Synthesis and thermal stability of mercury diacetylide Hg(C≡CH) 2

Article abstract of DOI:10.1016/j.poly.2013.06.005

Mercury diacetylide was synthesized and studied by FT-IR spectroscopy, both in the mid and far infrared, and electronic absorption spectroscopy. The spectral data are consistent with the structure H-C≡C-Hg-C≡C-H. Mercury diacetylide is insoluble in common solvents but shows a minimal solubility in ethanol and tetradecane. The thermal stability of mercury diacetylide was studied by thermogravimetry (TGA), differential thermal analysis (DTA) and differential scanning calorimetry (DSC). In an open crucible under nitrogen, mercury diacetylide undergoes an exothermal and explosive decomposition releasing 725.6 J/g with an onset temperature of about 250 °C and a peak temperature of 287 °C. The decomposition occurs at a higher temperature in a sealed crucible (onset 326 °C and peak 337 °C). Using the TGA-FT-IR analytical technique it has been found that the deflagration of mercury diacetylide produces elemental carbon, elemental mercury and acetylene. The enthalpy of formation of mercury diacetylide has been determined for the first time. The explosive parameters of mercury diacetylide have been compared with those of other common explosives showing that it is a dangerous and powerful explosive.

Full text of DOI:10.1016/j.poly.2013.06.005

Polyhedron 62 (2013) 42–50
Contents lists available at SciVerse ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Synthesis and thermal stability of mercury diacetylide Hg(C„CH)2
c
Franco Cataldo a,b, , Vlado Kanazirev
⇑
a Tor Vergata University, Via della Ricerca Scientifica, 00133 Rome, Italy
b Actinium Chemical Research srl, Via Casilina 1626A, 00133 Rome, Italy
c UOP LLC a Honeywell Company, 25 East Algonquin Road, Des Plaines, IL 60017-5017, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Mercury diacetylide was synthesized and studied by FT-IR spectroscopy, both in the mid and far infrared,
and electronic absorption spectroscopy. The spectral data are consistent with the structure H–C„C–Hg–
C„C–H. Mercury diacetylide is insoluble in common solvents but shows a minimal solubility in ethanol
and tetradecane. The thermal stability of mercury diacetylide was studied by thermogravimetry (TGA),
differential thermal analysis (DTA) and differential scanning calorimetry (DSC). In an open crucible under
nitrogen, mercury diacetylide undergoes an exothermal and explosive decomposition releasing 725.6 J/g
with an onset temperature of about 250 °C and a peak temperature of 287 °C. The decomposition occurs
at a higher temperature in a sealed crucible (onset 326 °C and peak 337 °C). Using the TGA–FT-IR analyt-
ical technique it has been found that the deflagration of mercury diacetylide produces elemental carbon,
elemental mercury and acetylene. The enthalpy of formation of mercury diacetylide has been determined
for the first time. The explosive parameters of mercury diacetylide have been compared with those of
other common explosives showing that it is a dangerous and powerful explosive.
Received 18 March 2013
Accepted 3 June 2013
Available online 17 June 2013
Keywords:
Mercury diacetylide
Acetylides
Spectroscopy
Thermal analysis
Explosive
Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction
to that of silver and copper acetylides, even in its instability and
explosiveness. Another work [12] mentioned by Frad [13] and
Copper and silver acetylides are useful compounds in organic
synthesis and this has ensured a constant interest in these com-
pounds [1]. Furthermore, the chemical structure of copper and sil-
ver acetylides was deeply investigated because the air oxidation of
these compounds leads a solid state coupling of the acetylide moi-
eties forming polyyne chains [2–5]. The synthesis of a polyyne
mixture with the submerged carbon arc technique [6] has given di-
rect access to polyyne chains having more than 16 carbon atoms
[7] and this has led to the synthesis and structural characterization
of copper and silver polyynides [8–10]. Particularly interesting, in
terms of the hazard in handling copper and silver acetylides and
polyynides, is the work of Cataldo and Casari which has been ded-
icated to the thermal stability and explosiveness of these com-
pounds [7].
