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The Crystal and Molecular Structure of the Tetra-n-butylammonium Salt of the Dianionic Dimer of Bis( 1,2,3,4-tetrachlorobenzene-5,6-dithiolato)cobaltate M. J. Baker-Hawkes, Zvi Dori, Richard Eisenberg, and Harry B. Gray' Contribution f r o m the Department of Chemistry, Columbia University, New York, New York 10027, and Contribution No. 3569 f r o m the Gates and Crellin Laboratories of Chemistry, California Institute of Technology, Pasadena, California 91109. Received September 11, 1967 Abstract: The crystal and molecular structure of the tetra-n-butylammonium salt of the dimer of the bis(1,2,3,4tetrachlorobenzene-5,6-dithiolato)cobaltateanion has been determined from three-dimensional single-crystal X-ray data collected by film methods. The structure has been refined by least-squares procedures to conventional and weighted R factors of 0.094 and 0.080, respectively, for 1588 nonzero reflections. The complex crystallizes in space group P21/~-C2ht~f the monoclinic system in a unit ce!l of dimensions: a = 13.95 i 0.03, b = 18.98 =t 0.04, c = 15.36 f 0.03 A, p = 114.8 f 0.4", and V = 3692 A3. An experimental density of 1.52 f 0.05 g/cm3 agrees well with a calculated value of 1.54g/cm3for two dimeric molecules in the unit cell. The [co2(s2c6c14)4]2-dimer units are centrosymmetric with the center of symmetry at the origin located midway between the Co atoms. Dimerization occurs through the formation of two Co-S bonds of length 2.f04 A, making each Co atom fivecoordinate. The estimated standard deviation for this Co-S distance is 0.00T A. The coordination about the Co atom is best described as square pyramidal with the metal atom raised 0.26 A out of the plane of the four basal S atoms toward the apical S of the second planar unit. The [ C O ( S ~ C ~ C ~ half-dimer ~ ) ~ ] - unit is only approximately planar with the ends of the benzene rings bent out of the plane, presumably because of repulsion effects from the adjacent half-dimer unit. Molecular orbital calculations on related Ni systems are used to provide an electronic explanation for the dimerization of the [Co(S2C6C14)2]- units. The electronegative chlorine substituents remove electron density from the central metal atom, particularly that associated with the metal 4p, orbital; the 4p, is thus available for forming axial u bonds with electron donor groups. The intermonomer bonding in the [Co2(SZC~cl&]2-dimer unit is attributed to two u bonds formed between the Co 4p, orbitals and occupied T orbitals which have large S components.
T
he remarkable stability of square-planar transition metal complexes containing unsaturated fivemembered metal chelate rings with sulfur donor atoms is well established.2 Recent studies aimed at the elucidation of the electronic structures of complexes of types I and I1 have indicated that the electronic nature of the substituent plays an important role in stabilizing these systems and in influencing their magnetic behavior.
r
R= CN, CF3,CsH5,CH3,H
r
1"
X = Y = Z = H,CH3,CI4 X = Y = H ; Z=CH3 X =H;Y = Z = CH3
(1) Author to whom correspondence should be addressed at Gates and Crellin Laboratories of Chemistry, California Institute of Technology, Pasadena, Calif. (2) (a) E. Billig, R. Williams, I . Bernal, J. H. Waters, and H . B. Gray, Inorg. Chem., 3, 663 (1964); (b) G. N. Schrauzer and V. P. Mayweg, J . A m . Chem. SOC.,87, 3585 (1965); (c) A . Davison, N. Edelstein, R.H. Holm, and A . H. Maki, Inorg. Chem., 2, 1277 (1963). (3) M . J. Baker-Hawkes, E. Billig, and H . B. Gray, J . A m . Chem. SOC., 88, 4870 (1966). (4) The following abbreviations are used to denote the respective ligand systems: bdt = benzene-1,2-dithiolate, X = Y = 2 = H ; tdt = toluene-3,4-dithiolate, X = Y = H, 2 = CH3; xdt = xylene-4,5dithiolate, X = H, Y = 2 = CHa; and pdt = prehnitene-5,6-dithiolate, X = Y = 2 = CH3.
