4
U.G. da Silva Jr. et al. / Thermochimica Acta 450 (2006) 2–4
Table 3
The infrared data (Table 1) shown that for cadmium adducts,
the ν(NH)as band exhibits “progressive” negative (cm−1) with
respect to the free ligand from Cl to I: 3340 (Cl), 3306 (Br) and
3298 (I), and that the Br and I compounds exhibit very close
shift values. So, it is possible that larger negative shifts of the
ν(NH)as band are associated with weaker metal-ligand bonds.
The same hypothesis it can be pointed out for the ν(NH)s band.
Based on the dissolution enthalpy values shown in Table 3.
The cadmium halides adducts are those with higher dissolution
enthalpy, suggesting that these compounds exhibit higher inter-
molecular forces, as also suggested by the melting temperatures.
On the other hand, comparing only the chloride compounds,
can be verified that the dissolution enthalpy values follows the
sequence: Hg > Cd > Zn.
Dissolution enthalpies for group 12 halide adducts with aniline
Process
ꢀHdiss. (kJ mol−1
)
an (l) + EtOH → an (sol)
0.24 0.06
−46.70 0.58
−53.18 0.14
−36.05 3.57
−24.25 0.39
−2.28 0.33
−7.58 0.30
−12.45 0.84
−43.39 1.33
−37.93 0.64
−48.23 0.50
−4.47 0.27
−46.16 0.34
ZnCl2 (s) + an (sol) → ZnCl2·2an (sol)
ZnBr2 (s) + 3/2an (sol) → ZnBr2·1.5an (sol)
ZnCl2·2an (s) + EtOH → ZnCl2·2an (sol)
ZnBr2·1.5an (s) + ZnBr2·1.5an (sol)
CdCl2 (s) + 2an (sol) → CdCl2·2an (sol)
CdI2 (s) + 2an (sol) → CdI2.2an (sol)
CdBr2 (s) + 2an (sol) → CdBr2·2an (sol)
CdCl2·2an (s) + EtOH → CdCl2·2an (sol)
CdBr2·2an (s) + EtOH → CdBr2·2an (sol)
CdI2·2an (s) + EtOH → CdI2·2an (sol)
HgCl2 (s) + 2an (sol) → HgCl2·2an (sol)
HgCl2·2an (s) + EtOH → HgCl2·2an (sol)
Considering the ΔrHmθ as well as the D(M − N) values in
Table 4, the following acidity sequence for the halides can be
established: Zn > Cd > Hg which is also related with the hardness
of the metal halides. That is, higher ΔrHmθ and D(M − N) values
are related with harder metal halides. Considering only the cad-
mium adducts, the higher ΔrHmθ value is observed for the iodide
compound, that is, larger and softer anions favor higher values
of ΔrHmθ , probably due to the fact that higher anions allows a
better insertion of the ligand into the crystalline network of the
metal salt. Such better insertion is not related, apparently, with
a shorter metal-ligand distance, since from Br to I compounds,
higher values of ΔrHmθ are not associated with higher values of
D(M − N).
Table 4
Thermochemical parameters for group 12 halide adducts with aniline
ΔrHmθ
ΔDHmθ
ΔMHmθ
ΔgHmθ
D(M − N)
ZnCl2·2an
ZnBr2·1.5an
CdCl2·2an
CdBr2·2an
CdI2·2an
−82.4 3.6
−77.1 0.4
−45.3 1.4
−45.2 0.7
−60.3 0.1
194
−343.0
−291.8
−338.1
−308.3
−309.6
−235.2
−287.2
−236.0
−282.3
−252.5
−253.8
−179.4
143.6
157.3
141.2
126.2
126.9
89.7
160.8
156.9
156.8
171.9
153.0
HgCl2·2an
A summary of the thermogravimetry data are shown in
Table 2. In all compounds the elimination of ligand molecules
occurs in a single step of mass lost. This suggests that, from an
energetic point of view, the two ligand molecules are located at
energetically equivalent coordination sites. With the exception
of zinc chloride, all salts sublimate completely after the release
of ligand molecules. For ZnCl2 the final residue is composed
by zinc oxychloride. For mercury compounds which exhibit a
simultaneous release of gaseous aniline molecules and mercury
halide sublimation, or the sublimation of the mercury halide-
aniline compound itself.
References
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The thermochemical data are summarized in Tables 3 and 4.
For the cadmium halide adducts the metal-ligand bond enthalpy
is greater for the chloride compound and that the Br and I
compounds exhibits very closer metal-ligand enthalpy values.