In contrast to copper and silver acetylides, there is very little
knowledge about mercury diacetylide and the potential risks in
handling such a compound. The literature reports on mercury
diacetylide are limited to very old works done without any spectral
and thermal analysis. For example, in a work dated 1894, Plimpton
and Travers [11] report on the formation of mercury(II) acetylide
and describe this compound as having a behavior almost similar
Aylett [14] distinguishes between mercurous and mercuric
acetylide. The former was obtained as a white precipitate by pass-
ing acetylene into an aqueous suspension of mercurous acetate,
but was too unstable and decomposed during the recovery. On
the other hand mercuric acetylide (afterwards referred as mercury
diacetylide) was quite easily accessible, for example by passing
acetylene into alkaline solutions of K2HgI4 or K2Hg(CN)4 [13,14].
Nesmejanov [15] rationalized the reactivity of acetylene and al-
kynes with mercury compounds pointing out that alkaline condi-
tions are necessary to ensure the formation of mercury acetylide,
otherwise other products ranging from organomercury chlorovinyl
derivatives to aldehydes are formed.
Mercury diacetylide is described as a whitish powder, insoluble
in common solvents and with a density of 5.3 g/cm3, that explodes
violently by rapid heating or under mechanical shock [14]. The
present work is dedicated to the study of mercury diacetylide
and particular attention is dedicated to its thermal stability and
explosive decomposition.
The renewed attention to mercury diacetylide is due to the fact
that mercury and organomercury compounds are widespread in
the environment [16]. Some oil and gas streams from refineries
and petrochemical production do contain mercury and mercury
compounds along with acetylene and alkynes [17]. In addition,
mercury catalysts are still used in the industrial synthesis of vinyl-
chloride and acetaldehyde from acetylene. Consequently, there is
⇑
Corresponding author at: Tor Vergata University, Via della Ricerca Scientifica,
00133 Rome, Italy. Tel.: +39 06 94368230.
E-mail address: franco.cataldo@fastwebnet.it (F. Cataldo).
0277-5387/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.poly.2013.06.005

F. Cataldo, V. Kanazirev / Polyhedron 62 (2013) 42–50
43
still the possibility of mercury diacetylide or mercury alkynides
formation under given process conditions. Therefore, there is the
need to know better the safety and the explosive properties posed
by these organomercury compounds.
at room temperature. The yellow–white powder obtained the next
day was used for further characterization by infrared spectroscopy
and DSC thermal analysis.
A number of mercury aryl- and alkylacetylides have been well
studied since they were employed for the isolation and identifica-
tion of alkynes before the advent of the spectroscopy for the same
purposes [18–23]. Mercury alkynides have also been used to syn-
thesize other organomercury compounds and to study the relative
reaction kinetics [24–27]. There are also reports about the hazards
in handling such compounds [28]. For example, the bis(alkynyl)
mercurial (i-Pr–C„C)2Hg was found to be easily explosive [28]
and the hazard in handling copper and silver acetylides was as-
sessed very recently [7,29].
It is important to underline that the research on transition
metal acetylides is flourishing as demonstrated by the very recent
review on homoleptic acetylides [30], which also include mercury-
based derivatives. The interest in complex metal acetylides resides
in the potential application of these species as luminescent mate-
rials or as molecular components in microelectronics [31–34].
Thus, assessing the potential hazard in handling metal alkynides
is very important as a necessary step in any application. In the
present work we are focusing on the simplest mercury alkynide:
mercury diacetylide.
2.2. Synthesis of mercury diacetylide Hg(C„C–H)2 from HgCI2
Mercury dichloride (HgCl2, 650 mg) was completely dissolved
by stirring in 35 ml of distilled water inside a gas washing bottle.
The pH of the resulting solution was 4.5. The passage of a stream
of acetylene did not produce any precipitate. 4.5 g of KI were then
added in one shot. The precipitation of red HgI2 was briefly ob-
served before it dissolved quickly in the excess KI, yielding a trans-
parent and homogeneous solution. The pH of this solution was 7.0,
neutral. A stream of acetylene was then passed through this solu-
tion, but once again no precipitate formation was observed. 50 mg
of sodium hydroxide (NaOH) were then dissolved into the solution
and acetylene was again passed through the solution, yielding an
abundant precipitate of mercury diacetylide. The precipitate was
collected and washed as described in the previous Section 2.1;
the diacetylide yield was about 410 mg.
