Thermochemistry of adducts of some bivalent transition metal bromides with aniline

Article abstract of DOI:10.1016/j.tca.2005.11.026

The compounds [MBr₂(an)₂] (where M is Mn(II), Fe(II), Co(II), Ni(II), Cu(II) or Zn(II); an = aniline) were synthesized and characterized by melting points, elemental analysis, thermal studies, and electronic and IR spectroscopy. The

Full text of DOI:10.1016/j.tca.2005.11.026

Thermochimica Acta 441 (2006) 1–7
Review
Thermochemistry of adducts of some bivalent transition
metal bromides with aniline
Pedro Oliver Dunstan∗
Instituto de Qu´ımica, Universidade Estadual de Campinas, C.P. 6154, CEP 13084-971, Campinas, Sa˜o Paulo, Brazil
Received 15 July 2005; received in revised form 4 November 2005; accepted 15 November 2005
Available online 27 December 2005
Abstract
The compounds [MBr2(an)2] (where M is Mn(II), Fe(II), Co(II), Ni(II), Cu(II) or Zn(II); an = aniline) were synthesized and characterized by melt-
ing points, elemental analysis, thermal studies, and electronic and IR spectroscopy. The enthalpies of dissolution of the adducts, metal(II) bromides
and aniline in methanol, aqueous 1.2 M HCl or 25% (v/v) aqueous 1.2 M HCl in methanol were measured. The following thermochemical parame-
ters for the adducts have been determined by thermochemical cycles: the standard enthalpies for the Lewis acid/base reactions (ꢀrH◦), the standard
enthalpies of formation (ꢀfH◦), the standard enthalpies of decomposition (ꢀDH◦), the lattice standard enthalpies (ꢀMH◦) and the standard enthalpies
◦
¯
of the Lewis acid/base reactions in the gaseous phase (ꢀrH (g)). The mean bond dissociation enthalpies of the M(II)–nitrogen bonds (D(M N)) and
the enthalpies of formation of the adducts from the ions in the gaseous phase: M2+(g) + Br−(g) + an(g) → [MBr2(an)2](g), (ꢀfiH◦) have been estimated.
© 2005 Elsevier B.V. All rights reserved.
Keywords: Metal(II) bromides; Transition metals; Thermochemistry; Metal(II)–nitrogen bonds; Dissolution enthalpies
Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2. Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.1. Chemicals. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2. Analytical. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.3. Adducts synthesis. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.4. Infrared spectra . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.5. Thermal studies. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.6. Calorimetric measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.7. Electronic spectra . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3. Results and discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.1. Infrared spectra . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.2. Thermal studies. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.3. Electronic spectra . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.4. Calorimetric measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1
2
2
2
2
2
2
2
2
3
3
3
4
4
6
1. Introduction
by metal halides or pseudo-halides with aniline are not found in
the literature. In earlier papers [1–10], the preparation and the
spectroscopy in the UV, visible and IR spectra of these com-
pounds are described. Also, dielectric measurements have been
made on them [6].
Thermochemical parameters related to transition metal–
nitrogen coordinated bonds in coordination compounds formed
∗
The present paper is a calorimetric study of adducts of some
divalent transition metal bromides and aniline, with the pur-
Tel.: +55 19 37883023; fax: +55 19 37883088.
E-mail address: dunstan@iqm.unicamp.br.
0040-6031/$ – see front matter © 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.tca.2005.11.026

2
P.O. Dunstan / Thermochimica Acta 441 (2006) 1–7
Table 1
Yields in %, appearance and analytical data on the compounds
Compound
Yield mpa (K) Appearance
C
H
N
Br
M
Calculated Found Calculated Found Calculated Found Calculated Found Calculated Found
[MnBr2(an)2]n 82
558
448
532
552
443
531
wh. pw.
l. br. pw.
l. bl. cr.
l. gre. cr.
d. br. pw.
wh. cr.
