Thermochemistry of pyridine- and picoline-N-oxide complexes of arsenic trihalides

Thermochimica Acta, 181 (1991) 143-154 Elsevier Science Publishers B.V., Amsterdam

143

THERMOCH.EMISTRY OF PYRIDINE- AND PICOLINE-N-OXIDE COMPLEXES OF ARSENIC TRIHALIDES

P.O. DUNSTAN Institute de Quimica, Universidade Estadual de Campinas, C.P. 0154, CEP 13.081, Campinas, Srio Paulo (Brazil) (Received 7 September 1990)

ABSTRACT The complexes AsX,-nL (where L = pyridine-N-oxide (pyNO), (Y, /3,or y-picoline-N-oxide ((Y, /3-, or y-piCONO), n =1 or 2 for X = Cl, n = 1, 3/2 or 2 for X = Br and n =l, 3/2 or 2 for X = I) were prepared and characterised by melting point, elemental analysis, thermogravimetric studies and infrared spectroscopy. From the enthalpies of dissolution in ethanol or 50% 2.4 N HCl in ethanol of the complexes, arsenic trihalides and ligands at 298.15 K, the standard enthalpies (A,H*) for the Lewis acid/base reactions were determined. From the A,H* values, the standard enthalpies of formation of arsenic trihalides and the standard enthalpies of formation of the ligands, the standard enthalpies of formation of the complexes ( Ar H * ) were calculated. The standard enthalpies of decomposition of the complexes (AoH*), as well as the lattice standard enthalpies (AMH “) and the standard enthalpies of the Lewis acid/base reactions in the gaseous phase (A,H *(g)), were calculated by means of thermochemical cycles, and the mean standard enthalpies of the arsenic-oxygen bonds (B (As-O)) were estimated. The mean energies of the As-O bonds range from 98 to 125 kJ mol-’ in these complexes. The thermochemical data suggested the order AsCl, > AsBr, > AsI, for acidity and y-piCON0 > pyN0 > cu-piCON for basicity. Correlation between several of the thermochemical parameters, and between the thermochemical parameters of these complexes and parameters of the complexes of non-oxidized ligands, were established.

INTRODUCTION

In a recent article [l], we described the synthesis, characterisation and thermochemistry of complexes of arsenic trihalides with pyridine, and pand y-picoline, for which mean arsenic-nitrogen coordinate bond energies were determined. This paper describes the interaction of arsenic trihalides with pyridine-N-oxide, and a-, p- and y-picoline-N-oxide, forming complexes in which the oxygen atom is the coordinating atom of the ligands. The strength of the As-O bond in amide complexes of arsenic trihalides ranges from 89 to 150 kJ mol-’ [2]. In the present paper, we report the calorimetric measurements made on a series of trihalide complexes, which 0040-6031/91/$03.50

0 1991 - Elsevier Science Publishers B.V.

144

are analogous with the complexes of pyridine and picoline, in order to study the influence of introducing an oxygen atom into the ligand structure and to compare the strengths of the As-N and As-O coordinate bonds.

EXPERIMENTAL

Due to the moisture sensitivity and toxic nature of the compounds involved, all preparations were carried out in polyethylene glove bags under a dry nitrogen atmosphere. Chemicals The arsenic trihalides were prepared as described before [l]. The ligands were prepared by methods outlined in the literature [3-51. Solvents were purified by distillation. Analytical Carbon, hydrogen and nitrogen were determined by microanalytical procedures. Halogen analysis was by gravimetry using N/10 AgNO, solution [6], following dissolution of the complexes in water. Arsenic content was determined by redox titration of the aqueous solutions of the complexes with standard 0.05 M iodine solution, until the starch indicator turned blue

