Thermochemistry of adducts of tin(IV) bromide with ligands containing amide or thioamide groups

Article abstract of DOI:10.1016/S0040-6031(02)00011-4

The compounds [SnBr₄(L)₂] (where L is formamide (fa), acetamide (a), N,N-dimethylacetamide (dma), benzamide (ba), thioacetamide (ta), N,N-dimethylthioformamide (dmtf), N,N-dimethylthioacetamide (dmta) or thiobenzamide (tba)) were synthesized and characterized by melting points, elemental analysis, thermal analysis and IR spectroscopy. The enthalpies of dissolution of the adducts, tin(IV) bromide and ligands in methanol were measured and by using thermochemical cycles the following thermochemical parameters for the adducts have been determined: the standard enthalpies for the Lewis acid/base reactions (Δ_rH), the standard enthalpies of formation (Δ_fH⁰), the standard enthalpies of decomposition (Δ_DH⁰), the lattice standard enthalpies (Δ_MH⁰) and the standard enthalpies of the Lewis acid/base reactions in the gaseous phase (Δ_rH⁰(g). The mean bond dissociation enthalpies of the tin(IV)-oxygen (D_(Sn-S)) and tin(IV)-sulphur bonds (D_(Sn-O)) have been estimated.

Full text of DOI:10.1016/S0040-6031(02)00011-4

Thermochimica Acta 389 (2002) 25–31
Thermochemistry of adducts of tin(IV) bromide with
ligands containing amide or thioamide groups
Pedro Oliver Dunstan*
Instituto de Quımica, Universidade Estadual de Campinas, C.P. 6154, CEP 13083-970 Sa˜o Paulo, Brazil
´
Received 3 August 2001; received in revised form 28 December 2001; accepted 1 January 2002
Abstract
The compounds [SnBr4(L)2] (where L is formamide (fa), acetamide (a), N,N-dimethylacetamide (dma), benzamide (ba),
thioacetamide (ta), N,N-dimethylthioformamide (dmtf), N,N-dimethylthioacetamide (dmta) or thiobenzamide (tba)) were
synthesized and characterized by melting points, elemental analysis, thermal analysis and IR spectroscopy. The enthalpies
of dissolution of the adducts, tin(IV) bromide and ligands in methanol were measured and by using thermochemical cycles the
following thermochemical parameters for the adducts have been determined: the standard enthalpies for the Lewis acid/base
reactions (DrHy), the standard enthalpies of formation (DfHy), the standard enthalpies of decomposition (DDHy), the lattice
standard enthalpies (DMHy) and the standard enthalpies of the Lewis acid/base reactions in the gaseous phase (DrHy(g). The
ꢀ
ꢀ
mean bond dissociation enthalpies of the tin(IV)–oxygen (DðSnÀSÞ) and tin(IV)–sulphur bonds (DðSnÀOÞ) have been estimated.
# 2002 Elsevier Science B.V. All rights reserved.
Keywords: Tin(IV); Tin(IV) bromide; Sn–O and Sn–S bonds; Amide and thioamide; Thermochemistry
1. Introduction
thioures derivatives [1] and those obtained here for
the adducts of acetamide and thioacetamide are estab-
lished. Correlations between the bond energies and
those obtained for the adducts of tin(IV) chloride with
the same ligands [2] with the acidity of the tin(IV)
halides, are also established.
In a recent article [1] we described the preparation,
characterization and thermochemistry of adducts of
tin(IV) bromide with substituted urea and thiourea. In
this article, we describe the synthesis of adducts of
tin(IV) bromide with substituted acetamide and thioa-
cetamide, and hence obtain the enthalpies at formation
of the adducts. The effect of substitution of hydrogen
atoms in acetamide or thioacetamide by methyl groups
and the substitution of the methyl group by a phenyl
group or hydrogen on the energy of the Sn–O or Sn–S
bonds is also studied. Correlations between the bond
energies and the basicity of the ligands and between
the bond energy values obtained for the urea and
2. Experimental
Due to the moisture sensitivity of the compounds,
all preparations and manipulations were carried out
under a dry nitrogen atmosphere.