35.94
35.86
35.59
35.61
35.19
35.03
35.87 3.52
35.75 3.51
35.48 3.48
35.48 3.49
35.16 3.45
35.08 3.43
3.47
3.63
3.36
3.56
3.32
3.40
6.99
6.97
6.92
6.92
6.84
6.81
6.94
6.89
6.88
6.81
6.80
6.81
39.85
39.76
39.46
39.48
39.02
38.84
39.75 13.70
39.70 13.89
39.40 14.55
39.47 14.50
39.00 15.51
38.85 15.89
13.65
13.85
14.60
14.49
15.43
15.90
[FeBr2(an)2]n
[CoBr2(an)2]
[NiBr2(an)2]n
[CuBr2(an)2]
[ZnBr2(an)2]
8
27
54
73
66
l., light; d., dark; wh., white; br., brown; bl., blue; gre., green; pw. powder; cr., crystals.
a
Melting with decomposition.
pose of determining the energy involved in the formation of
the coordinated metal–nitrogen bonds and to determine several
thermochemical parameters for the adducts. The knowledge of
the thermodynamic properties of these compounds is impor-
tant to help the understanding of the coordinated metal–nitrogen
bonds, and to characterize and understand the properties of these
coordination compounds, which eventually could be useful in
determining their potential applications in catalysis or in the
chromatographic separation of metal ions.
desiccator over calcium chloride. In all cases a molar ratio of
salt/ligand of 1/2 was used.
2.4. Infrared spectra
Spectra were obtained with samples in KBr. For the liquid
ligand, a film sandwiched between NaCl plates was used. A
Perkin-Elmer 1600 series FT-IR spectrophotometer in the region
4000–400 cm−1 region was used.
2.5. Thermal studies
2. Experimental
TG/DTG and DSC measurements were obtained in argon
atmosphere in a Du Pont 951TG analyzer with the samples
varying in mass from 3.02 to 6.84 mg (TG/DTG) and from
2.24 to 4.25 mg (DSC), and a heating rate of 10 K min−1 in
the 293–513 K (DSC) and 298–1243 K (TG/DTG) tempera-
ture ranges. TG calibration for temperatures was made with
metallic aluminum as a standard (mp = 660.37 ◦C). The equip-
ment carried out the calibration for mass automatically. The
DSC calibration was made with metallic indium as a standard
2.1. Chemicals
´
Aniline (+99.5% Quımica Fina Ltd.) was purified by distil-
lation through an efficient column. All the anhydrous metal(II)
bromides used in the synthesis of the adducts were of reagent
grade. Solvents used in the synthesis of the compounds and in
the calorimetric measurements were purified by distillation and
˚
stored over Linde 4 A molecular sieves.
l
(mp = 165.73 ◦C, ꢀs H◦ = 28.4 J g−1).
2.2. Analytical
2.6. Calorimetric measurements
Carbon, hydrogen and nitrogen contents were determined
by microanalytical procedures. The metal contents were deter-
mined by complexometric titration with 0.01 M EDTA solution
[11] of the aqueous solution of the adducts. Bromide analysis
was obtained by gravimetry with standard 0.1 M AgNO3 solu-
tion, after the adducts have been dissolved in water [12]. The
capillary melting points of the adducts were determined in a
UNIMELT equipment from Thomas Hover.
All the solution calorimetric measurements were carried
out in an LKB 8700-1 precision calorimeter as previously
described [13]. The solution calorimetric measurements were
performed by dissolving samples of 5.5–110.3 mg of the adducts
or metal(II) bromides in 100 mL of methanol, aqueous 1.2 M
HCl or 25% (v/v) aqueous 1.2 M HCl in methanol and the ligand
aniline in this latter solution maintaining the molar relation of
1/2, equal to the stoichiometry of the adduct. The accuracy of the
calorimeterwasverifiedbydeterminingtheheatofdissolutionof
tris[(hydroxymethyl)amino]methane in 0.1 mol dm−3 HCl. The
result 29.78 0.03 k J mol−1, is in agreement with the value rec-
ommended by IUPAC (−29.763 0.003 kJ mol−1) [14].
2.3. Adducts synthesis
The adducts were prepared by the reaction of the anhy-
drous metal(II) bromides dissolved in hot ethanol (acetone
for Fe(II) bromide) with aniline. A typical procedure is given
bellow.
2.7. Electronic spectra
To a solution of 1.5 g of MnBr2 (6.98 mmol) in 40 mL of hot
ethanol, 1.3 mL of aniline (13.96 mmol) was added, slowly and
drop-wise with stirring. The white solid that formed was filtered
and washed with tree portions of 10 mL of petroleum ether. The
product was dried for several hours in vacuum and stored in a
Spectra in the 350–2000 nm region were obtained with a
UV–vis–NIR Varian-Cary 5G spectrophotometer, with a stan-
dard reflectance attachment for obtaining the spectra of the solid
adducts.

