Vol. 2, No. 4, August, 1963 The plot of pH vs. Elj2 for the reduction of Mo(V1) to Mo(V) indicates two electrode reactions are involved, depending on pH. From pH 2.5 to 5.0 it is probable that two hydrogen ions are involved in the reduction (slope = 1.82) while from p H 5.0 to 7.0 only one hydrogen ion is involved (slope = 0.88). This difference undoubtedly reflects changes in the state of polymerization of molybdate with pH. A similar phenomenon was observed by Pecsok and Sawyer3 with EDTA complexes of Mo(V1).
STRUCTURE OF CsHsNiC6H5C2(CO2CH& 713 It is clear that under the proper conditions of p H and ligand concentration, gluconic acid will serve satisfactorily in the polarographic determination of Mo(V1). Furthermore, the data reported here indicate the possibility of the existence of other complexes with a-hydroxy acids, some of which may be of biochemical significance. Acknowledgment.-Thanks are expressed by the authors to the Public Health Service (Grant GM08347-1, Division of General Medical Sciences) for financial support.
CONTRIBUTION FROM THE DEPARTMENT OF CHEMISTRY, OF WISCONSIN, MADISON 6, WISCONSIN UNIVERSITY
Structure and Nature of Bonding of C,H,NiC,H,C2(C02CH,)21
Received December 21, 1962 The structure of C5H5NiC6HbC2( CO2CH3)2has been determined from an X-ray anslysis. The crystalsocontain four moleyules in a monoclinic unit cell of symmetry Cc and dimensions a = 13.40 f 0.02 &, b = 13.78 f 0.02 A., c = 7.88 f 0.01 A., and p = 94’45’ f 10’. Final least-squares refinement has yielded discrepancy factors of RI = 6.8% and RZ = 7.5y0 for 978 observed reflections. The structure consists of discrete molecules in which a Ni(I1) is covalently bonded to a rcyclopentadienyl anion and a bicyclo[2.2.1]heptadienyl (Le., norbornadienyl) anion which coordinates to the metal uia a u-bond from the bridged carbon atom and a p-bond from the olefinic group to which the methyl carboxylate substituents are attached. The C5HeNiC6HbC2 fragment possesses an approximate mirror plane which passes through the nickel, one carbon of the cyclopentadienyl ring, and the bridge carbon and mid-points of the olefinic bonds of the norbornadiebyl represents the first known value for a nickel bonded to a moiety. The Ni(I1)-bridged carbon distance of 1.964 =t 0.012 1. tetrahedral carbon atom. The unconjugated olefinic bond is symmetrically linked to the Ni(I1) with an average h3-C distance of 1.970 f 0.0009 d.; the nickel distance to the mid-point of the olefinic bond is 1.83 A. The olefinic C-C distance of 1.450 h 0.016 A. is significantly longer than the c-C distance of 1.330 f 0.021 A. for the uncoordinated olefin (of normal double bond length). A detailed comparison of the structural features of the norbornadienyl group in C5HsNiC ~ H ~ C ~ ( C O ~is C made H ~ ) Zwith those of the norbornadiene molecule itself and the molecule in chelated form as norbornadienedichloropalladium(11). Structural evidence for a small localized interaction between the Ni( 11) and the cpclopentadienyl anion is given from the bond lengths and resulting molecular conformation. This specific interaction is ascribed to the perturbation of the r-electron density on the CsH6- anion by the ring’s MO interaction with the dsp*-type hybrid orbitals of the Ni( 11). The nature of this bonding is discussed with respect to recently found localized metal interactions with other n-organic systems.