2.1. Chemicals
* Tel.: þ55-19-37883088; fax: þ55-19-37883088.
Tin(IV) bromide (99%, Aldrich) was purified by
distillation through an efficient column (mp ¼ 30 8C).
E-mail address: dunstan@iqm.unicamp.br (P.O. Dunstan).
0040-6031/02/$ – see front matter # 2002 Elsevier Science B.V. All rights reserved.
PII: S 0 0 4 0 - 6 0 3 1 ( 0 2 ) 0 0 0 1 1 - 4

26
P.O. Dunstan / Thermochimica Acta 389 (2002) 25–31
Formamide (99%, Carlo Erba), dimethylformamide
(99% Riedel-deHaem), dimethylacetamide (99% z.s.,
Merck), and dimethylthioformamide (99%, Aldrich )
were purified by distillation through an efficient
column (bp ¼ 207, 150, 162 and 55 8C per 3 mmHg,
respectively). Acetamide (99%, Carlo Erba), benza-
mide (98% z.s., Merck), thioacetamide (99% p.a.,
Merck), dimethylthioacetamide (East-man) and thio-
benzamide (Aldrich ) were purified by recrystallation
from suitable solvents (mp ¼ 78À80, 123–124 , 109–
110 and 70–71 8C, respectively). Solvents used in the
synthesis of the adducts and calorimetric measure-
ments were purified by distillation and stored over
2.4. Infrared spectra
Spectra were obtained with samples in KBr matrix
for adducts and solid ligands. For liquid ligands, a
film of the ligand sandwiched between NaCl plates
was used. A Perkin-Elmer 1600 series FTIR spectro-
photometer in the 4000–400 cmÀ1 region was used.
2.5. Thermal studies
TG/DTG and DSC measurements were obtained in
an argon atmosphere in a Du Pont 951 TG analyzer
with samples varying in mass from 3.88 to 10.94 mg
(TG/DTG) and from 3.06 to 4.73 mg (DSC) and a
heating rate of 10 K minÀ1 in the 298–673 K (DSC)
and 298–1173 K (TG/DTG) temperature ranges. TG
calibration for temperature was made using metallic
aluminum as a standard (mp ¼ 660:37 8C) and the
calibration for mass was carried out automatically by
the equipment. The DSC calibration was made using
metallic indium as a standard (mp ¼ 165:73 8C,
Dgs Hy ¼ 28:4 J gÀ1).
˚
Linde 4 A molecular sieves.
2.2. Analytical
Carbon, hydrogen and nitrogen contents were
determined by microanalytical procedures. Tin was
determined by gravimetry as stannic oxide following
precipitation of hydrated stannic oxide by ammonia
solution from the nitric acid solutions of the adducts.
The precipitate was ignited to stannic oxide. The
filtrate was used for the determination of bromine
content as silver bromide by using N/10 AgNO3
solution. Samples where amide or thioamide inter-
fered with the estimation of tin were initially reacted
with concentrated HNO3 prior to analysis. Bromine in
these samples was determined as silver bromide after
refluxing for several hours with sodium carbonate
solution.