P.O. Dunstan / Thermochimica Acta 441 (2006) 1–7
3
Table 2
data for N H stretching and bending frequencies of adducts and
ligand.
Infrared dataa for aniline and complexes
Compound
N
Hstretching (cm−1
)
N
Hdeformation
)
(cm−1
3.2. Thermal studies
an
3433s, 3355s
1617s
[MnBr2(an)2]
[FeBr2(an)2]
[NiBr2(an)2]
[CoBr2(an)2]
[CuBr2(an)2]
[ZnBr2(an)2]
3302m, 3241m
3384m, b, 2859m, b
3361m, 3315m
3257m, 3211m
3296m, 3230m
3263m, 3219m
1607m
1600m
1603s
1604s
1601m
1604m
Thermogravimetry and derivative thermogravimetry of the
adducts showed that the associated thermal dissociation process
were of different types, with the lost of mass in three steps.
Some of these steps consist of two successive decomposition
processes. They lose part of the ligand in the first two steps (Cu
adduct), or all the ligand and part of the bromine (Mn, Fe and Ni
adducts) or all the ligand, all the bromine and part of the metal
(Zn adduct). In the third step, the adducts of Mn, Co and Ni lose
part of the bromine. The adduct of Fe lost the rest of the bromine
and part of the metal in the third step. The adduct of Cu lose the
rest of the ligand, all the bromine and part of the metal in the third
step. The adduct of Zn lost part of the metal in the third step. In all
cases a residue is observed that is part of the metal or a mixture of
metal and metal bromide. For the heating of metal(II) bromides
the loss of all the bromide and part of the metal or part of the
bromine has been observed, leading to a residue of the respective
metal or a mixture of the metal and metal bromide [22].
The DSC curves of the adducts are consistent with the
TG/DTG data and show endothermic peaks due to melting with
the partial elimination of the ligand, partial elimination of lig-
and or partial elimination of the ligand together with the partial
elimination of bromine. Table 3 presents the thermoanalytical
data on the adducts.
a
Intensity of bands: s, strong; m, medium; b, broad.
3. Results and discussion
All adducts obtained were solids. The yields ranged from
27 to 82%. The yields, melting points, colors, appearance and
analytical data are summarized in Table 1. All of the compounds
melted with decomposition.
3.1. Infrared spectra
The infrared spectra of the adducts is similar to that of free
aniline. There is a shift of the N H stretching and bending modes
of the adducts to lower frequencies, indicating a weakening of
this bond after coordination of the nitrogen atom to the metal
ion [10, 15–21]. Table 2 presents the extracted infrared spectral
Table 3
Thermal analysis of the compounds
Compound
Mass lost (%)
Calculated
TG temperature range (K)
Species lost
DSC peak temperature
368
ꢀH◦ (kJ mol−1
)
Observed
[MnBr2(an)2]
6.97
41.48
36.86
7.8
331–342
416–450
858–976
−0.3 L
57.83
41.48
36.19
15.25a
−0.7 L–0.1 Br
−1.85 Br
[FeBr2(an)2]
[CoBr2(an)2]
[NiBr2(an)2]
[CuBr2(an)2]
48.33
9.96
29.21
48.35
10.80
29.21
11.44a
421–477
477–574
590–787
−2 L–0.1 Br
− 0.5 Br
449
98.51
−1.4 Br–0.1 Fe
43.70
37.81
0.58
44.20
37.74
0.62
17.44a
451–487
827–931
1067–1102
−1.9 L
307
330
452
0.37
0.56
0.06
−0.1 L–1.8 Br
−0.03 Br
17.26
36.66
29.61
17.73
36.53
29.95
15.79a
360–328
328–484
820–918
−0.75 L
335
397
9.68
96.78
−1.25 L–0.4 Br
−1.5 Br
12.51
14.78
61.09
12.40
14.80
61.24
11.56a
369–389
389–552
389–961
−0.55 L
370
423
23.14
12.40
−0.65 L
−0.8 L–2 Br–0.25 Cu
[ZnBr2(an)2]
33.95
58.89
2.38
33.97
58.73
2.30
5.00a
455–497
497–724
724–908
−1.5 L
335
20.01
−0.5 L–2 Br–0.55 Zn
−0.15 Zn
a
Residue at 1243 K.