3. Results and discussion
3.1. Properties, infrared and UV spectra of mercury acetylide
2. Experimental
As mentioned in the introduction, mercury diacetylide was re-
ported in the old literature at the end of the 19th and at the begin-
ning of the 20th century. We have selected the most effective
experimental conditions which are known to produce mercury
diacetylide. These conditions require the use of complex mercury
HgI2 and HgCl2 were purchased from Sigma–Aldrich and used
as received. All the other reagents and solvents were analytical
grade materials obtained from Sigma–Aldrich or from Fluka. The
FT-IR spectra were recorded on a Nicolet 6700 spectrometer from
Thermo-Scientific. The mid infrared spectra were recorded in the
transmittance mode with the samples embedded in a KBr pellet
or in the reflectance mode using a ZnSe crystal and a horizontal
attenuated total reflectance attachment. The far infrared spectra
were obtained on the same spectrometer using the samples in
CsI pellets. The thermogravimetric analysis of the samples was per-
formed on a Linseis TGA model L-81 + DTA at a heating rate of
10 °C/min under a nitrogen flow (20 L/h). The combined TGA–FT-
IR analysis was performed by connecting the TGA apparatus with
a transfer line to the FT-IR gas cell of 10 cm path length and
equipped with BaF2 windows. The differential scanning calorime-
try (DSC) study was performed on a Mettler-Toledo DSC-1 Star
System. Different heating rates were used, as stated in the text,
and the samples were heated under a nitrogen flow of 3 L/h using
either a conventional aluminum pan with a hole or completely
sealed medium pressure stainless steel crucibles. In both cases
the sample size was 5 mg.
2ꢀ
salts, such as HgI4 and Hg(CN)42ꢀ, in a weakly alkaline medium
[13–15]. The passage of acetylene through such solutions causes
the formation of
a white–yellow precipitate described as
(–Hg–C„C–)x by Aylett [14], suggesting a polymeric nature to this
compound, which is reported to be insoluble in any common sol-
vent, has a density of 5.3 g/ml and is explosive.
In Section 2.1 we have produced a whitish precipitate by pass-
ing acetylene through an alkaline solution of K2HgI2 and in
Section 2.2 it was shown that a mercury diacetylide precipitate
occurs only when NaOH was added to the solution (basic condi-
tions). Neither passing acetylene through a slightly acidic solution
of HgCl2 nor through a derived solution of K2HgI2 prepared by add-
ing KI in excess to HgCl2 resulted in Hg diacetylide.
The chemical structure of mercury diacetylide can be easily
understood from its FT-IR spectrum, reported in Fig. 1. The spec-
trum shows a very sharp and clear acetylenic H–C„C– stretching
at 3280 cmꢀ1, while the bending of the same moiety occurs at
670 and 637 cmꢀ1. On the other hand the triple bond stretching
2.1. Synthesis of mercury diacetylide Hg(C„C–H)2 from HgI2
is located at 2013 cmꢀ1
.
Concerning the C–Hg–C bond stretching, literature [35] reports
the asymmetric stretching of the linear molecule dimethylmercury
at 538 cmꢀ1, the symmetric stretching at 515 cmꢀ1 and the bend-
ing at 160 cmꢀ1. In the case of diethylmercury, the asymmetric
Mercury diiodide (HgI2, 760 mg) was suspended by magnetic
stirring in 40 ml of distilled water and then potassium iodide (KI,
2.5 g) and potassium hydroxide (KOH, 186 mg) were added simul-
taneously under stirring. There was a gradual dissolution of HgI2,
which otherwise is insoluble. Once the solution was homogeneous,
it was transferred into a gas washing bottle and acetylene was bub-
bled with continuous magnetic stirring. Mercury diacetylide
started to precipitate immediately as soon as acetylene was passed
into the solution. The precipitate was whitish-yellow and could be
collected by filtration through filter paper, grade ‘‘Analitica A’’.