[71Synthesis

of the complexes

The complexes were prepared as described in previous papers [1,2]. A typical procedure is given here for AsCl, - pyN0. A solution of 2.27 g of pyN0 (23.8 mmol) in 20 ml of benzene was added slowly, dropwise with stirring to a solution of 2.0 ml of AsCl, (23.8 mmol) in 20 ml of benzene. The stirring was maintained for at least three hours. After removal of the solvent, the white solid formed was washed with three 20 ml portions of petroleum ether, and then dried for several hours in a vacuum. The compound obtained was stored in a desiccator over calcium chloride. For the synthesis of the AsBr, complexes, chloroform, carbon tetrachloride or 50% Ccl,-n-hexane were used as solvents, and for the AsI, complexes, CS, was used. Infrared spectra The spectra were obtained with complex and free ligand sample mulls in Nujol sandwiched between NaCl plates, using a Perkin-Elmer 180 spectrophotometer.

145

Thermogravimetric

studies

These were carried out in a nitrogen atmosphere in a Du Pont 1090 thermogravimetric analyser, with samples varying in weight from 2.41 to 9.41 mg and a heating rate of 10 K mm-‘. Calorimetric measurements All the solution calorimetric determinations were carried out in an LKB 8700-l precision calorimeter as previously described [1,2].

RESULTS AND DISCUSSION

Nearly all the arsenic trihalide complexes were prepared with a molar ratio of 1: 1 between AsX, and the ligands. In the preparation of AsBr, 2pyNO and AsI,. 2y-piCON0, a molar ratio of 1: 2 was used because products of indefinite stoichiometries, were obtained with a 1 : 1 molar ratio. The yields of the complexes obtained ranged from 60 to 100%. The melting points, colours and analytical data of the complexes are summarised in Table 1. Infrared data The infrared spectra of pyN0 complexes showed a small dislocation, splitting or decrease in intensity of some bands when compared with the spectrum of free pyN0. Coordinated pyN0 is distinguished from free pyN0 by a decrease in the frequency of the NO stretching vibration (at 1250 cm-’ in the free ligand) upon coordination [8]. The AsI, complex showed the smallest decrease. The main infrared bands and their assignments [8-lo] for pyN0 and its complexes are presented in Table 2. The infrared spectra of cw-piCON complexes showed a decrease in the frequency of the NO stretching vibration (at 1252 cm-’ in the free ligand) upon coordination. There were also small dislocations of some of the other bands. The spectra of the mono- and bis-complexes of AsBr, are similar; only small differences were observed. The NO stretching vibration was split into two bands and a new strong band appeared at 762 cm-’ in the bis-complex. Table 3 shows the main infrared bands and their assigments [ll] for a-piCON and its complexes. The infrared spectra of P-piCON complexes are similar to that of the free ligand, except for small dislocations and a decrease in intensity of some bands when compared with the spectrum of free a-piCON0. Appreciable dislocations toward lower frequencies are observed after coordination for the absorption attributed to NO stretching vibration in the free ligand

Yield

65 64 67 51 50 60 68 78 88 51 100 57 60

(%I

%C

21.73 24.82 36.08 24.82 23.79 17.01 27.05 17.01 22.60 18.60 12.76 17.45 21.39

Calc. 20.80 24.75 36.20 24.98 24.00 16.46 26.70 16.80 21.70 18.50 13.03 17.20 21.19

Found

Found 2.00 2.62 3.90 2.61 2.26 1.97 3.00 1.70 2.30 1.70 1.47 1.40 2.19

1.82 2.43 3.53 2.43 2.00 1.66 2.65 1.66 2.21 1.56 1.25 1.14 2.09

%H Calc.