4
P.O. Dunstan / Thermochimica Acta 441 (2006) 1–7
3.3. Electronic spectra
observed in pseudo-tetrahedral Cu(II) compounds [26,28], the
Cu(II) ion being surrounded by two nitrogen atoms from two
ligand molecules and by two bromide ions. Table 4 contains the
band maxima assignments and calculated ligand field parame-
ters for the adducts.
The ligand field parameters for the Co(II) adduct were cal-
culated according to Lever [23]. According to the number and
position of the bands [24,25] and considering the magnitude of
thecrystalfieldparametersascomparedwiththatofBolster [26],
it is concluded that the Co(II) ion is pseudo-tetrahedrally sur-
rounded [8] by two nitrogen atoms from two ligand molecules
and by two bromide ions. The ligand field parameters for the
NI(II) adduct were calculated according to Reedijk et al. [27]
and Lever [23]. According to the number and position of the
observed bands, and considering the magnitude of the crys-
tal field parameters as compared with that of Bolster [26], it
is concluded that the Ni(II) ion is pseudo-octahedrally [8] sur-
rounded by two nitrogen atoms from two ligand molecules and
by four bromide ions. For the adduct of Mn(II) since only
spin-forbidden bands can be observed in the electronic spec-
tra of high-spin(II) complexes, it is impossible to determine
with accuracy the ligand field parameters. It is however, pos-
sible to deduce the local symmetry, which is pseudo-octahedral
[8,24,26] with two nitrogen atoms from two ligand molecules
and four bromide ions surrounding the Mn(II) ion. The ligand
field parameters for the Fe(II) adduct were calculated according
to Bolster [26]. It is concluded by the position of the absorp-
tion band and considering the magnitude of Dq that the Fe(II)
is pseudo-tetrahedrally surrounded by two nitrogen atoms from
two ligand molecules and by two bromide ions. For the Cu(II)
adduct, the electronic spectrum showed a rather broad band with
two maxima. The intensity and position correspond with those
3.4. Calorimetric measurements
The standard enthalpies of dissolution of metal(II) bromides,
aniline and adducts were obtained as previously reported [13].
The standard enthalpies of dissolution were obtained according
to the standard enthalpies of the following reactions in solution:
MBr2(s) + calorimetric solvent → solution A, ꢀ1H◦
2an(l) + solution A → solution B, ∆2H◦
(1)
(2)
[MBr2(an)2](s) + calorimetric solvent → solution C, ∆3H◦
(3)
Solution B → solution C, ∆4H◦
(4)
The application of Hess’ law to the series of reactions (1)–(4)
gives the standard enthalpies of the acid/base reactions (ꢀrH◦)
according to the reaction:
MBr2(s) + 2an(l) → [MBr2(an)2](s), ꢀrH◦
where ꢀrH◦ = ꢀ1H◦ + ꢀ2H◦ − ꢀ3H◦
(5)
Table 4
Band maxima and calculated ligand field parameters for the compounds
Compound
Band maxima (×103 cm−1
)
d–d
Intra-ligand + charge transfer
28.4, 29.2
[MnBr2(an)2]n
[CuBr2(an)2]
18.5, 23.5
10.5, 12.4
Band maxima (×103 cm−1
)
d–d
Intra-ligand + charge transfer
28.0
ν1
Dq (cm−1
1238
)
[FeBr2(an)2]n
12.38a
Band maxima (×103 cm−1
)
Dq (cm−1
)
B (cm−1
)
Dq/B
β+
d–d
Intra-ligand + charge transfer
ν1
ν2
7.07b
12.9e
ν4
ν3
[CoBr2(an)2]
[NiBr2(an)2]n
15.9c
23.8g
28.4
27.5, 28.6
275
834
983
535
0.28
1.56
1.01
0.52
8.34d
14.1f
β+ = B/B0; B0 = 971 cm−1 (Co2+); B0 = 1030 cm−1 (Ni2+); reference [26].
a
b
c
d
e
f
5
ν1 = 5Eg ← T2g
.
ν2 = 4T1(F) ← A2.
4
ν3 = 4T1(P) ← A2.
4
ν1 = 3T2g ← A2g
.
3
ν2 = 3T1g(F) ← A2g
.
.