Once the filtration was completed, the filtrate was washed twice
with distilled water and then dried in air. The total yield was
380 mg. About 20 mg of the wet mercury diacetylide were trans-
ferred with a spatula into a small agate mortar to dry overnight
C–Hg–C stretching occurs at 515 cmꢀ1
, the symmetric at
488 cmꢀ1 and the skeletal bending Hg–C@C at 267 cmꢀ1, while
the C–Hg–C bending appears at 140 cmꢀ1. The C–Hg–C bond
stretching in divinylmercury H2C@CH–Hg–HC@CH2 occurs at 541
and 513 cmꢀ1. Since for the elements P and Ge the passage from
the divinyl derivatives to the diethynyl derivatives implies a shift
to lower wavenumbers of the carbon–metal stretching band by
Dm
= 70 to 80 cmꢀ1 [35], the asymmetric C–Hg–C bond stretching
in mercury diacetylide is expected at 464 cmꢀ1 and the symmetric
stretching at 435 cmꢀ1, as indeed is seen in Fig. 1. The other three
bands in the far infrared portion of the spectrum of Fig. 1 are

44
F. Cataldo, V. Kanazirev / Polyhedron 62 (2013) 42–50
1.1
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
3500
3000
2500
2000
1500
1000
500
Wavenumbers (cm-1)
Fig. 1. FT-IR spectrum of mercury diacetylide (red line). The spectrum is dominated by the C„C–H stretching at 3280 cmꢀ1 and the C„C–H bending mode at 670 and
637 cmꢀ1. Other important infrared bands are at 2013 cmꢀ1 due to the triple bond stretching and, in the far infrared at 464 and 435 cmꢀ1 due to the C–Hg–C stretching and at
281, 208 and 103 cmꢀ1 due to the C–Hg–C and Hg–C„C bending modes. The blue line shows the gas phase spectrum of pure acetylene as a reference. The position of the
„C–H stretching band of acetylene is almost identical to that of mercury diacetylide. (For interpretation of the references to colour in this figure legend, the reader is referred
to the web version of this article.)
certainly assignable to the Hg–C„C bending at 281 cmꢀ1 and to
the C–Hg–C bending at 208 and 103 cmꢀ1
To explain the insolubility and the lack of volatility of mercury
acetylide it is necessary to invoke a interaction between the
.
p
All these spectral data are completely in line with the expected
chemical structure:
aceylenic groups and the mercury atom in a homoleptic interaction
[30]. Cataldo and Casari have already proposed such an interaction
to explain the insolubility of silver and copper acetylides, and espe-
cially polyynides [7]. Evidently this interaction is the reason for the
insolubility also observed in the case of mercury diacetylide.
In addition, there is evidence concerning an unusual aggrega-
tion of certain mercury alkynides in the solid state [36,37]. In such
systems the Hg–Hg distances were found in the range 0.37–
0.40 nm, possibly indicating weak mercuriophilic interactions,
but the main driving force for clustering appears to be the interac-
tions between the Hg atoms and the C„C bonds of adjacent mol-
ecules [36,37]. In this context it is important to look at the
HACBCAHgACBCAH
ð1Þ
However, mercury diacetylide is insoluble in common solvents
and, as will be shown in the following sections, it is neither fusible
before decomposition nor volatile. For example, prolonged shaking
of mercury diacetylide in benzene (15 mg in 4 ml) does not lead to
any dissolution. Only sonication in an ultrasonic bath causes the
formation of a milky dispersion of mercury diacetylide in benzene.
The suspension gradually separates a white precipitate of mercury
diacetylide without any decomposition. Similar results are ob-
served when mercury diacetylide is sonicated for 15 min in
tetradecane, toluene or anhydrous ethanol. The mercury diacety-
lide is finely dispersed by ultrasonic treatment in these solvents,
giving a milky-white suspension which on standing releases as a
precipitate the whitish and fluffy insoluble diacetylide. However,
as soon as the tetradecane solvent is filtered from the acetylide
suspension, it displays the electronic absorption spectrum shown
in Fig. 2 and is characterized by an absorption maximum at
204 nm and a series of weak shoulders at 220, 234 and 266 nm;
a similar spectrum is observed also in ethanol. This implies that
a very small fraction of mercury diacetylide dissolves in these
solvents.
position of mC„C in the spectrum of Fig. 1. The main band is located
at 2013 cmꢀ1 while the free acetylenic group band should occur at
2111 cmꢀ1 [35]. The shift
between the acetylenic group and the Hg atoms of adjacent mole-
D
m
ꢁ 100 cmꢀ1 is due to the
p interaction
cules [35].
In the older literature, mercury diacetylide is reported as a
monohydrated compound.
HACBCAHgACBCAH ꢂ H2O
ð2Þ
Indeed, the FT-IR spectrum of Fig. 1 also shows a relatively weak
and broad stretching band for water at about 3453 cmꢀ1. This
infrared band cannot be removed even after prolonged heating at

F. Cataldo, V. Kanazirev / Polyhedron 62 (2013) 42–50
45
Fig. 2. Electronic absorption spectrum of the minimal fraction of Hg(C„CH)2 soluble in tetradecane. The spectrum resembles that of diacetylene, see text for further
discussion.
