June20, 1961
COMMUNICATIONS TO THE EDITOR
emerges from the synthesis of quite varied types of indirect structural evidence. Proof of sevencoordination is now provided by the direct structural analvsis of sDectrometricallv recorded I hkZ1 X-ray d a t i from a’single crystal df the salt, RbFi(OHz)Y.H20. The monoclinic unit containing 4RbFe(pHz)Y . H2O has a = 14.17, b = 7.73, c = 14.63 A., 6 = 90.44’. Calculated and experimental densities agree a t 1.93 g./cc. The space group is P2/a. A crystal specimen small enough to obviate the need for absorption corrections has given 2983 { h k l ) intensities measurable with MoKa radiation in the range, 0 < sin(O/k) < 0.77. Some 96y0 of the amplitude data are presently included in a threedimensional Fourier synthesis which yields both configuration and a structurally consistent set of bond parameters. The remaining amplitudes, mostly for reflections giving counts comparable with background, are gradually being added to the synthesis in conjunction with the refinement (just begun) of thermal parameters. Of present import is the indication from a Fourier synthesis of calculatqd amplitudes, using a single isotropic B of 3.25 A.2, that Fe-0 and Fe-N bond distances are known already to within 0.01 A. on the average. The discrepancy index R for all 2983 forms is presently 0.165 (for the h0Z zone, 0.121). On the basis of (1) minimizing ring strain, (2) minimizing steric repulsion within the inner coordination group, and (3) permitting puasi-equivalence of ligands, the evident choice for a sevencoordinate Fe(OH2)Y- containing sextet Fe(II1) with its five unpaired electrons3 is2 the NbF,= configuration4 of maximum symmetry C*,-rnm. In fact, howevert the experimentally established configuration is loosely describable as pentagonal bipyramidal, in disregard of the assumptions (2) and (3). Fe-0 bond lengths average to 2.055 .k., very nearly the sum of the ionic rsdii, whereas the Fe-N bonds are 2.30 and 2.32 A. In the sexadentate octahedral COY-, with which (a presently hypothetical) F e y - containing doublet Fe(II1) should be virtually isodimensional, othecorresponding bond lengths are5 l.SX and 1.92 A,, much smaller atid much less different from one another than with sextet Fe(II1). The very long Fe-N bonds in Fe(OH2)T- make it easier for two nitrogen and three oxygen atonis (one from H20) to form a very roughly planar girdle about iroti with three not yreasonably long Fe-0 bonds a t 2.08, 2.09, 2.10 The other pair of Fe-0 bonds, closed along the “pentagonal” axis without semblance of steric iFterference, are coriipensatingly short : l.OS, 2.02 A. The free anion may have a t most a twofold axis passing through HzO, Fe, and the center of the C-C bond of the ethylenediamine ring. The ring bond angles a t the central atom in Fe(OH2)Yare uniformly smaller (11-15’) than in CoxT-; apart from the angles and bonds which involve (2) Cf. J. I,. Hoard, Gordon S. Smith and Maurice Lind, in “Advances in the Chemistry of Codrdination Compounds,” @ Stanley Rirschner, 1961, T h e Macmillan Company, New York, N. Y.,in press. (3) W Klemm, Z . nizovp ailgenr Chem., 252, 226 ( I n $ $ ) . (4) J. L. Hoard, J . A m . Chcm. Soc., 61, 1252 (l!I39). ( 5 ) H. A. Weakliem and J. L. Hoard, ibid., 81, 549 (1929)
2771
Q
Fig. 1.-Skeleton model in perspective of Fe(OH2)Y -: the short Fe-0 bonds involve 01 and 0 2 .
the central atoms, the ring geometries of the two complexes are similar. The possibility that packing relations in the crystal control the selection of configuration for the bulky and presumably flexible Fe(OH2)Y- is not dismissed readily. A configuration rather more like that of NbF7= is indicated for an analogous Mn(I1) complex now under study in this laboratory. The machine computations of this paper were carried out a t the Cornel1 Computing Center through the courtesy of Mr. Richard C. Lesser, Director. Our thanks go also to Mrs. Diana Chantry and Dr. Gordon S. Smith for assistance in the early stages of the study. (6) Fellow of t h e John Simon Guggenheim Memorial Foundation. 1960. (7) Predoctoral Fellow of t h e Monsanto Chemical Company, 19601861.