2.6. Calorimetric measurements
All the solution calorimetric determinations were
carried out in an LKB 8700-1 precision calorimeter
as described elsewhere [3]. The solution calorimetric
measurements were performed by dissolving sam-
ples of 4.6 – 204.6 mg of the adducts or tin(IV)
bromide in 100 ml of methanol and the ligand in
this latter solution, maintaining a molar relation
equal to the stoichiometry of the adduct. The accu-
racy of the calorimeter was carried out by determining
the heat of dissolution of Tris[(hydroxymethyl)amino]
methane in 0.1 mol dmÀ3 HCl. The result (À29:78 Æ
0:03 kJ molÀ1) is in agreement with the value reco-
2.3. Synthesis of the adducts
The adducts were obtained by the reaction of
tin(IV) bromide and ligands in solution. A typical
procedure is SnBr4–dma. On mixing a solution of
2.4 g of SnBr4 (5.48 mmol) in 20 ml of petroleum
ether with 1.10 ml of dma (11.8 mmol), a light yellow
solid appeared. This was stirred for several hours. The
solid was filtered, washed with three portions of 10 ml
of petroleum ether, dried for several hours in vacuum
and stored in a desiccator over calcium chloride. All
the adducts were prepared with a molar relation
tin(IV) bromide/ligand of 1/2. Most of the adducts
were washed with petroleum ether. A half of the
adducts were prepared from chloroform or carbon
tetrachloride solutions.
mmended by IUPAC (À29:763 Æ 0:003 kJ molÀ1
)
[4].
3. Results and discussion
All the adducts obtained were solids. The yields
ranged from 37 to 91%. The yields, melting points,
colours, appearance and analytical data are summar-
ized in Table 1.

P.O. Dunstan / Thermochimica Acta 389 (2002) 25–31
27
Table 1
Yields, melting points, appearance and analytical data of the adducts
Compound
Yield (%) Mp (8C)a Appearanceb
C
H
N
Sn
Br
Calculated Found Calculated Found Calculated Found Calculated Found Calculated Found
[SnBr4(fa)2]
[SnBr4(a)2]
37
79
85
83
91
41
89
240
ye. pas. s.
hy. ye. pw;
l. ye. Pw.
l. ye. Pw.
wh. pw.
ye. pw.
4.55
8.63
4.55 1.14
8.45 1.81
12.50 2.41
15.40 2.96
24.66 2.07
8.25 1.71
11.71 2.29
14.75 2.81
23.37 1.98
1.20
–
–
22.46
22.13 60.49
20.99 57.44
19.99 54.68
19.58 52.18
17.66 46.79
19.97 54.31
19.15 51.83
18.62 49.58
16.46 44.85
60.84
57.04
54.26
51.95
46.42
53.98
51.60
49.88
45.00
155–156
100–102
201–203
190–191
95–97
2.01 5.03
2.55 4.79
2.98 4.57
2.07 4.12
5.13 21.33
5.10 20.31
4.48 19.38
4.17 17.44
[SnBr4(dmf)2]
[SnBr4(dma)2]
[SnBr4(ba)2]
SnBr4(ta)2]
12.33
15.69
24.71
8.16
1.80
–
–
20.17
[SnBr4(dmtf)2]
200–202
173–175
179–180
ye. pw.
11.69
14.90
23.59
2.30 4.54
2.79 4.35
1.96 3.93
4.55 19.25
4.35 18.41
3.84 16.65
[SnBr4(dmta)2] 80
[SnBr4(tba)2] 84
ye. pw.
ye. pw.
a Melting point with decomposition.
b
Key: l., light; pas., pastry; hy., hygroscopic; ye., yellow; wh., white; s., solid; pw., powder.
3.1. Infrared spectra
decrease of the CO or CS stretching frequencies and an
increase of the CN stretching frequency [5,6]. The NH
stretching frequency decreases on the formation of the
adduct, via the nitrogen atom and no appreciable
change will be observed when the adduct is formed
via the oxygen atom of the ligand [5,6]. The IR spectra
of the amide or thioamide adducts showed negative
shifts of the carbonyl or thiocarbonyl stretching fre-
quencies and positive shifts in the CN stretching fre-
quencies, both with respect to free ligands, suggesting
The more important IR bands are reported in Table 2.