70 °C for 7 h. Therefore the relatively weak band at 3453 cmꢀ1 is
compatible with a small amount of water trapped in Hg(C„CH)2,
but the amount of water is not necessarily in an equimolar ratio
with the diacetylide, as will be shown in the following section.
Hg(C„CH)2 is considerable more stable in air and even in hu-
mid air than the copper and silver diacetylides. The latter two un-
dergo slow changes on aging in air due to a solid state coupling
reaction of the acetylenic moieties [2–5]. This phenomenon does
not occur at all in the case of Hg(C„CH)2, which seems to be indef-
initely stable in air at room temperature, provided that it is not ex-
posed to intense light or direct sunlight. In the latter case a
distinctive yellowing of Hg(C„CH)2 was observed, confirming
the photosensitivity of this compound.
Turning back to the electronic absorption spectrum of
Hg(C„CH)2 in tetradecane, reported in Fig. 2, it is worth noting
that it resembles the spectrum of diacetylene HC„C–C„CH. In fact
diacetylene shows four bands at 214, 224, 234 and 246 nm [38]
which should be compared with the bands at 204, 220, 234 and
266 nm in the case of mercury diacetylide. This implies that the
that the decomposition process of mercury diacetylide is exother-
mal and the decomposition onset occurs at 250.5 °C. Indeed, the
explosive decomposition of mercury diacetylide was so violent
that a distinct detonation was heard, accompanied by the shatter-
ing of the quartz arm of the balance and breaking of the platinum
crucible filled with only 40 mg of mercury diacetylide.
During the thermogravimetric measurement, the TGA was con-
nected to an infrared gas cell of 10 cm path equipped with two
BaF2 windows. All the gases released by the mercury diacetylide
sample during the decomposition were passed into the gas cell
and a series of infrared spectra of the gas phase were recorded dur-
ing the decomposition, and these are reported in Fig. 4. It is evident
that acetylene was produced during the decomposition of mercury
diacetylide. Acetylene was easily recognized from the strong infra-
red absorption band at 730 cmꢀ1. The bottom of Fig. 4 shows the
reference spectrum of acetylene, while the other spectra of Fig. 4
were recorded at different temperatures to follow the thermal
decomposition of mercury diacetylide.
The release of acetylene is first evident at 150 °C, a much lower
temperature than that of the explosion onset recorded with the
DTA at 250.5 °C. Fig. 5 reports the amount of acetylene released
by the mercury diacetylide decomposition, as measured from the
area below the peak at 730 cmꢀ1 of the spectra of Fig. 4. As pointed
out, acetylene release starts at 150 °C, grows almost linearly with
temperature up to 250 °C when it reaches a maximum, which coin-
cides with the explosion. Afterwards there is a rapid drop in the
acetylene concentration in the gas phase.
The explosion of mercury diacetylide produces acetylene and
leaves behind a heavy dark carbon soot containing elemental mer-
cury. Therefore, the thermal decomposition of mercury acetylide
under inert atmosphere can be represented by the following
reaction:
Hg atom between the acetylenic groups does not interrupt the
p
conjugation, otherwise only the isolated acetylenic transitions,
which occur below 190 nm, should be observed. Since the insertion
of an Hg atom between two phenyl groups does not alter much
both the UV spectrum and the molar extinction coefficients of
the absorption bands [24], assuming that the same phenomenon
occurs in the case of mercury diacetylide, then we can estimate
the concentration of Hg(C„CH)2 in tetradecane and in ethanol
using an
e
ꢁ 300 L cmꢀ1 molꢀ1 [38,39]. The solubility of
Hg(C„CH)2 is estimated to be about 3.5 ꢃ 10ꢀ3 mol/l in tetrade-
cane and about double that value in ethanol.
3.2. Thermogravimetric analysis of mercury acetylide and TGA–FT-IR
analysis
HACBCAHgACBCAH ! Hg þ 2C þ HACBCAH
ð3Þ
Since we now know the decomposition reaction of mercury
diacetylide under an inert atmosphere, we can now measure the
decomposition heat and then derive the enthalpy of formation.