This influence can be seen by comparing the magnetic properties of two closely related monoanionic Co complexes. The bis(toluene-3,4-dithiolato)cobaltate complex has been found to have a stable spin-triplet ground state in both the solid and solution states for a wide variety of solvent^.^ Similar magnetic behavior has been observed for the related Co(bdt)?-, C ~ ( x d t ) ~ - , and Co(pdt)z- complexes. In contrast, the bis( 1,2,3,4tetrachlorobenzene-5,6-dithiolato)cobaltate complex is diamagnetic in the solid state and exhibits varying magnetic behavior in solution, depending on the coordinating ability of the solvent. In poorly coordinating solvents, such as cyclohexane and tetrahydrofuran, co(s2c6c14)?-is paramagnetic with magnetic moments indicating a full spin-triplet ground state (pes = 3.14 and 3.18 BM, respectively). In the stronger coordinating solvent, dimethyl sulfoxide, the magnetic moment is reduced to a value of 2.37 BM. In pyridine, a very strong coordinating solvent, co(s2c6cl4)?- is diamagnetic and there is evidence for addition of pyridine to give five- and six-coordinate species. The unusual magnetic properties of Co(S2C6C14)2can be explained in several different ways. First, it is possible that the spin-singlet ground state in the solid results from a sufficiently large separation of the highest occupied and lowest unoccupied energy levels in monomer units; the levels may then be significantly perturbed in solution to give a spin-triplet monomer species. A second possibility is the existence of strong pairwise interactions between spin-triplet monomer ( 5 ) R. Williams, E. Billig, J. H . Waters, and H. B. Gray, J . Am. Chem. SOC.,88, 43 (1966).
Baker-Hawkes, Dori, Eisenberg, Gray J Bis(l,2,3,4-teti.achlorobenzene-5,6-dithioIato~cobaItate Complex
4254 units in the solid resulting in diamagnetism. This possibility has been partially realized in the monoanionic complex bis(maleonitriledithio1ato)nickelate (I with R = CN). The reduced magnetic moment observed for this complex in the solid state (peff 1.0 BM) has been shown to result from magnetic interactions between pairs of nearby planar anion units.6 In the extreme, strong pairwise interactions can lead to structurally identifiable dimeric species in the solid state. The neutral bis(cis- 1,2-trifluoromethylethene1,2-dithiolato)cobalt complex has been shown to have a dimeric structure.’ Thus, the effect of the crystalline coordination environment on the various Co systems must be unequivocally established before any meaningful conclusions about their electronic structures can be made. The present study is the second part of our investigations on the structures of monoanionic cobalt complexes. In the first pqper, the results of a structure determination on the triphenylmethylarsonium bis(toluene-3,4-dithiolato)cobaltate complex were presented.8 The Co(tdt)z- anions were found to be essentially square planar with the anion units well separated (Co-Co distance 10.21 A). The spin-triplet ground state was explained in terms of two close-lying out-ofplane T orbitals.Y In this paper, we present the results of an X-ray structure determination on the tetra-n-butylammonium salt of the bis( 1,2,3,4-tetrachlorobenzene-5,6-dithiolato)cobaltate complex. This structure determination confirms the existence of a dimeric anion in the solid state. A comparison of the structure with related Co structures is presented, and a model is proposed for the electronic nature of intermonomer bonding in the dimeric structures. Unit Cell and Space Group Determination. The complex [(n-C4H,)4N],[Co~(SzC8C14)4] was synthesized according to the procedure previously described3 and shiny blue-black crystals of the complex were obtained by recrystallization from methylene chloride solution. On the basis of precession and Weissenberg photographs taken with Mo K a radiation, the crystals were found to belong to the monoclinic system. The cell has the dimensions: a = 13.95 + 0.03, b = 18.98 i 0.94, c = 15.36 + 0.03 A, p = 114.8 f 0.4”, V = 3692 A3. An experimental density of 1.52 rt 0.05 g/cm3, obtained by the flotation method using a mixture of hexane and carbon tetrachloride, is in good agreement with a density of 1.54 g/cm3 calculated from the X-ray data for four monomeric molecules or two dimeric units i n the unit cell. The systematic extinctions, OkO for k odd and A01 for I odd, indicate the centrosymmetric space group Czh5-P21/cand thus imply the probability that all atoms for one monomeric unit are in general positions of the space group. However, crystallographic symmetry conditions impose a center of symmetry on the dimer. Collection and Reduction of the X-Ray Data. Intensity data were collected at room temperature by the multiple film equiinclination Weissenberg technique,
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(6) J. F. Weiher, R. R. Melby, and R. E. Benson, J . A m . Chem. SOC., 86,4329 (1964). ( 7 ) J . H. Enemark and W . H. Lipscomb, Iiiorg. Chem., 4, 1729 (1965). (6) R . Eisenberg, Z. Dori, H. B. Gray, and J. A . Ibers, ibid., 7, 741 ( 1968), (9) M. J. Baker-Hawkes, Ph.D. Thesis, Colunibia University, 1967.