If coordination occurs through the nitrogen atom of the
ligand, the double bond characterof CO or CS increases
and that of CN decreases, resulting in an increase of the
CO or CS stretching frequencies and a decrease of the
CN stretching frequency. Coordination via oxygen
atom leads to the decrease of the double bond character
of CO or CS and an increase of CN resulting in a
Table 2
Main IR absorption bands (cmÀ1) of complexes and free ligands
Compound
Assignmenta
nNH
nco/ncs
Phenyl
nCH þ nNH
nCN
fa
3325vs
3412vs
3191s
1685vs
1682vs
1666s
1605s
1091m
1110w
1006w
1042w
1133s
[SnBr4(fa)2]
a
1553m
1609sh
1558m
[SnBr4(a)2]
dmf
3361s
1652vs
969s
[SnBr4(dmf)2]
dma
924s
1140s
1645vs
1601vs
1660vs
1636vs
720s
1013m
1026m
1030m
1076m
1304s
[SnBr4(dma)2]
ba
3171s
3334s
3303s
3376s
1580vs
1600s
1627vs
1527vs
n.o.
[SnBr4(ba)2]
ta
[SnBr4(ta)2]
dmtf
697vs
1483m
1363s
969s
1133s
[SnBr4(dmtf)2]
dmta
924s
1140s
864m, 655m
844m, 644m
687s
1125m
1181m
1280m
1305m
[SnBr4(dmta)2]
tba
3360m
3379m
1598m
n.o.
1520sh
1469m
[SnBr4(tba)2]
651s
a Key: n, stretching; d, angular deformation; phenyl, nCC in the phenyl group. Intensity of bands: vs, very strong; s, strong; m, medium; sh,
shoulder; n.o., not observed.

28
P.O. Dunstan / Thermochimica Acta 389 (2002) 25–31
coordination through the oxygen or sulphur atoms.
Positive shifts of the NH stretching frequencies were
also observed [7].
III. ½SnBr4ðLÞ2Š ! 0:9 SnBr4 þ ½SnBr4Þ0:1ðLÞ2Š
½ðSnBr4Þ0:1ðLÞ2Š ! pyrolysis.
The adducts of fa, a, dmf, dma, ba, dmtf and tba
followed process I with pyrolysis of the adduct in one
step of mass loss. Only in the case of the adduct of dmf
or dma, the pyrolysis is in three or two successive steps
of mass loss. Some of them leaved a residue that is
probably tin or a mixture of tin with carbon [8]. The
adduct of ta followed process II with elimination of
0.5 mol of SnBr4 in a first step of mass loss, followed
by the pyrolysis of the intermediate product in three
3.2. Thermal studies
Thermogravimetry and derivative thermogravime-
try of the adducts show the associated thermal dis-
sociation processes of different types.
I. ½SnBr4ðLÞ Š ! pyrolysis.
II. ½SnBr4ðLÞ2Š ! ½ðSnBr4Þ ðLÞ Š þ 0:5SnBr4
2
0:5
2
½ðSnBr4Þ0:5ðLÞ2Š ! pyrolysis.
Table 3
Thermal analysis data of the compounds
Compound
Mass lost(%)
TG temperature range (K) Species lost
DSC peak temperature (K) DHy (kJ molÀ1
)
Calculated Observed
[SnBr4(fa)2]
[SnBr4(a)2]
100
100
98.4a
493–559
434–473
Pyrolysis
Pyrolysis
364
417
5.89
2.26
99.2b
362
396
418
À0.62
3.83
5.50
[SnBr4(dmf)2] 87.5
6.25
6.25
87.3c
6.9
450–485
485–752
752–825
–SnBr4–dmf
À0.5dmf
414
0.57
4.8
À0.5dmf
–SnBr4–1.5dma
[SnBr4(dma)2] 92.9
7.1
92.6
6.0
1.4d
446–472
472–800
372
411
454
19.59
1.08
À0.5dma
13.35
[SnBr4(ba)2]
[SnBr4(ta)2]
100
98.6
1.4d
467–502
Pyrolysis
492
45.91
65.27
37.2
37.2
19.1
6.4
37.2
34.8
21.3
5.9
408–532
532–571
571–763
763–1238
À0.5SnBr4
–0.5SnBr4
À1.5ta
382
À0.5ta
5.4d
[SnBr4(dmtf)2] 100
97.0b
3.0d
453–504
Pyrolysis
482
475
À95.27
[SnBr4(dmta)2] 62.6
63.2
25.5
5.6
453–531
531–549
À0.9SnBr4
30.06
24.7
4.8
À0.1SnBr4–1.2dmta
À0.3dmta
549–782
1.6
1.5
4.3d
1148–1191
À0.1dmta
[SnBr4(tba)2] 100
97.1
2.9d
462–484
Pyrolysis
451
62.36
a Three overlapping mass lost.