When mercury diacetylide is heated under a nitrogen flow in a
thermobalance it does not show any transition or any measurable
weight loss up to 250 °C. As stated in the previous section, the
amount of trapped water detected by FT-IR should be very small
since was not possible to quantify it by thermogravimetric analysis
(TGA). Fig. 3 shows the first derivative of the TGA, the DTG, which
indicated the explosive weight loss of the mercury diacetylide
sample at 253.4 °C. At that temperature the maximum decomposi-
tion rate of the sample has been reached. Moreover, the differential
thermal analysis (DTA) trace, also shown in Fig. 3, demonstrates
3.3. Differential scanning calorimetric (DSC) study on mercury
diacetylide
The thermal behavior of mercury diacetylide was studied also
with the DSC, both in a conventional punched aluminium pan
and in a sealed medium pressure stainless steel crucible. A heating

46
F. Cataldo, V. Kanazirev / Polyhedron 62 (2013) 42–50
Fig. 3. Differential thermal analysis (DTA, black line) and first derivative of the thermogravimetry (DTG, blue line) of mercury diacetylide measured under a N2 flow. The DTA
shows the exothermal peak when the onset of the explosion occurred at 250 °C, while the DTG shows the explosive weight loss of the sample. The maximum decomposition
rate occurred at 253.4 °C.
415-37 Hg(CCH)2 pyrolysis 70°C
0.005
415-37 Hg(CCH)2 pyrolysis 105°C
0.01
415-37 Hg(CCH)2 pyrolysis 150°C
0.01
415-37 Hg(CCH)2 pyrolysis 200°C
0.02
415-37 Hg(CCH)2 pyrolysis 245°C
0.04
0.02
415-37 Hg(CCH)2 pyrolysis 260°C
415-37 Hg(CCH)2 pyrolysis 280°C
415-37 Hg(CCH)2 pyrolysis cooling 1
Acetylene;Ethyne
0.04
0.02
0.03
0.02
0.04
0.02
1.0
0.5
1050
1000
950
900
850
800
750
Wavenumbers (cm-1)
Fig. 4. Gas phase FT-IR spectra collected during the mercury diacetylide decomposition of the TGA under a N2 flow. The peak at 730 cmꢀ1 is due to acetylene, as shown by the
reference spectrum reported at the bottom of the figure. The amount of acetylene released as function of temperature is reported in Fig. 5. The spectra shown were taken
respectively (from top to bottom) at 70, 105, 150, 200, 245, 260 and 280 °C, and in the cooling phase after the explosion.
rate of 10 °C/min was applied to the samples and all measurements
were made under a nitrogen flow. Fig. 6 shows the DSC traces of
the two Hg(C„CH)2 samples, heated in an open and sealed cruci-
ble respectively. In the case of the sample heated in the punched
crucible, no thermal transitions are observed up to 200 °C, which
could suggest water evaporation, melting of the sample or

F. Cataldo, V. Kanazirev / Polyhedron 62 (2013) 42–50
47
Fig. 5. Amount of acetylene released during the thermal decomposition of mercury diacetylide as measured with the TGA–FT-IR analytical technique. The amount of
acetylene was evaluated from the area of the peak at 730 cmꢀ1 from the spectra shown in Fig. 4. The absorbance area was measured using the Omnic software of the Nicolet
6700 spectrometer.
sublimation. Above 200 °C the decomposition of Hg(C„CH)2 starts
and the decomposition onset was found at 245 °C, in comparison to
250 °C measured with the TGA–DTA in Section 3.2. The decompo-
sition peak occurs at 287 °C, thus it is at a slightly higher temper-
ature than that measured in the TGA–DTA. The broadened shape of
the decomposition peak suggests a deflagration of the sample
rather than a sharp detonation. The total heat released in the ther-
mal decomposition is impressive: 727.1 J/g. Fig. 6 also shows that
the Hg(C„CH)2 sample heated in the sealed crucible to the decom-
position point releases virtually the same amount of heat: 724.2
J/g, but in this case the release of heat occurs in a sharp peak, sug-
gesting a detonation of the sample inside the crucible. Indeed the
crucible was found to be deformed, but not broken, after the mea-
surement. Because the Hg(C„CH)2 sample was heated in a sealed
crucible under pressure, it remained stable above 300 °C. In fact
the decomposition onset this time was found at 326 °C, while the
explosion peak was at 337.4 °C.