DEPARTMEKT OF CHEMISTRY J. L. HOARD^ CORNELL UNIVERSITY MAURICELISD’ ITKACA,NEW YORU J. V. SILVERTOS RECEIVED MAY16, 1961
A GENERALIZATION OF THE OCTANT RULE‘
Sir: During the past decade a considerable aiiiouiit o f experimental data has been accumulated by Coolison2 and others that indicates a marked heighteiiing of the long-wave length carbonyl transition iri certain B, y-unsaturated ketones, such as the santonides. Recently, Labhart and Wagni&re4 put forth a possible explanation of the phenomenon. We wish to point out that the same electric dipole transition moment responsible for the enhanced intensity of absorption must also give rise to ;I marked increase in the rotational strength of the same electronic promotion. Briefly, the latter (1) (a) Financial support was provided l>yNational Science J’oundation grants No. G-15710 (to K. R I . ) and h-u. G-G579 ( t o C. 11.). (b) This paper represents N o . 1,111 in t h e series “Optical Rotatory Dispersion Studies” by C. D . For paper L I I see Tetvaiiedvorz Lellers, 237 (1961). (2) R C. Cookson and N. S. Wariyar, J . Chem Soc., 2303 (1950); 14. Birnbaum, R. C. Cookson and N. Lemin, i b i d . , 1224 (1961). (3) R. B. Woodward and E. G. Kovach, J . .4m Chew. Soc., 72, 1009 (1950). (1) H . Labhart and G. Wagnihre, Ifria. C h i i n A c / u , 42, !21!) (19.59). I:ur a recent application of these views see 9 . Winstein, 1. de Vries and R. Orloski, J . A m . Chcm. SOL.,83, 2020 (10131).
COMMUNICATIOXS TO THE EDITOR
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shape of the partial rotatory dispersion curve is uniquely determined by the shape of the corresponding absorption curve. This means that in a compound such as that depicted in Fig. 1, where the electric dipole and magnetic dipole transition moments must be colinear, i t should be possible to construct the rotatory dispersion curve (if the background rotation is not too large) solely f r o m a knowledge of the absorption curve.6.9 Our calculations on this point, indicated in Fig. 1, bear out our contentions. I n a later paper the theory will be discussed in detail and examples will be given which point up the utility of the generalized octant rule. ...~~_I
i
(9) A . Moscowirz, chapter 12 in ref. 5 .
Fig. 1.-Rotatory dispersion of dimethyldibenzsuberone in isooctane: calculated; 0 experimental. Two dots ronnected by a vertical line indicate two separate measurements at the same wave length.
DEPARTMENT OF CHEMISTRY XETVYORKUXIVERSITY NEWYORK53, N. Y . DEPARTMENT OF CHEMISTRY
phenomenon inay be attributed to the formation of a twisted composite chromophore from the carbonyl and ethylenic (or other a-electronic) moieties ; there are indications from ultraviolet spectral and optical rotatory dispersion6 data that in the a-haloketones, chlorine, bromine and iodine may act in the same manner as the a-electronic groups. As has been noted previously,6 such i n herently twisted chromophores (e.g., hexahelicene) will in general be associated with rotational strengths of greater orders of magnitude than are their more symmetric counterparts (e.g., the carbonyl group in saturated ketones). The above discussion has important implications for structural organic chemistry. It permits a generalization of the octant rule7 to include P,-yunsaturated ketones, although the theoretical principles involved are somewhat different than for the case of the saturated alkyl ketones. Moreover, because the contributions to the optical activity associated with the inherent asymmetry so outweigh those that can be attributed to asymmetrically disposed substituents, such as the ring methylene and alkyl groups, the sign of the Cotton effect of the long-wave length transition will be determined solely by the orientation of the unsaturated group with respect to the carbonyl chromophore.8 In addition, because of the limitations placed on such orientations by the necessity of satisfying reasonable bond distances and bond angles, the handedness of twist (and hence the sign of the Cotton effect) can be correlated with the four far octants of the previously enunciated octant rule,7 provided that the relative geometry of aelectronic system and carbonyl group is fixed, as in rigid molecules. Conversely, conformations m a y be assigned to flexible molecules of this type o n the basis of absolute conJiguration and Cotton effect curve. I t is an important corollary to our work that for the type of compound under consideration the
MINNEAPOLIS 14, MINX.