b Two overlapping mass lost.
c Two overlapping peaks.
d Residue at 1238 K.

P.O. Dunstan / Thermochimica Acta 389 (2002) 25–31
29
successive steps of mass loss. The adduct of dmta
followed process III with elimination of 0.9 mol of
SnBr4 in a first step of mass loss followed by the
pyrolysis of the intermediate product in more three
successive steps of mass loss.
The DSC curves of the adducts are consistent with
the TG/DTG data and show endothermic peaks due to
partial elimination of ligands, melting or decomposi-
tion of ligands. Exothermic peaks are also observed
due to pyrolysis of ligands. Table 3 lists the thermo-
analytical data of the adducts.
The application of Hess’Law to the series of reactions
1–4 gives the standard enthalpies of the acid/base
reactions (DrHy) according to reaction 5:
SnBr4ðsÞ þ 2Lðs;lÞ ! ½SnBr4ðLÞ2ŠðsÞ;
DrHy ¼ D1Hy þ D2Hy À D3Hy
(5)
since the final state of reactions 2 and 3 is the same
andD4Hy ¼ 0. Table 4 gives the values obtained for
the enthalpies of dissolution of tin(IV) bromide
(D1Hy), ligands into the solution of SnBr4 (D2Hy)
and of the adducts (D3Hy). Uncertainty intervals given
in this table are twice the standard deviation of the
means of five replicate measurements. Combined
errors were calculated from the square root of the
sum of the square of the component errors.
From the values obtained for the standard enthalpies
of the acid/base reactions (DrHy) and by using appro-
priate thermochemical cycles [3,6], the following
thermochemical parameters were determined: the
standard enthalpies of formation (DfHy), the standard
enthalpies of decomposition (DDHy), the standard
lattice enthalpies (DMHy) and the standard enthalpies
of the Lewis acid/base reactions in the gaseous phase
(DrHy). These latter values can be used to calculate the
standard enthalpies of the Sn–O and Sn–S bonds [6],
3.3. Calorimetric measurements
The standard enthalpies of dissolution of tin(IV)
bromide, ligands and adducts were obtained as pre-
viously reported [8]. The standard enthalpies of dis-
solution were obtained according with the standard
enthalpies of reactions 1–4 in solution:
SnBr4ðsÞ þ methanol ! solution A; D1Hy
2Lðs;lÞ þ solution A ! solution B; D2Hy
(1)
(2)
½SnBr4ðLÞ2ŠðsÞ þ methanol ! solution C; D3Hy (3)
solution B ! solution C; D4Hy
(4)
Table 4
Thermochemical data for the compounds at 298.15 K
Compound