’ Uncorrected. b Key: wh, white; ye, yellow; or, orange; br, brown; s, slightly; pw, powder; cr, crystal.

syepw syepw syepw syepw syepw or pw or pw br pw or pw

wh cr wh cr wh cr whcr

AsCl,. pyNQ AsCl s- a-piCON AsCl seZ&piCONO A&I 3. y-piCON AsBr,-2pyNO AsBr,- a-piCON AsBr,,Za-piCON AsBr,.&piCONO 2AsBr,.3y-piCON As1 s- 2pyNO AsI,, a-piCON 2As1,. 3/I-piCON AsI,.Zy-piCON

71 98 120 70 99 65 60 67 79 99 71 71 93

Appearance b

Compound

Melting points, appearance, yield and analytical data of the complexes

TABLE 1

%N

5.07 4.82 7.01 4.82 5.55 3.31 5.26 3.31 4.39 4.34 248 3.39 4.16

Calc. 4.95 4.65 6.90 4.85 5.70 3.58 5.07 2.99 4.34 4.00 2.36 3.40 4.13

Found

%As

27.11 25.80 18.75 25.80 14.84 17.68 14.06 17.68 15.66 11.60 13.70 12.10 11.12

Calc.

27.22 26.02 19.01 25.49 14.72 17.74 14.36 17.55 15.57 11.68 13.61 12.21 11.39

Found

38.48 36.62 26.62 36.62 47.48 56.57 44.98 56.57 50.12 58.95 67.41 61.47 56.49

38.25 36.56 26.85 36.68 47.27 56.36 44.79 56.40 50.17 58.68 67.24 61.62 56.90

Found

% Halogen CaIc.

147 TABLE 2 Assignment

of infrared

PYNO

AsCl,.pyN0

AsBr,.2pyNO

AsI,.2pyNO

Assignment

3065m,sh

3056m,sh

3065s,sh

CH str. (v)

1613~ 1566m n.0.

1619~

Ring def. in-plane

n.0.

1508~

no.

1313w

n.0.

CH in-plane

def. (p)

1180s

1244m 115ow

NO str. (v) CH m-plane

def. (j3)

1010s 926~

3084m 3064m 3044m 1615m 1591m 1535m 1517m 1507m 1443m 1328m 1255m 1198s 1183s 1148~ 1096~ 1068w 1048~ 1023m 953w

1018~ 938~

Ring breathing CH out-of-plane

840s

820s

760s

769s

670s

665s

1598s 1551s 1498m

1250s,b 1168s

1073m

a Intensity of bands: n.o., not observed.

frequencies

(cm-‘)

of pyN0

and its complexes

a

(cy)

1143m 1090m 1058m 1020m 940s 850s 832s 810s 760s

680m 660s vs, very strong;

834~ 800m 765m 740m 720m 665s

s, strong, m, medium;

def. ( y)

NO def. (8) CH out-of-plane

def. ( y)

? Ring def. out-of-plane

w, weak; sh, shoulder;

(+)

b, broad;

(double band at 1280-1270 cm-‘). Table 4 shows the infrared data and their assigments [12] for /3-piCON and its complexes. The y-piCON complexes showed infrared spectra similar to that of free y-piCON0. The NO stretching vibration of the free ligand (1246 cm-‘) suffered dislocation toward lower frequencies upon coordination. A small dislocation and a decrease of some other bands were also observed. The main infrared bands and their assigments [13] for y-piCON and its complexes are shown in Table 5. Thermogravimetric

data

Thermogravimetry of the compounds indicated three different decomposition schemes: in the first, ligand and arsenic trihalide are lost together in a unique step; in the second, evolution of arsenic trihalide and part of the

148 TABLE

3

Assignment

of infrared

frequencies

(cm-‘)

of a-piCON

and its complexes

a

(YpiCON

AsCl,. apiCON

AsBr,. apiCON

AsBr,.2apiCON

AsI,.apiCON

Assignment

3030m 1612m 1555w 1491s 1252s

no. 1620m 1574w 1490m 1189s

3017m 1621s 1566~ 1487m 1190s

3040m 1618s 1571w 1483s 1210m

CH CC CC CC NO

str. and and and str.