using Zr-filtered Mo K a radiation. The crystal was mounted with a as the rotation axis, and the layers Okl to 8kl were recorded. The intensities of 2921 independent reflections accessible within the angular range &, < 20’were estimated visually from a calibrated intensity strip. The usual Lorentz-polarization factors were then applied to the intensities to yield FU2values (where F, is the observed structure amplitude). The F, values were subsequently brought to an approximate absolute scale using a modification of Wilson’s statistical method. N o corrections were made for absorption due to the small size of both the crystal (a parallelepiped of approximate dimensions 0.13 X 0.10 X 0.08 mm) and the linear absorption coefficient (p = 12.7 cm-I for Mo K a radiation). Solution and Refinement of the Structure. The positions of the cobalt and two sulfur atoms were determined from a three-dimensional Patterson function after much trial and error. The structure proved difficult to solve because of the large number of heavy atoms in the unit cell which resulted in many Patterson peaks of relatively high density. A three-dimensional difference Fourier based on phases obtained from these Co and S atom positions yielded the locations of one C1 and an additional S atom. The positions of the remaining S and seven C1 atoms were then determined from a second difference Fourier map. The positions of all 28 carbon atoms and the one cation nitrogen were located in subsequent difference Fourier maps based on phases obtained from the refined positions of the heavy atoms. The structure was refined by a least-squares procedure. The function minimized was Z w ( F , - F J 2 , where the weights w were assigned in the following way: I < 4, w = (I/4)2;4 < I 1 7 5 , = ~ (175/1)?, I being the average raw intensity for the particular reflection. The atomic scattering factors for the neutral atoms tabulated by Iberslla were used. The anomalous parts of the Co, S, and C1 scattering factors were obtained from Templeton’s tabulationllb and were included in the calculated structure factors. Initially, the refinement was carried out in which all atoms were assigned individual isotropic thermal parameters. This refinement of 177 positional, scale, and thermal parameters converged to a conventional R factor ( R = Z l IF, - IF, ‘ / 2 F,J) of 0.138 and a weighted R factor R’ (R’ = (Z:w(F, - F c ) * / ~ w F , 1 2 ) ~ ’ ~ ) of 0.136. A difference Fourier based on this refinement provided evidence for considerable anisotropic thermal motion of the heavy atoms. After the correction of several indexing errors, a final refinement was carried out in which the Co, S, and C1 atoms were allowed to vibrate anisotropically. In this refinement, the 12 independent C atonis of the anion, together with the 17 C and N atoms of the ( ~ Z - C ? H ~ )cation, ~ N + were restricted to isotropic vibration, Because of computer limitations, this partial anisotropic refinement was carried out in blocks. First the positional and thermal parameters of the (10) I n addition to various local programs, the main programs for the IBM 7094 computer used in this work were local modifications of Zalkin’s FORDAP Fourier program and the Busing-Levy ORFLS leastsquares program. C. I