3
3
ν4 = 1Eg ← A2g
.
g
3
ν3 = 3T1g(P) ← A2g

P.O. Dunstan / Thermochimica Acta 441 (2006) 1–7
5
Table 5
Enthalpies of dissolution at 298.15 K
Compound
Calorimetric solventa
Number of experiments
i
ꢀiH◦ (kJ mol−1
)
MnBr2(s)
an(l)
[MnBr2(an)2](s)
FeBr2(s)
an(l)
[FeBr2(an)2](s)
CoBr2(s)
an(l)
[CoBr2(an)2](s)
NiBr2(s)
an(l)
[NiBr2(an)2](s)
CuBr2(s)
Methanol
2:1 MBr2–methanol
Methanol
Methanol
2:1 FeBr2–methanol
Methanol
9
4
4
7
6
4
9
4
4
9
4
4
5
5
5
12
4
1
2
3
1
2
3
1
2
3
1
2
3
1
2
3
1
2
3
−76.58 1.78
−4.62 0.10
−17.97 0.90
−79.21 2.10
−11.42 0.45
25.63 1.14
Methanol
−105.99 1.22
−4.73 0.15
−13.16 0.36
−58.75 1.05
−58.88 1.53
−24.18 1.16
−25.41 0.23
−45.62 0.88
9.30 0.50
2:1 CoBr2–methanol
Methanol
1.2 M HCla
2:1 NiBr2–1.2 M HCla
1.2 M HCla
25% 1.2 M HCl–methanolb
2:1 CuBr2–25% 1.2 M HCl–methanolb
25% 1.2 M HCl–methanolb
Methanol
an(l)
[CuBr2(an)2](s)
ZnBr2(s)
an(l)
[ZnBr2(an)2](s)
−48.39 0.54
−4.93 0.09
43.90 0.49
2:1 ZnBr2–methanol
Methanol
4
a
1.2 M HCL = aqueous 1.2 M HCl.
25% 1.2 M HCl–methanol = 25% (v/v) aqueous 1.2 M HCl in methanol.
b
Since the final thermodynamic state of reactions (2) and (3) is
tice enthalpies (ꢀMH◦) and the standard enthalpies of the Lewis
acid/base reactions in the gaseous phase (ꢀrH◦(g)). These lat-
ter values can be used to calculate the standard enthalpies of
the same and ꢀ4H◦ = 0. Table 5 gives the values obtained for the
enthalpies of dissolution of MBr2 (ꢀ1H◦), aniline into the solu-
tion of MBr2 (ꢀ2H◦) and of the adducts (ꢀ3H◦). Uncertainty
intervals given in this table are twice the standard deviation of
the means of 4–12 replicate measurements. Electronic spectra
revealed that the adducts of Mn(II), Fe(II) and Ni(II) in the solid
state are polymeric structures with chains formed by bridges of
bromide ions linking the metallic ions [8]. The thermochemical
parameters were calculated for monomeric hypothetical adducts
of Mn(II), Fe(II) and Ni(II) adducts in the solid phase. From
the values obtained for the standard acid/base reactions (ꢀrH◦)
and by using appropriate thermochemical cycles [13,29,30],
the following thermochemical parameters for the adducts were
determined: the standard enthalpies of formation (ꢀfH◦), the
standard enthalpies of decomposition (ꢀDH◦), the standard lat-
the M N bonds [30], being equal to D(M N) = −ꢀrH◦(g)/2.
¯
Table 6 lists the values obtained for all these thermochem-
ical parameters. Combined errors showed in this table were
calculated from the square root of the sum of the component
errors.
For the determination of ꢀrH◦(g) it was necessary to assume
that the molar standard enthalpies of sublimation of the adducts
were equal to the enthalpy of vaporization of one mol of the lig-
and [30,34] as the melting points and/or thermal studies showed
that the adducts decomposed on heating and were not found in
the liquid phase, and probably not in the gaseous phase.
