COMMUNICATIONS TO THE EDITOR
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Figure 1. The excess enthalpies of CeHa at 25' (0)and 45' (A), C& -I- C&H at 25' (0)and 42' and Cd& f 1,2,4,5-tetrafluorobenzeneat 25' (+) and 39 ( X ) ; 2 2 is the mole fraction of the fluorochemical in each case.
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range and a t two temperatures (Figure 1). The calorimeter used is similar to that described by Larkin and McGlashan2 and is described elsewhereO3 The C6H6 CBFP, results near x2 = 0 and those for CeH6 C6FsHnear z2 = 1 have been carefully checked and the sign change in the heat of mixing has been definitely established in each case. Since runs a t these composi-, tions jnvolve no electrical compensation, the sign of the temperature change unequivocally determines the sign of RE. The S-shaped curves found in the system l-hydron-perfluoroheptane acetone4 were interpreted in terms of the combination of a symmetric exothermic "chemical" interaction arising from the formation of a 1: 1 hydrogen-bonded complex and a skewed endothermic "physical" interaction arising from the mixing of hydrocarbon and fluorocarbon groups. The former contribution was obtained independently from nmr data and the latter inferred from the system CTF16 acetone. We believe the S-shaped curves for C6H6 C6F6 and C6H6 CeFsH may be similarly explained.
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There is strong evidence suggesting charge-transfer complex formation between C6H6, acting as donor, and C6F6, acting as acceptor. The freezing point diagrams shows the formation of a 1: 1 solid complex. The predominantly exothermic results that we obtain for this system can be understood in terms of such a complex; the positive temperature dependence of BE must be due to a decrease in complex formation with increasing temperature. The more endothermic results for C6H6 C6F6Hpresumably arise from a smaller exothermic contribution; i.e., C6FsH is a poorer acceptor than C6F6 and forms a weaker complex. Any temperature dependence in this case is small, less than the experimental error. With C ~ H B4- 1,2,4,5-tetrafluorobenzene, the further increase in RE again reflects further decrease in complex formation. The negative temperature dependence for this system corresponds to the normal behavior6 of systems with positive RE; indeed, there is no clear evidence for any complex formation in this case. Additional measurements with other fluorine sub~, CaHsF) are stituted benzenes (e.g., C B H ~ FCdU?", underway. (2) J. A. Larkin and M. L. McGlashan, J. Chem. Soc., 3425 (1961). (3) J. A. Larkin, D. V. Fenby, T. S. Gilman, and R. L. Scott, to be published. (4) D. L. Anderson, R. A. Smith, D. B. ,Myers, S. K. Alley, A. G. Williamson, and R. L. Scott, J.Phys. Chem., 6 6 , 621 (1962). (5) F.L.Swinton, private communication. (6) M. L. McGlashan, Pure Appl. Chem., 8 , 157 (1964).
DEPARTMENT OF CHEMISTRY UNIVERSITY OF CALIFORNIA Los ANGELES,CALIFORNIA90024
DAVIDV. FENBY IANA. MCLURE ROBERT L. SCOTT
RECEIVED DECEMBER 27, 1965
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Bond Lengths in Iron Pentacarbonyl Sir: On the basis of an electron diffraction study of gaseous iron pentacarbonyl, Davis and H. P. Hanson recently reported' that the axial Fe-C bonds were shorter than the trigonal by 0.045 A, the values being 1.797 and 1.842 A, respectively. They stated that the existence of the shorter axial bonds was corroborated by X-ray crystallographic studies of A. W. Hansoq2 whose results gave axial bonds of 1.785 and 1.807 A and trigonal bonds of 1.827, 1.827, and 1.837 A. Davis and H. P. Hanson were apparently unaware that the crystal structure refinement by A. W. Hanson (1) M.I. Davis and H. P. Hanson, J . Phys. Chem., 69,3405 (1965). (2) A. W.Hanson, Acta Cryst., 15, 930 (1962).
Volume 70, Number 8 February 1966
604
was carried out using an incorrect space group: when the correct space group is used,s the molecule has crystal symmetry (32-2, with two equivalent axial bonds having length 1.810 f 0.020 A and trigonal bonds of length 1.797 =k 0.017 A (two equivalent) and 1.763 f 0.034 A where the uncertainties given are the standard derivations. The X-ray results thus do not provide support for the axial bonds being shorter, but, if anything, suggest that they are longer (as might be expected), although, on the basis of the usual significance test,6 neither of the differences between the axial bond length and the two trigonal bond lengths is in the “significant” class. Furthermore, we doubt whether the electron diffraction data are capable of detecting a difference of only 0.045 A between the two kinds of Fe-C bonds because of the high correlation between that difference and the vibrational amplitudes. In the refinement’ by the matrix least-squares method described by Hedberg and Iwasaki,6 Davis and Hanson used assumed values for the vibrational amplitudes, a procedure which virtually fixed the bond length difference before the refinement began, It is interesting that in the investigation, by electron diffraction, of the structure of 2,3-dimethylbuta-1,3-diene17 Hedberg and Hedberg were unable to distinguish between models with the bond lengths CmeLhyl-C equal to C2-Ca with vibration amplitudes 0.046 A and models having a difference of 0.05 A between the two bond lengths with vibration
The Journal of Physical C h a i a t r y
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amplitudes 0.039 A. In the present case a satisfactory fit was obtained with a bond length separation of 0.045 A and assumed vibration amplitudes of 0.041 and 0.059 A. An equally good fit could probably have been obtained with no bond length separation and somewhat larger vibration amplitudes or even a reversal in the bond length split and different vibration amplitudes. The correlation among these quantities is certainly so great as to preclude their independent determination by the electron diffraction method.
Acknowledgment. This work was supported by a grant from the National Science Foundation. We wish to thank Professor Kenneth Hedberg for helpful discussion. (3) J. Donohue and A. Caron, Acta C r y t . , 17, 663 (1964). (4) These three nonequivalent Fe-C bond lengths and their standard derivations were recalculated from the data of ref 3, and are given here to one more significant figure than were given therein. (5) D.W.J. Cruickshank, Acta Cryst., 2, 65 (1949). (6) K.Hedberg and M. Iwasaki, ibid., 17, 529 (1964). (7) L.Hedberg and K. Hedberg, American Crystallographic Association Meeting, Gatlinburg, Tenn., July 2, 1965.
DEPARTMENT OF CHEMISTRY OF SOUTHERN CALIFORNIA UNIVERSITY Los ANGELES, CALIFORNIA 90007
JERRYDONOHUE
DEPARTMENT OF CHEMISTRY UNIVEFLSITY OF MASSACHUSETTS AMHERST,MASSACHUSETTS RECEIVED DECEMBER 14, 1965
AIMERY CARON
