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Molecular Structures of [ (C,H,) 2 P C ~ C 3 H jand ] [C,H, ) ,PNiC,H,] 2 . An Assessment of the Influence of a Metal-Metal Bond on the Molecular Geometry of an Organometallic Ligand-Bridged Complex' Judith M. Coleman?and Lawrence F. Dah13
Contribution from the Department of Chemistry, Unioersity of Wisconsin, Madison, Wisconsin 53706. Receiued August 23, 1966 Abstract: Structural determinations of the corresponding diamagnetic complexes [XCoC5H& and [XNiC6H5I2 (where X = (C6H&P) have provided the first known demonstration of the effect of an electron spin-coupling interaction on the molecular geometry of a binuclear organo-(transition metal) complex containing bridging groups. A separate metal-metal single-bond interaction, of bond strength no doubt equivalent to that found in dimeric organo(transition metal) complexes linked only by metal-metal bonds, is required for the cobalt dimer in order to explain the radical difference between the molecular geometries of the cobalt and nickel structures. Both molecular compounds crystallize in the monoclinic system with two dimeric molecules in a centrosymmetric unit cell. For [(C6H5)2PC~C5HS]2 the unit cell has the symmetry P2/b (C2h4,No. 13) and dimensions a = 8.64, b = 15.72, c = 10.31A, y = 90" 50', while for [(C6Hs)nPNiCaH5]2 the unit cell has the symmetry P24n (Cnh5,No. 14) and dimensions a = 9.46, b = 10.83, c = 16.78 A, y = 122" 28' (c axis unique in both cases). The diamagnetism of [(CsH&PCoC5H& is achieved by the formation of a bent metal-metal bond in which the metal-metal distance has decreased from a nonbonding value of 3.36 A in [(C6H5)2PNiC5H5]2 to a bonding value of 2.56 A in [(CsHs),PCoC,H,]2. The stereochemical consequence of this metal-metal bond (which occupies a regular coordination site about each cobalt atom) is a symmetrical bending deformation of the (PM)pframework along the P. . . P ' line from a planar (PN& system of dihedral angle 180" in [(C6H5)2PNiCSHS12 to a nonplanar (PCo), system of dihedral angle 105" in [(C6H5)?PCoC5H512. The degree of angular distortion is such as to decrease the bridging M-P-M' angle by 30" from a normal unstrained value of 102.4' in the nickel complex to a sharply acute angle of 72.5" in the cobalt analog. The detailed molecular features of these two compounds are compared to those of [C2HjSFe(C0)3]2 and other related complexes. The close resemblance of the molecular configurations of the electronically equivalent complexes [XCoC5H& (X = (C6H&P) and [XFe(CO)J (X = C2HsS)is clearly illustrated by the essential invariance of the (XM)U bridging system on replacement of the three carbonyl groups by the cyclopentadienyl ring which occupies three coordination sites. The observation of a possible cyclopentadienyl ring distortion in [[C6H,)2PNiCsH5]2 and its theoretical implications are discussed. Qualitative inferences are drawn from the structural features concerning differences in the bond energy of a x-CgH5-CoGS. a n-CsHS-Niinteraction and in the x-bonding character of a doubly bridged M-(SR)-M' us. M-(PR+M' system for the first-row group VI11 transition metals. ince 1957 when several polynuclear metal carbonyl complexes first were reported from X-ray diffraction studies4,: to be held together only by metal-metal bonds, increasing attention has been focused around the large number of transition metal compounds for which metal-metal bonds have been detected. Although a wide variety of structural types with metal-metal interactions are now known,fi little direct structural data are available concerning the degree of dependence of the molecular configuration on the metal--metal bond (except for those complexes stabilized by only metal-metal interactions). The recent preparation and characterization by Hayter7r8 of [(CgHj)~PCoC5H& and [(C6H5)2PNiC5Hj]2 offered the first real opportunity t o examine the actual
S
(1) Presented in part at the 149th National Meeting of the American Chemical Society, Detroit, Mich., April 1965. (2) This article is based in part on a dissertation submitted by J. M. Coleman to the Graduate School of the University of Wisconsin in partial fulfillment of the requirements for the degree of Doctor of Philosophy, 1966. (3) Alfred P. Sloan Research Fellow, 1963-1965. (4) F. C. Wilson and D. P. Shoemaker, J . Chem. Phys., 27, 809 (1957). ( 5 ) L. F. Dahl, E. Ishishi and R. E. Rundle, ibid., 26, 1750 (1957); L. F. Dahl and R. E. Rundle, Acta Crqsf., 16, 419 (1963). (6) C . (a) J. Lewis in "Plenary Lectures of VIIIth International Conference on Coordination Chemistry," Butterworths, London, 1965, pp 11-36: (b) J. Lewis and R. S. Nyholm, Sci. Progr., 52, 557 (1964). (7) R. G. Hayter, Inorg. Chem., 2, 1031 (1963). (8) R. G. Hayter and L. F. Williams, J . Inorg. Nucl. Chem., 26, 1977 (1964).