Based on the ꢀrH◦ values for the adducts, the acidity
∼
order of the salts can be obtained: FeBr2 > CoBr2 ZnBr >
=
2
Table 6
Summary of the thermochemical results (kJ mol−1
)
ꢀrH◦
ꢀfH◦
ꢀsgH◦
ꢀMH◦
ꢀDH◦
ꢀrH◦(g)
¯
D(M
N)
Compound
MnBr2(s)
FeBr2(s)
CoBr2(s)
NiBr2(s)
CuBr2(s)
ZnBr2(s)
an(l)
−384.9a
−249.8a
−220.9a
−212.1a
−141.8a
−328.65a
206b
204b
183b
170b
182.4b
159.7b
55.8 1.1c
31.3 0.7c
[MnBr2(an)2](s)
[FeBr2(an)2](s)
[CoBr2(an)2](s)
[NiBr2(an)2](s)
[CuBr2(an)2](s)
[ZnBr2(an)2](s)
−63.23 2.00
−116.26 2.43
−97.56 1.28
−93.45 2.19
−80.33 1.04
−97.22 0.73
−384.5 3.2
−303.5 3.4
−255.9 2.8
−243.0 2.8
−238.6 2.6
−363.3 2.5
−379.7 3.6
49.4 3.0
227.9 3.3
209.2 2.5
18.2 3.1
31.3 2.4
208.8 2.3
−324.9 3.7
162.5 1.9
−432
−392
−375
4
3
4
−376
−336
−319
4
3
4
188
168
160
2
2
2
−374.3 3.1
−318.5 3.3
159.3 1.7
156.4 1.7
−49.1 3.1
−312.7 3.3
a
See reference [31].
See reference [32].
b
c
See reference [33].

6
P.O. Dunstan / Thermochimica Acta 441 (2006) 1–7
Table 7
Auxiliary data and enthalpy changes of the ionic complex formation in the gaseous phase (kJ mol−1
)
Compound
ꢀfH◦
ꢀrH◦
ꢀfIH◦
Br−
−219.07a
(g)
Mn2+
2522.0 0.1b
2751.6 2.3b
2841.7 3.4b
2930.5 1.5b
3054.5 2.1b
2781.0 0.4b
(g)
Fe2+
(g)
Co2+
(g)
Ni2+
(g)
Cu2+
(g)
Zn2+
(g)
[MnBr2(an)2](g)
[FeBr2(an)2](g)
[CoBr2(an)2](g)
[NiBr2(an)2](g)
[CuBr2(an)2](g)
[ZnBr2(an)2](g)
−330
5
5
4
5
−324.9 3.7
−2588
−2736
−2778
−2854
6
6
6
6
−248
−200
−187
−376
−336
−319
4
3
4
−103.7 4.4
−307.5 4.1
−318.5 3.3
−312.7 3.3
−2894.3 5.7
−2824.6 5.1
a
See reference [31].
b
See reference [32].
¯
assumption that the course of the variation of the enthalpy values
is linear in a hypothetical state without the influence of the ligand
field. The stabilization energies are the difference between the
real and the interpolated values. Thus, it is found that the stabi-
lization energies in the ligand field formed by two bromide ions
and two nitrogen atoms (from two ligand molecules) decreases
in the order: Ni(II) 125 kJ mol−1 > Cu(II) 110 kJ mol−1 > Fe(II)
100 kJ mol−1 > Co(II) 90 kJ mol−1. The adducts of Cu(II) bro-
mide, of the same stoichiometry, formed with pyNO [32], ␣-
picoNO [35] and ␤-picoNO [36] have stabilization energies of
107.5, 115 and 129 kJ mol−1, respectively. This means that the
order of stabilization energies of the ligands is: pyNO < an < ␣-
picoNO < ␤-picoNO.
NiBr2 > CuBr2 > MnBr2. Using the D(M N) values, the order is:
FeBr2 > CoBr2 > MnBr2 > NiBr2 > CuBr2 > ZnBr2.
The enthalpies for the process of an hypothetical complex
formation in the gaseous phase from metal(II) ions, bromide
ions and aniline molecules can be evaluated:
M2+(g) + 2Br−(g) + 2an(g) → [MBr2(an)2](g), ꢀfIH◦
(6)
with
ꢀfIH◦ = ꢀfH◦(adduct(g)) − ꢀfH◦(M2+(g)) − 2ꢀfH◦
(Br−(g)) − 2ꢀfH◦(an(g)).
Table 7 lists the values obtained for these enthalpy values.
The environment around the M(II) ions is pseudo-tetrahedral
(two Br ions and two N atoms). The correlation of the ꢀfIH◦
values with the metal atomic number is presented in Fig. 1. It
shows part of the double periodic variation profile. The ꢀfIH◦
values obtained depends on the electronic structure of the central
ion. The course of that relation allows determining graphically
the thermodynamic stabilization energy in the ligand field, on the
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