~
( 5 ) C. Djerassi. “Optical Rotatory Dispersion,” McGraw-Hill Rook Co., New York, N. Y., 19GO. ( 6 ) U’.hloffitt and A . Noscowitz, J. Chem. Phys., SO, 648 (1959). (7) W, hloffitt, R. B. Woodward, 9.hloscowitz, W. Klyne and C. Djerassi. J. Ant. Chein. S a c . , 8 3 , August (1961). See also chapter 13 in ref. 5. ( 8 ) .1; l l i i l i i i r . < l i d C. Djerasi, J. A m . Chem. Sor., 81,6247 (1960).
LTNIVERSITY O F MINNESOTA
KURTMISLOW
M.A. W. GLASS ALBERT~IOSCOTVITZ
DEP.4RTMEXT O F CHEMISTRY
STAXFORD UNIVERSITY CARLDJERASSI STAXFORD, CALIFORSIA RECEIVED MAY20, 1961 EQUILIBRIA BETWEEN LOW-MOLECULAR WEIGHT “LIVING” POLYMERS AND THEIR MONOMER’
Sir: Solutions of “living” polymers, ,Le., polymeric species retaining indefinitely their ability to grow,2 must be in equilibrium with their monomer. For a high molecular weight material the equilibrium concentration of the monomer is independent of the degree of polymerization.3,? However, this is not the case for a low molecular weight polymer. For each monor?ier-“liviIig” polynier systein there is a niiniiiiuin degree of poly-inerizntion n., such that Pno* inay grow but not degrade.5 The equilibrium concentration AT, is given, therefore, by the equation k&,.zP,* n
> no
=
k@‘,,* ;.e. Me = K - ’ ( W , * ) / @ P , * ) n>n, n t n , n > no
where k , and k d are the rate coiistniits of propagation and depropagation and K is the equilibrium constant of the polymerization, P,* 111 ~ r + cPn+l*. I t was suggested5 that no = 3 for the system amethylstyrene-poly-a-inethylstyrene and this structure was proposed for the “living” tetramer
+
CHI
CH3 CH3
CHI
Ph
Ph Ph
Ph
I l l I &a +,-C-CH2--CH*--C-C-CI-I2---C€I*-C-,?;af I I I I ’To show that the action of metallic sodium on diluted T H F solutions of a-methylstyrene produces the tetramer, and not a mixture of polymers of = 4, we studied the monomer-tetramer equilibrium. Increasing amounts of the monomer were added to the tetramer solutions and the mixture (1) This work was supported by the National Science Foundation. G r a n t KO.G14393 and by the Quartermaster Corp Grant No. DA10-129-OM-1297. (2) ;ZI Szwarc, Nature, 178, 1168 (1933); hf. Szwarc, M. Levy and R. Milkovitch, .I. A m . Chrm. Sor., 78, 26513 (1956). (3) H. W. McCorInick, J . P u l y m t v ScL, 26, 488 (1957). (4) D. J. Worsfold and S. Bywater, ibid., 26, 299 (1957). (5) hI. Szwarc, Makromolckuiare Chemie, 36, 132 (1960).