Journal of the American Chemical Society J 89:3
influence of a metal-metal interaction on the molecular geometry of an organometallic complex containing ligand-bridged groups in addition to the metal-metal interaction. These above two dimeric transition metal complexes, both of which possess bridging diphenylphosphine ligands, differ from each other only by the necessity of an electron spin-coupling interaction in [(C6Hj)ZPC~CjHj]Z as opposed to none being required in the nickel analog in order for each of the metal atoms to achieve a closed-shell electronic ground-state configuration in conformity with the diamagnetism of both compounds. For dinuclear metal complexes with bridging ligand groups, it has been pointed O U ~ that ~ ~ the , ~ observed diamagnetism may not necessarily be attributable to the formation of a distinct metal-metal bond per se but instead may be the result of either a super-exchange mechanism which operates through the bridging ligand groups or a high spin-orbit metal coupling constant. The spin-orbit coupling constants of cobalt and nickel are not sufficiently large to quench an electron spin moment. Because of the presence of two bridging diphenylphosphine groups in [(CsH&PCoCjH&, it (9) (a) Cf. B. N. Figgis and J. Leais, Progr. Inorg. Chem., 6 , 37 (1964), and references contained therein; (b) W. E. Hatfield and J. S. Pascha1,J. Am. Chem. Soc., 86, 3888(1964); W. E. Hatfield, Y .Mato, H. B. Jonassen, and J. S . Paschal, Inorg. Chem., 4, 97 (1965).
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543
is not indisputable that a separate metal-metal bond is the prime factor in determining its molecular structure without a direct comparison of its configuration with that of the corresponding nickel analog. A stereochemical consequence of the fact that these two complexes are representative members of the general type [XMC5H& (where M = Co or Ni and where X represents a bridging three-electron donor such as halogen, RS, R2P, R2As, or one-half of a bidentate group such as S2, SLC2R2,or RPCH2CH2PR)is that any of these related dimeric cobalt and nickel cyclopentadienyl complexes which contain metal-linked bridging atoms with a tetrahedral-like arrangement of four electron pairs undoubtedly are isostructural to a first approximation with the corresponding diphenylphosphinecobalt or -nickel cyclopentadienyl complex. Of these possible dimeric analogs, the compounds [RSCoC5H& (R = CH3),10[RSNiC5H& (R = CH3, G H S , and c6H6),I1[R2PNiCjH& (R = CH3),' and [R2AsNiC5H6]2 (R = CH$ have also been iso1ated;'Z the selection of the diphenylphosphine derivatives for crystallographic studies primarily was due to their relative stability as well as availability. An added incentive for this structural investigation was the opportunity to make a detailed comparison of the molecular features of an [XCoC5H5I2complex with that of the closely similar complex [XFe(C0)3]2 (where X = CzHbS) whose molecular geometry, established from an X-ray study, had been attributed to a bent metal-metal bond.13 Since the Fe(C0)3 group is electronically equivalent with the CoC5H5 group, it was of particular significance to ascertain the degree of alteration of the (XM), skeleton on replacement of the three carbonyl groups with the cyclopentadienyl group. Experimental Section Samples of [(CsH&PCoCsH& and [(CsHs)zPNiCsHs]z were generously supplied to us by Dr. R. G. Hayter of Mellon Institute (now at Shell Development Company, Emeryville, Calif.). For the cobalt compound, the black crystal utilized was a brickshaped fragment cut from a plate-like crystal to the approximate dimensions 0.5 X 0.5 X 0.4 mm and aligned inside a thin-walled Lindemann glass capillary with the 0.4-mm axis as the rotation axis. Multiple-film equiinclination Weissenberg data, obtained for ten reciprocal layers (Okl through 9kl), resulted in 1119 independent reflections. Timed-exposure precession photographs were taken from the hkO and hkl levels and merged with the Weissenberg data by least squaresI4 to give a total of 1180 reflections on a common scale; the weighted disagreement factor for the least-squares merging was 3.7x. For gathering intensity data on the nickel compound, a dark brown crystal of dimensions 0.4 X 0.4 X 0.5 mm was mounted
