X N : Arsenic(V) Chloride W. E. DASENT Victoria University College, WeKington, New Zealand
IT
IS not uncommon for textbooks2 to list arsenic(W chloride among the known compounds of arsenic, but the literature on AsCb makes it quite clear that the substance has never been prepared. Although salts of the cation AsC14+have recently been characterized, the fact that AsC16 itself has not been obtained, despite many attempts, is quite unexpected in view of the stability of PCls and 6,bCl.. The only known arsenic(V) halide is AsF5. Early published work (1-6) described several attempts to prepare the pentachloride by the action of chlorine on arsenic(II1) chloride. I n no case was any definite evidence of reaction obtained, although a t sufficiently low temperatures (-20' to -60°C.) much chlorine gas was absorbed, only to be given off again when the temperature was raised or when a stream of air was passed through the solution. Baskerville and Bennett (6) however, claimed to have isolated AsCls as a yellow crystalline solid by cooling AsC4 and C1, together in the correct proportions to -40°C. That their product was merely a mixture of the reactants was demonstrated by Emith and Hora (7) and by Biltz and Meinecke (S), who showed that the freezing points of AsCI3/C1, mixtures lay on a smooth curve, with no indication whatever of compound formation. More recently, Gutmann (9) has attempted unsuccessfully to stabilize the pentachloride as the anion AsC16-, by adding potassium chloride or tetramethylammonium chloride to a mixture of arsenic(II1) chloride and chlorine. No new compound resulted. I t was found possible, however, to isolate the addition compounds PClS~AsCI6 and SbCI6.AsCls. These water-sen-
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Suggestions of material suitable for this column are eagerly ought and will he acknowledged. They should be sent with as many details 8 8 possible to Karol .I.Mysels, Department of Chemistry, University of Southern California, Los Angelos 7, California. Contributors of discussions in a form suitable for publication directly will be acknowledged as guest authors. Since the plLrpOSe of this oolumn is to prevent the spread and continuation of errors and not the evaluation of individual texts, the source of the errors discussed will not be cited. The error must occur in at least two independent standard books to be p r e sented.
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VOLUME 34, NO. 11, NOVEMBER, 1952
sitive substances have been formulated by Gutmann as salts AsC14+ P C k and AsC14+SbC16-by analogy with the more stable compound PC15.SbC15,which is known t o give a conducting solution in molten iodine monochloride and to have the structure PCI4+SbC1,-. The alternative formulation of the arsenic compounds as PCla+ AsCle- or SbC14+ A s C k was regarded as nnlikely in view of the absence of any other evidence for the existence of the anion A s C k . Further evidence for the cation AsC14+ is the recent preparation of the substance AsF3Cl, by the action of chlorine on moist arsenic(II1) fluoride (10, 11). There is clear evidence that this compound is the salt AsC14+ AsF6-. for example the conductance in arsenic(II1) fluoride solution increases linearly with concentration, indicating a salt structure; furthermore the ion AsFesurvives alkaline hydrolysis and can be precipitated as its nitron salt. The greater susceptibility of bromides to oxidation makes the possibility of the existence of arsenic(V) bromide remote, and a study by Biltz and Jeep (1%)of the freezing points of AsBra/Br, mixtures confirmed the absence of any higher bromide. However, mention must be made of a kinetic study by Venkataraman (13) of the bromination of p,8-dimethylacrylic acid in acetic acid solution. The presence of AsBrs in the reaction mixture was observed to decrease the concentration of bromine available for addition to the unsaturated substance. This was Interpreted as evidence for the equilibrium AsBr8 Br2 AsBn in solution. This suggeation has been supported by Savithri (Id), who has shown that the densities and magnetic susceptibilities of the solutions obtained when solutions of AsBrS and Brr in acetic acid are mixed have values slightly less than those calculated. The non-existence of AsC16 appears to be only one illustration of a more general phenomenon: the reluctance of some elements of the$rst long period to assume the high covalences which are achieved by their group neighbors above and below them in the Periodic Table (15). Some examples are shown in the following
+
table. The compounds in parentheses are either nonexistent or of low stability: PC& (VCI,) NbCls
SFs (Cr4) MoFs
PCls (AsCI.) SbCls
SOs (SeOs) TeOa
HCIO, (HBrOd HI04
There is certainly no simple explanation of t,hese facts. A possible interpretation, discussed by Hugus (16),is that the effect is not due to any lowered stability in the highest oxidation states of the elements of the f i t long period, but rather to enhanced stability in elements of the second long period: the factor which stabilizes the latter is the contribution of 4f orbitals to bonding, a t least in the cases of antimony, tellurium, and iodine. Recent discussions (17) of the general feasibility of f-orbital hybridization hare not confuted this suggestion. LITERATURE CITED (1) ROSE,H., Poggendo$'s Ann., 52, 62 (1841). (2) MAYRHOFER, J., Ann., 158,326 (1871).
JANOWSKY, J. V., Z. anorg. Chem., 6,219 (1873). SLOAN, B. E., Chem. News, 44,203 (1881). BESSON,A,, Compl. rend., 109, 940 (1889). BASKERVILLE, C., A N D H. H. BEXUET~, J. Am. Chem. Sac., 24, 1070 (1902). (7) SMITH,W. R., AXD J. E. HORA,J. Bm. Chem. Soc., 26, 632 (1904). (8) BILTZ, W., A m E. MEINECKE,Z. anorg. Chem., 131, 1
(3) (4) (5) (6)
11023) \----,
(9) GUTMANN, V., Alonatsh., 82, 473 (1951).
(10) KOLDITZ, L., 2.anorg. Chem., 280, 313 (1955). J. Am. (11) DESS, R.M., R. W. PARRY,IUD G. L. VIDA~~E, Chem. Sac., 78,5730 (1956). (12) BILTZ,W., A N D K. JEEP,Z. ano~g.Chem., 162, 32 (1927). R., Proc. Indian .*cad: Sci., 13A, 259 (13) VENKATARAMAS, (1941). K., PPOC.Indian Aead. Sci., 16A, 196 (1942). (14) SAVITHRI, N. V., "The Chernioal Elements and Their Com(15) SIDGWICK, pounds," Oxford University Press, London, 1950, Vol. 2, p 1230. (16) HUGUS,Z Z., J. Am. Chem. Soc., 7 4 , 1076 (1952). (17) COWLSON, C. A,, ASD G. R. LESTER,I.Chem. Soe., 1956, 3650.
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