COMMUNICATIONS TO THE EDITOR
777
I
i
a t b
3.5
4.0
4.5
5.0
5.5
6.0
6.5
r, A.
Figure 1. The radial distribution curve of Fe(C0)6 in the region 3.2-6.5 A: 0 , experimental curve; 0.048 A; -r(Fe-C), - r(Fe-C)a = 0.020 A. Damping factor = exp( -0.008~~).
*, r(Fe-C)W -
r(Fe-C),, =
)
text, it is interesting to note that Donohue and Caron's value for the average Fe-C bond length differs by 0.03 A from ours. From the point of view of theory, if one compares the orbital description of PF5,in which the axial bonds are longer by 0.05 A, and Fe(CO)5, a number of important differences are encountered. In PF6 the d orbitals which might be used to stabilize the axial bonds are higher in energy than the s or p orbitals. Bartell has expressed doubts that the d-orbital contribution to bonding is at all significant, formulating his arguments in the context of the Rundle approach. In Fe(C0)5, on the other hand, the d orbitals are of lower energy than the p and may consequently be expected to add to the stability of the axial bonds. There is the further factor of back-donation in Fe(C0)s. A consideration of the respective symmetries of the occupied Fe orbitals and the antibonding CO orbitals show that multiple bonding is more favorable in the axial direction. On the basis of these considerations, a model in which the
axial Fe-C bond is shorter does not appear unreasonable. DEPARTMENT OF CHEMISTRY THEUNIVERSITY OF TEXAS AUSTIN,TEXAS 78712
M. I. DAVIS
DEPARTMENT OF PHYSICS THEUNIVERSITY OF TEXAS AUSTIN,TEXAS 78712
H. P. HANSON
RECEIVED SEPTEMBER 6, 1966
The Iron Pentacarbonyl Problem
Sir: The preceding letter by Davis and Hanson' contains a number of points which we feel deserve comment. First, we beg t o disagree that the most significant question posed by us2 was whether the electron dif(1) M. I. Davis and H. P. Hanson, J. Phys. Chem., 71, 775 (1967).
Volume 71,Number 9 February 1967
778
fraction data were detecting a difference of only 0.045 A between the two kinds of Fe-C bonds. Our principal point was (and is) that the crystal structure results3 did not provide confirmation of the structure with shorter (Fe-C)axisl bond lengths proposed by them.4 Davis and Hanson now1 call attention to the fact that our average Fe-C bond length differs from theirs by 0.03 A, with the implication that the molecular structures in the gas and the solid may not be the same. Perhaps so, but the evidence now available does not support this view: in the gas,’ (Fe-C),,,, = 1.823 f 0.0014 A, and in the solid,3 (F*C)mean = 1.795 A 0.020 A. The difference of 0.028 A, with a u of 0.020 A, is significant at the 8% level, or clearly in the “notsignificant” range, as defined by Cruickshank.6 Regarding the two closely contiguous and therefore unresolved peaks in the radial distribution function, no one can argue that there is not a high correlation between the separation between them and the vibrational amplitudes, as we have already pointed out.2 This difficulty may be obviated in certain special cases, such as the example of PFs6cited by Davis and Hanson. We are, however, unable to understand why they consider that the electron diffraction investigation of this molecule is germane to the present problem: the radial distribution function of PF5 consists of four rather sharp. well-separated peaks, only one of which is composite, and it was accordingly a simple matter for Hansen and Bartell to determine, with a high degree of precision, the two variables which characterize the molecule and to conclude that the (P-F)axialbond distance was greater than the (P-F)epuatorial bond distance by 0.043 h 0.008 A. This situation is in sharp contrast to that in Fe(C0)6,the radial distribution function of which has six peaks, two of which are rather well-defined shoulders and one of which, the outermost, is a rather ill-defined smear at about 5.9 A. Of these, five are composite and the position of the sixth, the outermost, does not depend directly on whether or not the two kinds of Fe-C bonds are equal in length. There is, accordingly, a serious resolution problem and the assumed vibration amplitudes, which have a marked effect not only on the width of the peaks but also on the positions of the shoulders, should be included as parameters in the refinement, together with the bond length difference. We agree that other electron diffraction groups should repeat the experiment and for that reason see no point in making detailed comments on some rather curious features of the radial distribution functions presented by Davis and Hanson. Finally, we feel that a quantum mechanical discussion of this molecule is premature. Given a correct The Journal of Physical Chemistry
COMMUNICATIONS TO THE EDITOR
molecular structure, an M O description which “explains” that structure can usually be found.’ I n the present case, it would seem more prudent to wait until the structure is firmly established. ~~
(2) J. Donohue and A. Caron, J . Phys. Chem., 70, 603 (1966). (3) J. Donohue and A. Caron, Acta Cryst., 17, 663 (1964). (4) M. I. Davis and H. P. Hanson, J . Phys. Chem., 69, 3405 (1965). (5) D. W. J. Cruickshank, Acta Cryst., 2, 65 (1949). (6) K. N. Hansen and L. S. Bartell, Inorg. Chem., 4, 1775 (1965). (7) See, for example, A. G. Turner and F. S. Mortimer, ibid., 5, 906 (1966).
DEPARTMENT OF CHEMISTRY UNIVERSITY OF PENNSYLVANIA PHILADELPHIA, PENNSYLVANIA DEP.4RTMENT OF CHEMISTRY
J. DONOHUE
A. CARON
UNIVERSITY CF M.4SSdCHCSETTS
AMHERST,MASSACHUSETTS
RECEIVED DECEMBER 1, 1966
Comments on the Paper, “Cation Exclusion from Gels,” by R. A. Horne
Sir: Hornel disagrees with us concerning our interpretation2 of the exclusion of aqueous ions from part of the pore volume of high surface area materials. The fact of exclusion has been known for a long time, but up until recently there has been very little discussion as to its explanation. Redfern and Patrick3 were unable to explain it. Plank4 observed it for certain ions, referring to “negative adsorption” in certain cases, and Konyushka6 reported it for aqueous nonelectrolytes in the pores of silica gel. We reported that when a silica gel adsorption column is just filled with aqueous hexachloroplatinic acid there is a depletion of the acid in the first part of the column with a corresponding enrichment in the latter part.6 We later made several studies of ion exclu~ion,~ usually with silica gel, and we concluded2 that the principal if not the only reason is a geometric one and that we consequently are measuring ion hydrate size (1) R. A . Horne, J . Phys. Chem., 70,1335 (1966).
(2) B. L. McConnell, K. C. Williams, J. L. Daniel, J. H. Stanton, B. N. Irby, D. L. Dugger, and R. W. Maatman, ibid.,68,2941 (1964). (3) S. Redfern and W. A. Patrick, ibid.,42,497 (1938). (4) C. J. Plank,ibid., 57, 584 (1953). ( 5 ) (a) I. M. Konyushka, Uch. Zap. Belorussk. Gos. Univ., 140 (1953); (b) ibid., 282 (1956); (c) Vestsi Akad. Navuk Belarusk. SSR, Ser. Fzz-Tekhn. Navuk, 111 (1956). (6) R. W. Maatman and C. D. Prater, Ind. Eng. Chem., 49, 253 (1957). (7) See ref 2 and our earlier papers referred t o therein.