Introduction Alkyne complexes of binuclear cyclopentadienylnickel have been reported3-5 in which an alkyne group as a distinct molecular entity utilizes its n-orbitals for bonding t o each of the two nickel atoms by an electronpair b-type bond.6 More recently a second type of cyclopentadienylnickel alkyne complex has been prepared by Dubeck7 by the reaction of dicyclopentadienylnickel with dimethylacetylene dicarboxylate. Elemental analysis and molecular weight measurements suggested a mononuclear nickel complex formed by a bimolecular coupling of the two reactants. On the basis of the compound’s diamagnetism, n.m.r. spectrum, (1) Presented in part at the 7th International Conference on CoBrdination Chemistry, Stockholm and Uppsala, Sweden, June, 1962. (2) Based in part on a dissertation submitted by Chin Hsuan Wei to the Graduate School of the University of Wisconsin in partial fulfillment of the requirements for the degree of Doctor of Philosophy. (3) J. F. Tilney-Bassett and 0. S. Mills, J. A m . Chem. Soc., 81, 4757 (1959). (4) M. Dubeck, { b i d . , 82, 502 (1960). (5) J. F. Tilney-Bassett, J . Chem. SOC.,577 (1961). Chem. Soc., 297 (1960). (6) P.L.Pauson, PYOC. 7) M. Dubeck, J. Am. Ckem. SOC.,82, 6193 (1960).
and degradative products, Dubeck7 proposed a molecular model arising from the interaction of a nickel(I1) with a n-cyclopentadienyl anion and with two electronpair donors, one donor an aliphatic carbon (formerly a cyclopentadienyl carbon atom) and the other a nonconjugated olefinic group (formerly the alkyne). The present X-ray study of C S H S N ~ C S H ~ C ~ ( C O ~ C H , ) ~ has confirmed the over-all configuration of Dubeck’s model. The molecular geometry provides evidence for a small localized interaction between the Ni(I1) and the cyclopentadienyl anion. The nature of this bonding is described, and the criteria presumably necessary for the formation of similarly localized cyclopentadienylmetal interactions are given. A detailed comparison of the molecular features of the compound with those of norbornadiene8 and norbornadienedichloropalladium(IIJ9is made. (8) C.F. Wilcox, Jr., S. Winstein, and W. G. McMillan, ibid., 82, 5450 (1960). (9) N. N. Baenziger, J. R . Doyle, G. F. Richards, and C. L. Carpenter, “Advances in the Chemistry of the Coardination Compounds,” ed. by S. Rirschner, The MacMillan Co., New York, N.Y.,1961, pp, 131-138.
714 LAWRENCE F. DAHL.4ND
CHIN
HSUANWE1
Inorganic Chemistry
Experimental Procedure ‘l’he compound was generously made available to us by Urs. Filbey and Dubeck of the Ethyl Corporation. The sample was recrystallized from toluene, and a small needle crystal of average width 0.06 mm. and length 0.26 mm. was used for obtaining intensity data without absorption correction. The density of the crystal mas measured by flotation in both 2-iodopropanetetrachloroethane solutions and 2-iodopropane-carbon tetrachloride solutions. The unit cell lengths were determined from both Weissenberg and precession photographs; p was measured from precession photographs. Multiple-film equiinclination photographs were obtained for nine reciprocal levels (hkO through hk8) with Zr-filtered Mo Ka radiation. Timed-exposure precession photographs of Okl data also were obtained with Mo Ka radiation. Both Weissenberg and precession intensity data consisting of 978 independent reflections were estimated visually with the aid of an internal standard and then corrected for Lorentz polarization effects. The corrected precession intensity data were used to place the corrected U’eissenberg intensity data on a common scale. Final adjustment of scale factors for different Weissenberg levels was made by least-squares refinement.
Results Unit Cell and Space Group.-Crystals of C,H&iCbHG(CO2CH3)2 (mol. wt. 331.0; m.p. ca. 85”)aremonoclinic with a = 13.40 i 0.02 B., b = 13.78 f 0.02 A,, c = 7.88 =t0.01 A., p = ~ ~ 3 3 10’; ’ volume of unit cell = 1450 *k3; pobsd = 1.30 g. ~ m . -us.~ pcalcd = 1.52 g. ~ m . for - ~ four molecules per unit cell. The total number of electrons per unit cell, F(000) = 688. The linear absorption coefficient for 110 K a radiation (A = 0.7107 A.) is 14 cm.-l. Systematic absences, { h k l ] for h k odd, (hOI} for I odd, and {OkO} for k odd, indicate either Cc or C2/c as the probable space group; the choice of Cc(C2) was confirmed by the structural determination. A11 atoms occupy the general fourfold set of positions (4a) : x,y 1 z; x, -y, ’/? z; +C centering.1° Determination of the Structure.-The structural analysis of the compound required the location of one nickel, sixteen carbon, and four oxygen atoms (and ideally the sixteen hydrogen atoms) which correspond to one molecule. The other three molecules in the unit cell are related by the space group symmetry. A three-dimensional Patterson function was computed from the corrected intensities. An interpretation of the resulting maps revealed the positions of the nickel atoms in the unit cell. Since the symmetry requirements of the space group Cc do not specify the origin of the unit cell in the a and c lattice-vector directions, the x and z coordinates of the asymmetric nickel atom arbitrarily were chosen a t I/da and I / ~ c , respectively, and were not varied, A three-dimensional isotropic least-squares refinement of the nickel’s y coordinate and its thermal parameter (initially given an estimated value of 2.0 A.2) was carried out. After two cycles a discrepancy index, RI = [21I F o ~- IFc/l/Z: 1 Foil X 100, of 26.9% was obtained, which indicated
+
+
(10) “International Tables for X-Ray Crystallography,” 1’01. I T h e Kynoch Press, Birmingham, England, 1952, p. 89.