Table 1 summarizes the decomposition temperature and heat of
the other explosive acetylides recently studied [7]: dicopper acet-
ylide (Cu2C2) and disilver acetylide (Ag2C2) in comparison to mer-
cury diacetylide. From Table 1 it is evident that Cu2C2 decomposes
at the lowest temperature, only 126.7 °C, but the decomposition
heat released in terms of kJ/mol is surprisingly coincident with
the decomposition heat released by Hg(C„CH)2: 185 kJ/mol for
Cu2C2 against 181.8 kJ/mol for Hg(C„CH)2. In terms of decomposi-
tion temperature, Ag2C2 is in the middle between dicopper acety-
lide
and
mercury
diacetylide:
169 °C.
However
the
decomposition heat released by Ag2C2 is ꢁ1.41 times the heat re-
leased in the deflagration of Hg(C„CH)2 and Cu2C2.
A useful value for the estimation of the hazard potential of an
explosive compound is the maximum (adiabatic) temperature in-
crease, which is given by the following equation [44]:
D
Tadiabatic ¼ Qexpl=Cp
ð7Þ
With the DSC of Fig. 6 we have measured the heat released by
reaction (3), which corresponds to an average value of 725.6 J/g.
Since the molecular weight of Hg(C„CH)2 is 250.63 Dalton, the
heat released from reaction (3) is 725.3 ꢃ 250.63 = 181.78 kJ/mol.
Since we know that the standard enthalpy of formation of acety-
lene is +226.75 kJ/mol [40], the free energy of formation of
Hg(C„CH)2 is:
where Qexpl is the heat released during the confined explosion in-
side the stainless steel crucible and Cp is the mean heat capacity
of the reaction contents, which is about 1.5 J/g for organic com-
pounds but only 0.14 J/g for elemental mercury [45]. Then, for mer-
cury diacetylide, 5183 K >
DTadiabatic > 484 K, which corresponds to
a hazard potential of ‘‘very high’’ [45].
The activation energy of decomposition of Hg(C„CH)2 can be
measured by DSC in a sealed crucible using different heating rates
[43,44]. As shown in Table 2, the decomposition temperature peak
is affected by the heating rate and is shifted to higher temperatures
for higher heating rates. The data in Table 2 can be elaborated
according to the Ozawa equation [45]:
D
HRð3Þ ¼ ½
D
HꢄfðHACBCAHÞ þ
D
HꢄfðHgÞ þ
D
HꢄfðCÞꢅ ꢀ ½
D
HꢄfHgðCBCAHÞ2ꢅ ð4Þ
and by substituting with values we have:
ꢀ181:78 ¼ 226:75 þ 0 þ 0 ꢀ ½
D
HꢄfHgðCBCAHÞ2ꢅ
ð5Þ
ð2:15LogbÞð1=TpeakÞꢀ1 ¼ ꢀE#=R
ð8Þ
so that
D
Hꢄ
¼ þ408:5kJ=mol
ð6Þ
fHgðCBCAHÞ2
or according to the Kissinger equation [45]:
Consequently, from the average bond dissociation energy of
divinylmercury [39] we have calculated Hf = + 363 kJ/mol for
½2:303Logðb=T2Þꢅð1=TpeakÞꢀ1 ¼ ꢀE#=R
ð9Þ
D
Hg(CH@CH2)2 in the gas phase, while the enthalpy of formation
of diphenylmercury in the gas phase is +392 kJ/mol [41]. As an-
other reference, the enthalpy of formation of the explosive mer-
cury fulminate Hg(NCO)2 is +941.56 kJ/mol [42,43].
This yields an activation energy for the thermal decomposition
of Hg(C„CH)2 of 153 kJ/mol.
Again, in comparison, it is interesting to note here that the acti-
vation energy for the decomposition of mercury fulminate is

48
F. Cataldo, V. Kanazirev / Polyhedron 62 (2013) 42–50
Fig. 6. Differential scanning calorimetric (DSC) measurement of mercury diacetylide. Both the DSC traces were recorded at a heating rate of 10 °C/min under a N2 flow. The
DSC trace in the upper figure was obtained for a sample heated in an aluminium crucible with a hole in the cap. The decomposition onset starts at 245 °C and the peak is
reached at 287 °C. The DSC trace in the lower figure was obtained for a sample heated in a sealed, medium pressure, steel crucible. The use of a sealed crucible retards the
decomposition onset, which occurs at 326 °C, while the decomposition peak is at 337 °C. However, in both cases the normalized heat released in the decomposition is
comparable: 727 J/g in the upper figure and 724 J/g in the lower figure.