1146~ 1109m 1049m 850s 755s

1151w llllm 1031m 837s 768s

1156m 1113m 1031m 837s 780~s

3040m 1618s 1576m 1486m 1190s 1178s 1156~ 1113s 1032m 838s 780~s 762~s

1157m 1109m 1032m 803s 762m 690m

CH CH CH CH

in-plane def. in-plane def. in-plane def. out-of-plane

(v) CN str. (v) CN str. (v) CN str. (v) (v) (p) (8) (/3) def. (7)

a See Table 2 for key.

ligand (or ligand and part of the AsI,) occurs in the first step followed by the loss of the rest of the ligand (or the rest of the AsI,) in the second step; in the third scheme, the loss of mass occurs in three steps: the first with elimination of part of the arsenic trihalide, the second with elimination of

TABLE 4 Assignment

of infrared

frequencies

(cm-‘)

of p-piCON

and its complexes

a

P-piCON

AsCl 3- 2/3piCON

AsBr,+PpiCON

2AsI 3’ 3/3piCON

Assignment

3047m 1602m 1562~ 1483m 1420m 1280s 1270s 1167sh 1160s 1015s 948m 907w 793s 748s 676s 665s

3029m 1612m 1575w 1486m 1423m 1248s

3055m 1618m n.0. 1494sh n.0. 1215~

n.0. n.0. n.0. no. n.0. 1260~

CH CC CC CC CC NO

1172s 1170m 1025m 931s 910w 813s 774s 742m 670s

1160~ 1150m 1015m 933w

1164~ 1145w 1028~ 922~

? CH in-plane str. (fl) Ring breathing CH out-of-plane str. (v) CH out-of-plane str. (v)

808w

797m 778m 718m 664s

a See Table 2 for key.

720m 664s

str. and and and and str.

(v) CN CN CN CN (v)

str. str. str. str.

(v) (v) (v) (Y)

149 TABLE

5

Assignment

of infrared

frequencies

(cm-‘)

of y-piCON

and its complexes

a

y-piCON

AsCl,. y-piCON

2AsBr,. 3y-piCON

AsI,. Zy-piCON

Assignment

3080m 1626~ 1487s 1246s 1180s 1122w 1041s 854s 830s

3030m 1629s 1488s 1206s 1189s 1127~ 1041s 862sh 833~s 782~s

3050sh 1627m 1484m 1202m 1188m n.0. 1038s 850~ 830m 792~

CH str. (v) CC and CN str. CC and CN str. NO str. (v) CH in-plane str. CH in-plane str. CH in-plane str. ? NO def. (6) CH out-of-plane

740s

757m

3020m 1629m 1491m 1207~ 1176~ 1114m 1034m 854~ 831sh 824s 800s 752m

750vs

(v) (v) (8) (8) (/I) str. ( y)

a See Table 2 for key.

the rest of the arsenic trihalide and part of the ligand, and the third with elimination of the rest of the ligand. The thermoanalytical data of the complexes is shown in Table 6. Calorimetric measurements The standard enthalpies of dissolution of arsenic trihalide, ligands and complexes ( AiH*) were obtained as previously reported [1,2]. Table 7 gives these standard thermochemical values. From the standard enthalpies of dissolution, the standard enthalpies of acid/base reactions ( A,H*) were determined. Using appropriate thermochemical cycles and applying Hess’s Law, we obtained the standard enthalpies of decomposition ( A,H *), the standard lattice enthalpies (A,H*) and the standard enthalpies of acid/base reactions in the gaseous phase ( A,H * (g)). From the A, H 8 (g) values, the mean standard enthalpy of the arsenic-oxygen bond (D(As-0)) was calculated [1,2]. The values of these parameters are listed in Table 8. The melting points and the thermogravimetric studies of the complexes showed that they decompose on heating. For the determination of A,H *(g), it was assumed that the molar standard enthalpy of sublimation of each complex ( AtH * ) was equal to the enthalpy of sublimation of one mole of ligand [2]. For the calculation of the standard enthalpies of formation of the complexes, it was necessary to calculate the enthalpies of sublimation of the ligands, (Y-, p- and y-piCON0, which are not available in the literature, using a group contribution method [17-191, adding the difference between the enthalpy of vaporisation of (Y-, /3- or y-picoline and pyridine [20] to the enthalpy of sublimation of pyN0.