inside a thin-walled glass capillary about a rotation axis which corresponded to one of the 0.4 mm directions. Nine layers of multiple-film equiinclination Weissenberg data, Okl through 8kl, were collected from which a total of 1229 independent diffraction maxima were obtained. All intensity data for both compounds were taken with Zr-filtered Mo Ka radiation. The intensities for both compounds were estimated visually by comparison with a calibrated set of intensities taken with the same crystals. Corrections for Lorentz-polarization effects and spot extension'6 were applied to all photographic data, but absorption corrections were neglected because of the small absorption coefficients of pRmax = 0.13 and 0.17 for the cobalt and nickel compounds, respectively. The standard deviations of the individual structure amplitudes were estimated as follows. If ~ ( ~ and [C2H5SFe(N0)2]2(2.27 A),2F roughly approximates CZvsymmetry (as found within and the doubly bridged Co-S distances in C O ~ ( C O ) ~ - experimental error for the [PCoC,H& fragment) is (SC,Hj): (2.26 A ) , 4 i CO~(CO)IO(SC~H:,)~ (2.24 A),47 indicated for the least-squares plane calculations in Table VI (c and d). The dihedral angle between the and SC06(CO)11(SC2H6)d (2.2 1 A). 4i The correspondtwo nearly perpendicular phenyl rings is 94.3’; the ing sums of Pauling’s covalent radii for Fe (1.16 A), dihedral angles formed between the idealized vertical Co (1.16 A), Ni (1.15 A), P (1.10 A), and S (1.04 A) mirror plane defined by P, P’, and the midpoint of give 2.26 A for the Co-P bond, 2.25 A for the Ni-P Co-Co’ and phenyl rings 1 and 2 are 5.3 and 80.8”, bond, 2.20 A for the Fe-S bond, and 2.20 A for the respectively. Co-S bond. Hence, even despite the fact that the estimated covalent radius of phosphorus is 0.06 A greater Because of the P - . . P ’ distance of 2.88 A, the symmetry-related phenyl rings 2 and 2’, which are required than that of sulfur, the doubly bridged metal-P(C6H5)z by the crystallographic twofold molecular axis to be distances are significantly shorter than the doubly parallel with each other, experience a mutual repulsion bridged metal-SC2H5 distances. These bond-length as evidenced by: (1) an increase in the separation of comparisons indicate substantially greater 7~ bonding for a bridging P(C8H& group than a bridging SC2Hj these phenyl rings (Figure 1) from 3.27 A for the phosphorus attached carbon atoms (C(12) and C(12’)), group. to 3.45 A for the ortho carbon atoms (C( 13) and C( 17’)), The average p--CG’rIC distance of 1.84 A for both comto 3.80 A for the meta carbon atoms (C(14) and Cpounds is in good agreement with the corresponding distances found in Co( P(C6Hj)2H]3Br2(1.83 A),46 (16’)), to 3.97 A for the para carbon atoms (C(15) and C(15’)); and (2) a clockwise rotation of 9” for IrC1CO(SO?)(P(C6H,),)? (1.84 A), 48 HRhCOtP(C6(44) (45) (46) (47) (48)
W. Oberhatisli and L. F . Dahl, Inorg. Chem., 4, 150 (1965). R. W. Perry, R. F. Heck, and L. F. Dahl, to be published. J. A. Bertrand and D. L. Plymale, Inorg. Chern., 5, 879 (1966). C. H. Wei and L. F. Dahl, to be published. S. J. LaPlncn and J. A. Ibers, Znorg. Chem., 5 , 405 (1966).
