibrated against carbon tetrachloride and indene. RESULTS

AND DISCUSSION

The vibrational spectrum of mercury(I1) cyanide has been assigned in the solid state [ 41 and in both aqueous and methanolic solutions [ 5). For a linear, isolated molecule, point group D lob,seven fundamental frequencies are predicted; 3 Raman active (vi, v,‘and r.~~) and 4 IR active (vg, v4, vg and Y,). The Raman active modes have been observed at vl = 2198, v2 = 412 and us = 276 cm-’ for a 0.4 M aqueous solution [5a] but the authors found no evidence for association effects or solvent interactions. Hg(CN)? is very soluble in liquid ammonia and a l/4.8 mole ratio solution shows the Raman bands listed in Table 1. Key parts of the spectra for both liquid NE& and PII& solutions are shown in IFigs.1 and 2, The absence of z&-N bands for free cyanide (cf. NaCN in liquid NH3 shows a strong polarized line at 2669 cm-l assigned to UC-N) indicates that cyanide groups remain directly co-ordinated to mercury(U). Apart from the bands expected for liquid ammonia itself, there is again definite evidence for the presence of co-ordinated ammonia molecules (see Table 1). Thus, the presence of co-ordinated solvent bands at 3170, 1166 and 342 cm-’ implies that the mercury@) cation has increased its co-ordination number. Some of the co-ordinated solvent bands for an ND, solution are clearly visible in Fig. 2. In view of the preference for 4 cooctagon shown by mercury(I1) when the halide [3] salts are dissolved in liquid NH3 and also for the case of [Hg(NH,),f [CIO,], [6], we propose that the most likely species present when Hg(CN)2 is dissolved in liquid m3

is WWW2

(m3)21-

molecule would be expected to exhibit two UC-N, two vHg-N and two vHg-C Raman active stretching modes assuming point group C,,. The assignments for these modes, made :‘I the basis of deuteration and polarization studies, are given in Table 1. The two vl2g-C and UC-N modes are clearly seen in Figs. 1 and 2 respectively. Only one vHg-N mode can be observed, however, and this has been assigned to the symmetric stretching The

303 TABLE 1 F&man frequencies (cm-‘) at ambient temperature

and assignments for mercury(H) cyanide in liquid NH, and ND,

HgUW,/NH,

WCWJND,

Assignment

ca. 245 w sh 264 m dp 342wshp 374 s p 518 w dp? ca. 1070 w p ca. 1166 mw p 1618 w dp 2152 mw dp? 2169 vs p ca. 3170 sh p 3223 s p -

ca

Hg-N bend Hg-C bend Hg-N sym. str. Hg-C sym. str. Hg-C asym. six. Ye sym. def. NH, coordinated solvent uq antisym. def. NH, C-N asym. str. C-N sym. six. co-ordinated solvent 2 vq NH, co-ordinated ND, y1 solvent V, solvent

Mole ratio l/4.8

3308 vs p 3387 s dp

Mole ratio l/5.3 235 264 328 370 510 ca. 815 ca. 900 1185 2158 2169 ca. 23 26 2352 2390 2405 2524

I

1750

4

1350

950 Wavenumber

550 (cm-’

150

)

Fig. 1. F&man spectrum of mercury(H) cyanide in liquid NH,.

mode since the Hg-N antisymmetric stretch is expected to give rise to a weak band and it is more likely to lie undetected beneath the intense vHg-C symmetric stretch, at 374 cm-‘. The position of vHg-N symmetric stretch (342 cm-‘) for the neutral species Hg[(CN),(NH&] is significantly lower than for the charged species [IHg(NH,),]’ (352 cm-‘) and [Hg(NH&] ‘+(401 cm-‘) [S] . The low position for the neutral species is anticipated due to the increased electron charge on the metal.

304

HgKN),

IN4

iigKN)z/ND3

4J / :.!c,k. 2600

2200

2190

2160

2130

Wavenumber (cm-‘)

Fig. 2. Raman spectraof mercury@) cyanide in liquid ND,. It is interesting to note, also, that the positions of the YHgS modes are affected to a small extent when liquid NH3 is replaced by liquid ND, (see Table 1). Initially one might expect that the change of solvent would have no effect on the positions of the YHg-C band, but it would lower YHg-N by ea. 20 em-i. The small but significant shifts for all three modes implies a mixing of the skeletal vibrational modes for Hg[ (CN),(NH&] Thus we conclude that the Raman spectra for Hg(CN)2 dissolved in liquid NH3 are entirely consistent with the presence of the species Hgf(CN), (NH3)*] a Solvent ammonia is not a strong enough ligand to displace CN- from the mercury central atom but the preferred co-ordination number of 4 for Hg(II) in liquid ammonia is achieved by two molecules of solvent directly co-ordinatin to the metal.

.