150 TABLE 6 Thermoanalytical data of the compounds AsX,.nL Compound

Weight loss (5%) Temperature Calcd.

Obs.

Attribution

mp (K)

ranBe (K)

AsCl s. pyN0

77.1 22.9 0

77.0 19.0 5.0

318-478 478-713 713-1223

-AsCl,, - 1/3pyNO - 2/3pyNO

344

AsCl a. cx-piCON

87.5 12.5 0

84.0 13.0 3.0

318-478 478-653 653-1223

-AsCl,, - 2/3o-piCON - 1/3a-piCON

371

AsCl,.ZP-piCON

50.8 49.2 0

50.0 47.0 3.0

368-463 463-578 578-1223

-AsCl,, - 1.SP-piCON - 9/5/3-piCON

393

AsCl a. y-piCON

74.9 25.1 0

74.0 23.0 3.0

368-483 483-913 913-1223

-AsCl,, - 1/3y-piCON - 2/3y-piCON

343

AsBr,.ZpyNO

65.6 15.6 18.0 0

68.0 14.0 15.0 3.0

343-518 518-583 583-873 87331223

- 3/4AsBr,, - 1/4AsBr, - lpyN0

- lpyN0

372

LOO 0

353-798 798-1000

- AsBr,, - cu-piCON

338

AsBr,. cx-piCON

100 0

AsBr,. 2o-piCON

79.5 20.5 0

85.0 15.0 0

318-503 503-803 803-1223

- AsBr,, - cx-piCON - cr-piCON

333

AsBr,.ji-piCON

66.8 13.9

68.0 11.0

373-508 508-513

340

19.3 0

20.0 1.0

513-943 943-1223

- 9/10AsBr, -l/10 AsBr,, - 1/4B-piCON - 3/4B-piCON

54.8 45.2 0

56.0 42.0 2.0

313-478 478-1118 1118-1223

- 5/3AsBr, - 1/3AsBr,,

352

AsI,. 2pyNO

85.9 14.1 0

88.0 9.0 3.0

343-488 488-613 613-1223

- ZpyNO, - 4/5AsI, - l/SAsI,

372

AsI,. wpiCON0

83.8 16.2 0

82.0 14.0 4.0

383-473 473-568 568-1223

- ar-piCON0, - 4/5AsI, - l/SAsI,

344

2As1,. 3/I-piCON

85.3 14.7 0

87.0 11.0 2.0

402-483 483-528 528-1223

- 3jSpiCON0, - 2/5AsI,

- 8/5AsI,

344

AsI,. Zy-piCON

83.8 16.2 0

82.0 14.0 4.0

368-498 498-568 568-1223

- 2y-piCON0, - l/SAsI,

- 4/5AsI,

366

2AsBr,.3y-piCON

- 3y-piCON

151 TABLE 7 Enthalpies

a of dissolution

at 298.15 K

Compound

Calorimetric

AsCl 3(1)

EtOH 1: 1 AsCl,-EtOH EtOH EtOH 1: 1 AsCl,-EtOH EtOH 2.4 N HCl-EtOH 2 : 1 AsCl,-2.4 N HCl-EtOH 2.4 N HCI-EtOH EtOH 1.1 AsCl,-2.4 N HCl-EtOH EtOH 2.4 N HCl-EtOH 2.1 AsBr,-2.4 N HCl-EtOH 2.4 N HCl-EtOH 2.4 N HCl-EtOH 1.1 AsBr,-2.4 N HCl-EtOH 2.4 N HCl-EtOH 2.4 N HCl-EtOH 1.2 AsBr,-2.4 N HCl-EtOH 2.4 N HCl-EtOH 2.4 N HCl-EtOH 1.1 AsBr,-2.4 N HCl-EtOH 2.4 N HCl-EtOH 2.4 N HCl-EtOH 3 : 2 AsBr,-2.4 N HCl-EtOH 2.4 N HCl-EtOH 2.4 N HCl-EtOH 2 : 1 AsI, -2.4 N HCI-EtOH 2.4 N HCl-EtOH 2.4 N HCl-EtOH 1: 1 AsI,-2.4 N HCl-EtOH 2.4 N HCl-EtOH 2.4 N HCl-EtOH 3 : 2 AsI, -2.4 N HCl-EtOH 2.4 N HCl-EtOH 2.4 N HCl-EtOH 2 : 1 AsI, -2.4 N HCl-EtOH 2.4 N HCl-EtOH