Calorimetric solvent
Number of experiments
DiHy (kJ molÀ1
)
SnBr4(s)
fa(l)
Methanol
2:1 SnBr4-methanol
Methanol
18
5
4
6
4
4
4
5
3
5
6
5
6
4
4
5
5
4
5
5
–79.15 Æ 1.44 (i ¼ 1)
5.29 Æ 0.12 (i ¼ 2)
15.64 Æ 1.00 (i ¼ 3)
27.50 Æ 0.5 (i ¼ 2)
À34.23 Æ 0.57 (i ¼ 3)
À133.50 Æ 1.99 (i ¼ 1)
À45.31 Æ 1.84 (i ¼ 2)
19.76 Æ 0.25 (i ¼ 3)
À7.36 Æ 0.26 (i ¼ 2)
33.71 Æ 0.46 (i ¼ 3)
29.36 Æ 0.83 (i ¼ 2)
À10.38 Æ 0.38 (i ¼ 3)
29.95 Æ 0.69 (i ¼ 2)
58.94 Æ 1.20 (i ¼ 3)
12.88 Æ 0.16 (i ¼ 2)
17.76 Æ 0.24 (i ¼ 3)
37.02 Æ 1.75 (i ¼ 2)
56.55 Æ 1.79 (i ¼ 3)
28.88 Æ 0.55 (i ¼ 2)
28.26 Æ 1.51 (i ¼ 3)
[SnBr4(fa)2](s)
a(s)
2:1 SnBr4-methanol
Methanol
1,2-Dichlorethane
[SnBr4(a)2](s)
SnBr4(s)
dmf(l)
2:1 SnBr4-1,2-dichlorethane
1,2-Dichlorethane
2:1 SnBr4-methanol
Methanol
[SnBr4(dmf)2](s)
dma(l)
[SnBr4(dma)2](s)
ba(s)
2:1 SnBr4-methanol
Methanol
[SnBr4(ba)2](s)
ta(s)
2:1 SnBr4-methanol
Methanol
[SnBr4(ta)2](s)
dmtf(1)
2:1 SnBr4-methanol
Methanol
[SnBr4(dmtf)2](s)
dmta(s)
2:1 SnBr4-methanol
Methanol
[SnBr4(dmta)2](s)
tba(s)
2:1 SnBr4-methanol
Methanol
[SnBr4(tba)2](s)

Table 5
Summary of the thermochemical results (kJ molÀ1
)
DrHy
DfHy
Dg1Hy or Dgs Hy
DMHy
DDHy
DrHy
DðSnÀOÞ or DðSnÀSÞ
ꢀ
ꢀ
Compound
SnBr4(s)
À511.3 [9]
39.8 [9]
fa(l)
À243.5 Æ 4.0a
À317.0 Æ 0.7 [10]
À239.3 Æ 1.1 [11]
À278.32 Æ 1.51 [12]
À202.6 Æ 1.1 [11]
À70.6 Æ 1.1a
À16.0g
52.5 Æ 2.6a
78.7 Æ 1.1 [10]
47.6 Æ 2.0 [11]
50.10 Æ 0.20 [13]
105.4 Æ 2.3a
83.26 Æ 0.34a
76.1g
a(s)
dmf(l)
dma(l)
ba(s)
ta(s)
dmf(l)
dmta(s)
À66.6g
78.62g
109.95 Æ 2.03a
tba(s)
À22.6 Æ 3.1a
À953.9 Æ 8.2
À1028.8 Æ 2.4
À1054.6 Æ 3.1
À1054.2 Æ 3.5
À619.4 Æ 2.3
À626.7 Æ 3.1
À493.4 Æ 2.0
À542.7 Æ 5.0
À501.1 Æ 6.7
[SnBr4(fa)2](s)
[SnBr4(a)2](s)
[SnBr4(dmf)2](s)
[SnBr4(dma)2](s)
[SnBr4(ba)2](s)
[SnBr4(ta)2](s)
[SnBr4(dmtf)2](s)
[SnBr4(dmta)2](s)
[SnBr4(tba)2](s)
À89.49 Æ 1.76
À17.42 Æ 1.64
À198.57 Æ 2.72
À120.22 Æ 1.53
À39.41 Æ 1.70
À108.14 Æ 2.00
À84.03 Æ 1.47
À98.68 Æ 2.89
À78.53 Æ 2.16
À257.3 Æ 5.9
À237.6 Æ 3.4
À356.6 Æ 5.3
À283.2 Æ 2.5
À313.0 Æ 3.4
À337.4 Æ 2.9
À299.0 Æ 3.2
À318.7 Æ 5.3
À361.2 Æ 5.0
194.5 Æ 5.5
174.8 Æ 2.7
293.8 Æ 4.8
220.42 Æ 1.58
250.2 Æ 4.9
274.66 Æ 2.11
236.2 Æ 2.5
257.9 Æ 4.9
298.43 Æ 4.60
À204.8 Æ 6.3
À158.9 Æ 3.6
À309.0 Æ 5.6
À233.1 Æ 2.5
À207.6 Æ 5.8
À254.2 Æ 2.9
À222.9 Æ 3.3
À240.1 Æ 5.7
À251.3 Æ 5.5
102.4 Æ 3.2
79.5 Æ 1.8
54.5 Æ 2.8
116.6 Æ 1.3
103.8 Æ 2.9
127.1 Æ2.0
111.5 Æ 1.7
120.1 Æ 2.9
125.6 Æ 2.7
a See text.