that the nickel atoms were placed correctly in the unit cell. A three-dimensional Fourier synthesis based only on the phases of the nickel atoms was computed next. A fundamental difficulty arose concerning the interpretation of the peaks obtained on the maps, X1though the structure is not centrosymmetric, the nickel atoms are related to one another by centers of symmetry, and hence their phase contributions to the structure factors are such as to result in a superposition of electron-density peaks for two structures-the partially correct structure and its centrosymmetrically related image. On the basis of presumed bond distances and probable conformation, an assignment of eight Fourier peaks as a nickel atom, a five-membered cyclopentadienyl ring, and two carbon atoms was made. Two cycles of least-squares refinement of the positional and thermal parameters of these eight atoms resulted in a decrease of Iil t o 22.17,. Another three-dimensional Fourier function was computed from the refined phases for the eight atoms. By this procedure, which magnifies the peaks corresponding to the coordinates of the other atoms in the correct structure and eliminates the pseudosymmetrically related images, nine new peaks were identified. This “true-structure magnifying” procedure was repeated once more to locate the remaining atoms. Finally, another isotropic leastsquares refinement was carried out for all twenty-one non-hydrogen atoms. After six cycles positional and thermal parameters remained constant well within the limits of their standard deviations. The final discrepancy values, XI and Rz = [Zw( 1 Fol - l Fcl z / Z W ~ F ~ Ix~ ]100, ~ ’ ~are 6.8 and 7.5%) respectively, for the 978 non-zero reflections; the final value of the error of fit function, [ W ~ ~-F I~ FJ ~ I / ’/ (n~) ~ ! ’is~ 1.137.11 , In these calculations the Sly-Shoemaker Fourier program1* was used for the Patterson and Fourier syntheses, and the Busing-Levy full matrix least-squares program’3 was employed for the least-squares refinements. The weights14 for the individual reflections were assigned as
1
4%= 1O/F,
if Fo 2 4Fo,mi,j
11% = FJ/1.6Fu2,zntn if FO < 4F0,,,,,
The refined structure was confirmed by a calculation of a three-dimensional difference synthesis which revealed no residual peak heights greater than 0.6 elect r ~ n / A . ~KO . attempt was made to identify any of these peaks as due to hydrogen atoms. For the calculation of structure factors the atomic scattering factors of (11) Calculated and observed structure factors are deposited as Document No. 7414 with t h e American Documentation Institute, -4uxiliary Publication Project, Photoduplication Service, Library of Congress, Washington 25, D . C. A copy may be secured by citing t h e Document S u m b e r and remitting S1.25 for photoprints or 81.25 for 35.“. microfilm in advance payable t o : Chief, Photoduplication Service, Library of Congress. (12) W. G. Sly and D . P. Shoemaker, “Two- and Three-Dimensional Crystallographic Fourier Summation Program for t h e I B h l 701 Computrr,” MIFRI (19601. (13) W. R . Busing and H. A. Levy, “ A Crystallographic Least-Square@ Refinement Program for t h e I B h l 704,” Oak Ridge National Laboratory Report 59-4-37 (1969). (14) E. W. Hughes, J . A m . Cizznt. Soc., 63, 1737 (1941) L. I