pyNO(s) AsCl,.pyNO(s) AsCl,(l) a-piCONO(s) AsCl 3. cY-piCON AsCl s(1) /?-piCONO(s) AsCl,.Z/I-piCONO(s) AsCl,(l) y-piCONO(s) AsCl 3. y-piCONO(s) AsBr,(s) pyNO(s) AsBr,.2pyNO AsBr,(s) ol-piCONO(s) AsBr,- cu-piCONO(s) AsBr,(s) cY-piCONO(s) AsBr,.2a-piCONO(s) AsBr,(s) /SpiCONO(s) AsBr,.&piCONO(s) AsBr,(s) y-piCONO(s) 2AsBr,. 3y-piCONO(s) AsI, pyNO(s) AsI,.ZpyNO(s) AsI, cr-piCON As1 s. cY-piCON AsI 3(s) P-piCONO(s) 2As1,. 3b-piCON AsI, y piCONO(s) AsI,-2y-piCON a Five experiments

were performed

solvent

AiH*

(kJ mol-‘)

-111.82~0.12 (i=l) 11.56 + 0.42 (i=2) (i = 3) -35.46k0.42 - 119.14+ 0.49 (i=l) (i = 2) 16.96 + 0.31 (i = 3) - 47.63 f 0.05 - 62.56 * 0.69 (i=l) - 16.82 * 0.28 (i=2) (i=3) 40.37 + 0.43 - 115.72kO.60 (i=l) 12.08 f 0.37 (i=2) - 12.79 f 0.34 (i=3) (i=l) - 34.33 f 0.55 (i=2) 5.54* 0.10 (i=3) 36.04 k 0.62 - 34.64 f 0.52 (i=l) (i = 2) -2.22k0.17 8.94 f 0.07 (i=3) (i=l) - 31.58 fO.O1 - 12.28 f 0.71 (i=2) 24.12 f 0.22 (i=3) - 34.64 k 0.52 (i=l) (i = 2) - 9.18 f 0.36 (i = 3) 20.82kO.11 - 32.16 + 0.47 (i =l) 3.24 + 0.21 (i=2) 18.67 f 0.85 (i=3) (i=l) 14.97 kO.17 - 12.91 f 0.74 (i=2) 58.55 f 0.92 (i = 3) 14.97 f 0.17 (i=l) (i=2) - 3.17 f 0.09 42.23 f 0.46 (i=3) (i=l) 11.96 & 0.70 (i = 2) -11.14*0.11 47.19 f 0.12 (i = 3) (i=l) 14.97 kO.17 36.99 f 0.54 (i=2) 97.83 & 0.83 (i=3)

in alI cases.

Considering the Ar H * values for complexes of the same stoichiometry with a fixed ligand, we obtained the acidity order: AsCl, 7 AsBr, 7 AsI,. The basicity order, y-piCON 7 pyN0 7 cY-piCON0, was obtained if AsCl,

a b ’ d

Ref. 14. Ref. 15. Ref. 16. See text.