(49) S. J. LaPlaca and J. A. Ibers, Acta Cryst., 18, 511 (1965). J. A. Ibers and S. J. LaPlaca, J . A m . Chern. Soc., 87, 2581 (1965). S. J. LaPlaca and J. A. Ibers, Inorg. Chem., 4, 778 (1965). R. Eisenberg and J. A. Ibers, ibid., 4, 773 (1965). C. E. Wilkes and R. A. Jacobson, ibid., 4, 99 (1965). S. F. Watkins and L. F. Dahl, to be published.
(50) (51) (52) (53) (54)
Journal of the Americun Chemical Society / 89:3 / February I , 1967
551
0
b
I
Figure 3.
[Ool] projection of the unit cell of [(CGH~)ZPCOCSHSIZ.
each of these phenyl rings about its P-C6H5 axis from the orientation perpendicular to the idealized vertical mirror plane defined above (viz., the one passing through P, P’, and the midpoint of Co-Co’) in order to decrease inter-ring 7r-orbital overlap. The known methylmethyl and methylene-methylene van der Waals conI/4 I14 tact distance of 4.0 A,55 as contrasted to a considerably Figure 4. [OlO] projection of the unit cell of [(C6H5)2PNiC5H&. shorter aromatic interaction distance of 3.4 A,55indicates that unless a pronounced deformation occurs in the [PCoC5H& moiety such as to cause a significant increase by 0.5 A in the separation of the C(12) and For [(C6H5)2PNiC5H5]2 the two independent phenyl C( 12’) carbon atoms, cyclic pdialkylphosphidocobalt rings, which make a dihedral angle of 116.5” with each cyclopentadienyl dimers should not be obtained. None other, are situated well away from the other atoms in is known to date, and attempts by Hayter and Williamss the molecule. The twistings of the phenyl rings 1 and to prepare the methyl analog of [(C6H5)2PCoC5H& 2 with respect to the planar (PNi)2 system are such that by the corresponding reaction of tetramethylbiphosthe dihedral angles are 67.3 and 75.0°, respectively; phine with C 5 H 5 C ~ ( C 0 )were 2 unsuccessful; instead, the corresponding dihedral angles with respect to the a noncyclic dinuclear cobalt complex of formula Comean plane comprised of the atoms P, P’, C(6), and (CO)C5Hj(P2(CH3),)Co(CO)C5H5was obtained. C(12) (Table VI, b’) are 135.7” for phenyl ring 1 and However, the existence of the analogous electroni145.4’ for phenyl ring 2. cally equivalent bis(p-dialkylphosphido)bis(tricarbonylFigure 3 shows the [OOl] unit cell projection of [(c6iron) complexes (with methyl and ethyl g r ~ u p s ) ~ ~ J ’H5)2PCoC5H5I2,while Figure 4 gives the [OlO] unit cell together with various related phosphine-substituted projection of [(C6H5)zPNiC6H5]z.The smallest interderivativesb8 does show that a sufficient nonrigidity of molecular H C . . CH contact distances in both comthe [XFe(C0)3]2skeleton exists such that bridging dipounds are 3.5 A which indicate that these molecualkylphosphido groups can formally replace bridging lar crystals are held together by the usual van der Waals mercapto groups. In this regard, a detailed comforces. parison of the nonbonding intramolecular distances of Other known diamagnetic pairs of ligand-bridged [(C6H5)2PCoC5H5]z us. those of [C2H6SFe(C0)&shows organometallic dimers of adjacent first-row transition that a drastic skeletal rearrangement of the (XFe)2 metals, which differ from each other in configuration system involving a considerably larger dihedral angle only by the necessity of a metal-metal interaction for than the 95” found for [C2H5SFe(C0)3]2 is necessary in one of the complexes (viz., the number containing the order to increase the closest syn methylene-methylene two