Table 1
Table 2
Key decomposition properties of acetylides.
E# For Hg(C„CH)2 decomposition.
Dec. T
(°C)
Dec.
heat
(J/g)
Molecular
weight (Dalton)
Dec. heat
(kJ/mol)
Refs.
Heating rate
(°C/min)
Peak dec.
temp. (°C)
E# (kJ/mol) Ozawa
equation
E# (kJ/mol) Kissinger
equation
5
10
20
40
320.45
337.38
347.30
360.69
152.9
152.9
152.9
152.9
153.6
153.6
153.6
153.6
Cu2C2
Ag2C2
Hg(C„CH)2
127
169
287
1225.2
1070.4
725.6
151.11
239.76
250.63
185.1
256.6
181.9
[7]
[7]
this work
105 kJ/mol, while it is 167 kJ/mol for silver azide and 160 kJ/mol
for lead azide [41]. Mercury fulminate is reported to be much more
sensitive to impact and friction detonation than silver and lead
azide due to its lower activation energy for the decomposition
reaction [41].
The detonation velocity and the detonation pressure are other
parameters which can be easily estimated using the Kamlet and
Jacobs equations [46,47] once the heat released during the thermal
decomposition of Hg(C„CH)2 is known.

F. Cataldo, V. Kanazirev / Polyhedron 62 (2013) 42–50
49
Table 3
atoms and the CBC bonds of the adjacent Hg(C„C–H)2 molecules,
leading to a homoleptic structure [30] similar to that suggested for
the dicopper and disilver acetylides [7].
Detonation velocity and pressure of Hg(C„CH)2.
Density
(g/cm3)
Detonation velocity
(mm/s)
Detonation pressure
(kbar)
It is confirmed that Hg(CBC–H)2 undergoes an explosive and
exothermal deflagration when heated either in an open aluminium
crucible or in a sealed stainless steel crucible under a nitrogen flow.
The thermal decomposition of Hg(C„C–H)2 releases 725.6 J/g with
an onset temperature of about 250 °C and a peak temperature of
287 °C. The decomposition occurs at a higher temperature in a
sealed crucible (onset 326 °C and peak 337 °C). As shown in Table 1,
the amount of heat released in the explosive decomposition of
Hg(CBC–H)2, 181.9 kJ/mol, is the same as that released by the
explosive decomposition of dicopper acetylide (Cu2C2).
Mercury
fulminate
Lead azide
TNT
RDX
HMX
HNB
3.3
4.5
n.a.
3.8
1.6
1.8
1.9
2.0
2.0
2.1
4.5
6.8
8.8
9.1
9.4
n.a.
190
338
390
406
420
500
CL-20
ONC
9.4
10.1
Hg(C„CH)2
5.3
10.5
760
Using the TGA–FT-IR analytical technique it has been deter-
mined that the thermal decomposition of Hg(C„C–H)2 occurs un-
der N2 according to reaction (3), with the formation of elemental
carbon, elemental mercury and acetylene. Through reaction (3)
and the amount of heat produced in the deflagration of mercury
diacetylide it was possible to calculate, for the first time, the en-
Note: All the data in the table are from Refs. [42] and [47], with the exclusion
of the data on Hg(C„CH)2 which are the result of the present work. TNT =
trinitrotoluene; RDX = cyclotrimethylenetrinitramine; HMX = cyclotetramethyl-
enetetranitramine; HNB = hexanitrobenzene; CL-20 = hexanitrohexaazaisowurtzi-
tane; ONC = octanitrocubane.
thalpy of formation of mercury diacetylide:
D
Hf
=
Hg(C„C–H)2
Detonation velocity½mm=msꢅ ¼ 1:01ðNM1=2Q1=2
Þ
1=2ð1 þ 1:30q0Þ
+ 408.5 kJ/mol. The activation energy for the thermal decomposi-
tion of mercury diacetylide was ascertained to be about 153
kJ/mol, a value comparable to that of silver and lead azides. Such
a relatively high activation energy makes mercury diacetylide rel-
atively insensitive to friction and shock-induced explosion, as in
the case of the mentioned azides.
ð10Þ
Detonation pressure½kbarꢅ ¼ 15:58
q
20NM1=2Q1=2
ð11Þ
where N = moles of detonation gases per gram of explosive,
M = average molecular weight of these gases, Q = chemical energy
of the detonation [cal/g], and q
0 = density of the explosive [g/cm3].
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