g [6].

P.O. Dunstan / Thermochimica Acta 389 (2002) 25–31
31
being equal to DðSnÀOÞ or DðSnÀSÞ ¼ ÀDrHyðgÞ=2
Comparing the bond energy values obtained here
with the values obtained for the adducts of tin(IV)
chloride with the same ligands [2], the acidity order of
the tin(IV) halides is: SnCl4 > SnBr4as would be
expected from an inductive effect, due to chlorine
atoms having greater electronegativity values than the
bromine atoms. The mean tin(IV)–oxygen coordinate
bond dissociation energies of the adducts of tin(IV)
bromide with amides are weaker than the tin(IV)–
sulphur bonds in the thioamide adducts.
The mean energies of the Sn–O and Sn–S bonds
in adducts of tin(IV) bromide with amides or thioa-
mides that are derivatives of acetamide or thioaceta-
mide are weaker than those of the adducts of tin(IV)
bromide with ligands that are derivatives of urea or
thiourea [1].
ꢀ
Table 5 lists the values obtained for all these thermo-
chemical parameters. For the determination of
DrHy(g) it was necessary to assume that the molar
standard enthalpies of sublimation of the adducts were
equal to the enthalpies of sublimation or vaporization
of one mol. of the respective ligand [15,16], as the
melting points and 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 DrHy values, the basicity order
is: dmf > dma > ta > dmta > fa > dmtf > tba >
ꢀ
ꢀ
ba > a. Using the values of DðSnÀOÞ andDðSnÀSÞ
dmf > ta > tba > dmta > dma > dmtf > ba > fa >
a. The expected order on the basis of an inductive
effect would be that in which the amides are stronger
bases than the thioamides due to the electronegativity
value of the oxygenatom begreater than thevalue of the
sulphur atom. Among the amides or thioamides, those
with greater substitution of hydrogen atoms by methyl
groups, would be stronger bases than the unsubstituted
amide or thioamide, due to the electron-donating char-
acter of the methyl group. Those amides with substitu-
tion of a hydrogen atom by the electron-withdrawing
phenyl group, would be weaker bases than the unsub-
stituted amide or thioamide. Thus, dma > dmf >
a > fa > ba and dmta > dmtf > ta > tba. The seq-
uence is observed here but with the inversion between
some members of the expected series. The participation
of another kind of interaction like hydrogen bonds or
the phenyl group taking part of the bonds formed
between tin and oxygen or tin and sulphur atoms.
According to hard/soft acid/base (HSAB) [14–16], it
is expected the amides to be hard, the thioamides to be
soft and tin(IV) bromide to be soft. Then, the thioa-
mides are stronger bases for tin(IV) bromide. Overall, it
is so observed, when comparing one amide with its
thio-derivative. A exception is found in the case of
dmf and dmtf. The presence of methyl groups leads to a
relative softness of the amide. Then, dmf is soft and
dmtf is softer. If tin(IV) bromide is borderline, it would
form the stronger bond with dmf.
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