Mercury(Il) nitrate The Raman spectrum of mercury(R) nitrate has been reported previously by Gans and Gill [‘7]_ They were concerned mainly with locating the position of the vHg-N mode which they report at 415 cm-“. Their result was interpreted in terms of a four co-ordinate mercury species [Hg(NH3),]*+. Apart from confirming the presence of the yHg--N mode (on our instrument at 401 cm-‘) we have shown that the remainder of the spectrum is entirely consistent with the presence of solvated cations and anions. Our observed frequencies together with the proposed assignments are shown in Table 2. The frequencies can be separated into ttvo different groups - those due to the internal modes of the nitrate ion, where the positions of the bands remain unaltered when the solvent is changed from liquid NH3 to ND3, and the remaining frequencies which alter in position by the expected amount

305 TABLE

2

F&man frequencies

(cm-“)

QiWW,INH, Mole ratiol/32 401vsp 7oowp 1098wp 1218 mw p 1636mdp 3170&w 3217s~ 3299vsp 3376mdp

709

(380)a (547) (850) (938) (1192) (2320) (2349) (2400) (2520)

and assignments

403sp 72lshvw IlOOwp 1218mwp 1626mwdp 317oshw 3220s~ 3297 vsp 3378mdp 220-270pbr 743sp 936vw 2061~s~ 2110wp

frequencies

in liquidNH,

(330) (553) (850) (939) (1192) (2320) (2350) (2402) (2517) (220-270)

Metal-N sym. str. co-ordinated NH, rock Y, NH, sym. def. co-ordinated NH, def. wqNH, asym. def. co-ordinated NH, str. 2 v4 NH, v, NH, sym. str. us NH, asym. str. Hg-S sym. sty. 7 C-S stretch 2 x C-S bend C-N

str.

u, NO,-asym. bend NO,- sym. str. vg NO,- asym. str. 2 vi NO,-

w

trations.

and Hg(CNS),

Assignments

Hg(CNS),/NH, Moleratio l/l4

1045 ws p 133Sf 1378 mw dp 1661 m a( ) indicate

for Hg(NO,),

v,

(cm-‘)

of bands for liquid ND, solutions

at comparable

CNS

modes

_J 1 NO,- modes

concen-

when the deuterated solvent is used. The bands due to nitrate anions in liquid ammonia have been well character&d previously [S] and they will not be discussed further. The bands due to solvent ammonia have also been discussed at length [S] and the corresponding frequencies reported in Table 2 require no further comment. The most prominent feature due to co-ordinated ammonia is the strong polarized band located at 401 cm-“. Tbis band moves to 380 cm-’ in liquid ND3 and we have assigned it to vHg-N. The band occurs in the same position for HgCl* and HgBr, dissolved in liquid NH3 E3J and hence, by comparison, we have interpreted our result in terms of the species [Hg(NH3),]**. The remaining co-ordinated ammonia bands at ca. 700,1218 and 3170 cm-’ again leave no doubt that the mercury(E) cation is strongly solvated. Mercwy(II) thimyanate In aqueous solution the linear thiocyanate ion CNS exhibits a three band spectrum: vI (C-N stretch) at 2066 cm‘-‘; v2 (SCN- bend) at 470 cm”’ and u3 (C-S stretch) at 747 cm-’ [$I]. The authors gave no evidence for either ~ation~on or anionsolvent interactions. Tbe thiocyanate anion can act as an ambidentate ligand, depending upon the metal ion in solution and for the complex [ Hg(CNS),] =-, PIELIEand co-workers [lo] have interpreted the Raman spectra for aqueous solutions in terms of sulphur-bonded thiocyanate

groups (uHg-S = 236 cm-‘; z&‘-S = 717 cm-l and Z-N = 2114 cm-l). Lemley and Lagowski [ll] have examined NH&NS and LiCNS in liquid ammonia. They concluded that there are two thiocyanate species present in solution (a) SCN-‘ with a considerable degree of solvation at both ends of the anion and (b) where the Li”-GNS interaction is of considerable impOrta.YlC@.

Our observed frequencies and proposed assignments for mercury(lI) thiocyanate dissolved in liquid ammonia are given in Table 2. The spectrum shows convincing evidence for strong solvent co-ordination to the mercury@) cation. A very prominent feature is the strongly polarized band at 403 cm-’ (vI_ig-N) but much weaker co-ordinated NH, bands are observed at ‘721,1218 and 3170 cm- ‘. As in the case of mercury(I1) nitrate solutions, the positions are indicative of the four co-ordinate cation [Hg(NH3)J2”. The CNS modes for HgfCNS), dissolved in liquid NH3 show a single polarized YC-S mode at 143 cm-l but two distinct YC-N modes are observed (2061 and 2110 cm-‘). The lower frequency YC-N band, which is very strong and polarized, indicates the presence of unto-ordinated thiocyanate groups and the position of the vC-S mode supports this conclusion. The weak polarized band at 2110 cm-‘, however, suggests the presence of a small amount of undissociated mercury(II) thiocyanate in liquid ammonia solution although a shoulder on the low frequency side of the 743 cm-’ band, which would be expected for undissociated Hg(CNS)* was not detected (presumably due to its low intensity). Finally, a weak broad feature was observed at 220-270 cm-’ for Hg(CNS), in both liquid NH3 and ND3 and this is in the region anticipated for vHg-S [ 121. Thus we conclude that most of the mereury(II) thiocyanate has dissociated in liquid NH3 to form ~Hg(NH~)~]‘+ and solvated (CNS) anions. A very small proportion, however, remains undissociated. Presumably the mercury atom in the undissociated form again increases its co-ordination number to 4 by co-ordinating two NH3 molecules but we have no direct spectroscopic evidence for this addition. CONCLUSIcmf

The cationic species produced when mercury(U) salts are dissolved in liquid ammonia depend upon the relative co-ordinating ability of the solvent and the anion. For Hg(NO& the cation is completely solvated to give the species fHg(NHs)4]2”, In the case of Hg(CN)2, where a more covalent bond

exists between the mercury and the cyanide, the ammonia is unable to displace C-W. The solvent can, however, increase the co-ordination number

of the mercury to four to produce Hg[ (Nl&),(CN),] . Hg(CNS)z behaves in a more intermediate fashion in liquid ammonia and it is present both as a weak I:1 thiocyanate- metal complex and as solvated CRTS- and [Hg(NH3)4]2+ ions.

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