As&(s) pyNO(s) a-piCONO(s) @piCONO(s) y-piCONO(s) AsCl,.pyNO(s) AsCl,. a-piCONO(s) A&I,-2fi-piCONO(s) AsCl s. y-piCONO(s) AsBr,.ZpyNO(s) AsBr, . a-piCONO(s) AsBr,.2a-piCONO(s) AsBr,.j%piCONO(s) ZAsBr,. 3y-piCONO(s) AsI,.ZpyNO(s) AsI,. a-piCONO(s) 2AsI s. 3@piCONO(s) AsI,. 2 y-piCONO(s)

AsCl s(l) AsBr,(s)

Compound

- 64.80 f 0.61 - 54.55 & 0.58 - 119.75 kO.86 - 90.80 f 0.78 - 64.83 f 0.83 -45.8OkO.55 - 67.98 f 0.74 - 64.64 + 0.64 - 47.59 * 0.99 - 56.49 f 1.19 - 30.30 f 0.50 - 46.37 + 0.72 -45.87_+1.00

A,P AfH* 43.5 a 67.5 = 95.0 = 81.9k1.5 b 84.2 + 2.3 d 86.2 + 2.2 d 87.Ok2.1 d

A,H’ - 305.0 a - 197.5 a - 58.2 a 7.7kO.8 b 10.3kO.8’ 8.8 + 0.7 = 12.9kO.9’ - 362.1 - 349.3 - 407.2 - 383.0 - 246.4 - 232.5 - 244.4 - 252.8 - 225.2 - 99.3 - 78.3 91.4 - 78.3

Summary of the thermochemical results (kJ mol-i)

TABLE 8

-

190.2 182.3 335.7 221.4 291.6 197.5 303.9 218.3 245.6 315.3 209.6 270.7 314.9

AMH*

146.7 f 1.6 138.8k2.4 292.2 _+2.4 177.9 & 2.2 228.6 f 1.7 130.3 + 2.4 236.4 k 2.4 150.8 f 2.3 178.1+ 2.3 220.3 + 1.9 114.6 f 2.4 175.7 k 2.3 219.9f2.3

A,,H*

- 108.3 - 98.1 - 249.5 - 134.4 - 214.2 - 113.3 - 219.7 - 132.1 - 158.9 - 233.4 - 125.4 - 184.5 - 227.9

A.,H*(g)

108.3 98.1 124.8 134.4 107.2 113.3 109.9 132.1 105.9 116.7 125.4 123.0 114.0

~(As-0)

153

was used as the common acceptor. However, this basicity order seems to invert when AsBr, or AsI, are used as the common acceptor. The data indicate that y-piCON is the best base for AsCl, but that pyN0 is the best base for AsI, and ar-piCON is the best base for mono-complexes of AsBr,. The acidity order obtained is that expected on the basis of an inductive effect. The expected order for basicity on the basis of an inductive effect is y-piCON > p-piCO_NO = a-piCON > pyN0 [I]. Considering the D(As-0) values obtained, the same basicity order is observed although the acidity order is inverted. This means than in the gaseous phase, AsI, is the best acid for a-piCON and AsCl, is the worst. The parameters A,H* (complexes), A,H* and A,H* also showed this behaviour. Comparing the mean energy of the As-O bonds obtained for these complexes with the mean energy of the As-N bonds obtained for pyridine and picoline complexes of arsenic trihalides [l] of the same stoichiometry, it is observed that the introduction of an oxygen atom in the ligand structure tends to weaken the arsenic-ligand interaction. The As-N bond is always stronger than the corresponding As-O bond. It would be expected, on the basis of an inductive effect, that the oxygen atom should form a stronger bond to the arsenic atom than the nitrogen atom. Also, in terms of the hard/soft acid/base (HSAB) [21-231 concept, because of the slighty harder oxygen atom, the As-O bond should be stronger than the As-N bond for AsCl, complexes and weaker for AsI, complexes. Clearly, the nature of the bond formed between the donor and acceptor atoms is important in determining the relative strength of the interaction [1,2-41.

ACKNOWLEDGEMENTS

The author thanks CNPq and FAPESP

for financial support.

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