689
The Crystal Structures of Rubidium Hydrogen Croconate and Ammonium Hydrogen Croconate N. C. Baenziger and D. G. Williams
Contribution f r o m the Department of Chemistry, University of Iowa, Iowa City, Iowa. Received September 20, 1965 Abstract: The crystal structures of RbHC505and NH4HCsOswere determined by single crystal methods. The monobasic anions form long planar chains through the cell due to hydrogen bond formation. The hydrogen bond formation destroys the expected mirror symmetry of the anion, and ~s a result the double bond appears not to be delocalized completely. The 0-H. .O distances are 2.50 and 2 , 4 6 A. e
C
roconic acid is a dibasic acid first prepared by Gmelin.' As a result of early studies on this compound, some differences in opinion as to the positions of the hydroxyl groups arose. From the small difference in pKl and pK2 values, Carpeni2 concluded that the earlier idea of nonadjacent hydroxyl groups3 was correct (I). Hirato, et al.,4 first pictured croconic acid as a mixture of structures I, 11, and 111. Later, Hirato, Mizuno, and Yamadaj showed that their ultraviolet spectroscopic measurements could be explained if form 11 exists in acid solution and form IV in basic solutions (dianion stabilized by resonance).
/-'c 0
\
I
OH
/"-c
HO
2-c? O H 0
II \OH
PH
\
/c-c\o
I
O E Z
?H
P
West, et a1.,6-s prepared salts of the croconate ion and a number of other related species. From infrared and Raman data they found that the croconate anion should possess D5h symmetry. They also carried out simple molecular orbital calculations on a whole series of oxygenated anion^.^ The crystal structures of diammonium croconate1° and copper croconate' ' showed conclusively that the dianion exists with D,h symmetry. '0 The structure of the Zn salt of the croconate ion,'l however, showed a distorted anion of an enediol form, due to the partially covalent bonds formed to zinc. (1) L. Gmelin, Ann. Phjs., 4, 31 (1825). (2) G. Carpeni, J . Chim. Phys., 35, 193 (1938). (3) R. Nietzki and T. Benckiser, Ber., 19, 293 (1886). (4) Y. Hirato, K. Inukai, and T. Tsujiuchi, J . Chem. Soc. Japan, Pure Chem. Sect., 69, 63 (1948). ( 5 ) I
