Green Copper(l1) Chloride Dihydrate Is Not Autoionized George 6. Kauffman California State University. Fresno, CA 93740
Christian Klixbiill Jhensen University of Geneva, CH 121 1 Geneva 4. Switzerland In their otherwise imaginative and ingenious ion flotation demonstration. "Whv Cuoric Chloride Cwstals Are Green", Sabba and ~ebrowiki(i)state that their demonstration shows green CuCly2H20 to consist of blue C U ( O H Z ) ~and ~+ yellow C U C Pions. According to Alfred Werner's classification scheme (2), this could constitute a case of "polymerization isomerism" (3), the most common examples of which are Vauquelin's "pink salt", [Pd(NH3)4][PdCL](4). which exhibits the same stoichiometry as the yellow cis (5) or trans (6)isomers of Pd(NH3)~C12 (7). and Magnus's "green salt", [Pt(NHs)4][PtCL] (8, 9), which exhibits the same stoichiometry as the yellow cis (10) or trans (11) isomers of Pt(NH3)zClz (12). Unfortunately, however, Sabba and Zebrowski's conclusion is not in accord with other experimental evidence. The fundamental flaw in Sabba and Zebrowski's arguments is the fact that even if autoionized CuCly2HzO could be predicted to begreen (by superpositionof blueand yellow colors), it does not follow that the observed green cdor nrovesautoionization. The direct Cu-CI contacts in the crystal (but not in dilute solution) provide intense electron transfer bands (13) in the near-ultraviolet region, stretching slightly into the visible, as is known for CUC~,(OH~),~in media containing high concentrations of chloride ion (14, 15). Furthermore, the crystal structure (16) of CuCly2Hz0, whichsbows four close neighbor atoms in a plane (two internuclear distances Cu-0 1.96 A,two Cu-C12.29 A,and two Cu-C12.94 A,the latter chlorides bridging a t a distance of 2.29 A to another copper ion), does not support Sabba and Zebrowski's contention. "+
In general, it is very difficult to evaluate convincingly the composition of complexes in solution by comparison with species extracted in organic solvents or with precipitated salts. A typical case is thallium(II1) in hydrochloric acid, which precipitates instantaneously with CO(NH&~+as alC ~ ~analogous ] to salts of most insoluble [ C O ( N H ~ ) ~ ] [ T I(17) indium(III), bismuth(III), and rhodium(II1) (18) or of equal amounts of PbCk2- and PbCk4- ions. In solution Raman spectra (19) show predominakly tetrahedral TlC14- althoueh there mav be small amounts (20) of h i ~ h e rchloro com~lexesprese& in hydrochloric acid a t highconcentrations. In the present case, the chloro complexes of copper(I1) in aqueous solution have very low formation constants (14,151. The dodecylammonium ion simply precipitates a particular complex ion from among the several involved in the labile equilibria, which are thereby shifted toward the formation of more of the precipitated ion in accordance with Le ChBte76
Journal of Chemical Education
lier's principle. The particular ion which is precipitated from Cu(I1)-C1- solutions is markedly dependent on the counter cation present (21,22). As a case in point, even the C U C I ~ ~ ion (23), although present in exceedingly low equilibrium concentration, can be precipitated by the Cr(NHah3+ ion. Thus Sabba and Zebrowski's demonstration is simply an example of Basolo's principle of stabilizing a complex ion by selecting an appropriately charged or appropriately sized counter ion (24), which accounts for their observations. These facts. of course. do not ~recludethe occurrence of canes of autoion~zedcrystals, as, for example, when the gaseous molecules PC16 and PBri condense to PCln'PClh- and to PBra+Br-, respekively. A minor error in Sabha and Zebrowski's demonstration deserves to be mentioned. Although they state that "a saturated solution of cupric chloride" is used, their directions call for a solution of 13 g of CuC12.2H20 in 50 mL of H20, a concentration much less than that of a saturated solution (110 g/100 mL at 0 'C) (25). Acknowledgment The authors are indebted to the reviewer for valuable suggestions and additions. Llteralure Cited
4. Vauquelin,L.N.Ann.chim.phya. 1813,88,186. 5. Grinberg, A. A. Iruest. Iwt. Izueheniiu Plofiny i Drug. Blogomdn. Mefol. 1933,11,
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II R . ~ . . J ch,m , h \ a 18tr.[3]1,. ,I: rompr.rond. wr.18.1101 12 Ka tfhan.(;. B..Vman U 0 Inurd Sin 1963,.- >3> 13 I q m $ e n . C K Pros lnnn C h e m 1970.12. 101. I 4 Bprrum.J ; S k ~ b s k d . l .H Arrn Chom Zrond 1977.AJl.673. I nl.lrum,J. ~ r r cnem o srond. I)L.AII. 1% 16 P-6Ocrg. h *,-re Chrm Lund. 1WO.M. .Ail< , ~ d ~ f f e tlcattl.. .~ I H lna,. 1' I ,\lnlrtnn K J rhem soc I % ~ . . L I ~ 17 nnrr I8 I l r \ . P Ilall.1 n .I Chem . # r t >T. CI lnors. I !.em. 1965.1 11. 21 (:rccn.xd,h )r . F a r n h h s x . ~Vh~mrarrroltheElrm~nrr.I'crgm,n O.ford 19.44'
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