transition metal atoms of lower atomic number), distance to a value greater than 3.6 A to render the include the dimeric systems [XM(N0)2]2(M = Fe, Co) complex energetically stable. and [XM(CO)& ( M = Cr, Mn) where X again designates a three-electron donating bridging group such as (55) L. Pauling, “The Nature of the Chemical Bond,” 3rd ed, Cornel1 halogen, SR, SeR, PR2, etc. A direct stereochemical University Press, Ithaca, N. Y . , 1960, pp 261-262. (56) R. G. Hayter, Znorg. Chem., 3, 711 (1964). comparison of corresponding representative members (57) J. Chatt and D. A. Thorton, J . Chem. Soc., 1005 (1964). of these two molecular classes is not available as yet, (58) D. T. Thompson, J . Organometal. Chem. (Amsterdam), 4, since structural studies have been reported only for 74(1965).
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[C2HjSFe(N0)2]2(for which the observed Fe-Fe bonding distance of 2.72 A is compatible with the electronspin pairing)26 and for [BrMn(CO)& (for which a molecular D2h configuration with a nonbonding intramolecular M n . . . M n distance of 3.74 A was found).jg However, the geometrical consequences of the metalmetal bonds in these two dimeric systems are apparent from stereochemical considerations. The dimeric cobalt nitrosyl complexes n o doubt will retain a similar symmetrical configuration to the one ascertained in the known mercaptoiron nitrosyl dimer (in which the two N O groups and two S atoms form a tetrahedral-like environment about each iron atom) except for a longer nonbonding metal-metal distance of magnitude 3.3-3.4 A (together with normal bridging ligand angles of approximately looo). Similarly, outside of the expected decrease in metal-metal distance to a single bond value of 3.0 A (as found in the related bridging complex (59) L. F. Dah1 and C. H. Wei, Acta C r j s r . , 16, 611 (1963).
[C5HXrS2C4F& for which a proposed chromiumchromium single bond is in agreement with the observed diamagnetism),60the [XCr(CO)4]2species should remain invariant in its general shape relative to that of [BrMn(CO)4]2. Structural work is in progress to verify these deductions. Acknowledgments. We are extremely grateful to Dr. Roy Hayter for furnishing us with samples of these two complexes and for his interest in this work. We also are pleased to acknowledge the Air Force Office of Air Research and Development Command for their financial support of this research. The use of the CDC 1604 and 3600 computers at the University of Wisconsin Computing Center was made possible by partial support of N.S.F. and W.A.R.F. through the University Research Committee. (60) S. F. Watkins and L. F. Dahl, Abstracts ofPapers, 150th National Meeting of the American Chemical Society, Atlantic City, N. J., Sept 1965, p 23-0; to be published.
Aminopolycarboxylic Acid Ligand-Exchange Kinetics of Selected Alkaline Earth Ions’ Richard J. Kula and Dallas L. Rabenstein
Contributionf r o m the Department of Chemistry, University of Wisconsin, Madison, Wisconsin 53706. Received June 15, 1966 Abstract: The ligand-exchange kinetics of the strontium-ethylenediaminetetraacetic acid system and the cal-
cium-N’-(2-hydroxyethyl)ethylenediamine-N,N,N’-triaceticacid system have been evaluated in aqueous solution using proton nmr line-broadening techniques. Exchange occurs by both first-order dissociationof the metal-ligand species and second-order processes involving free ligand and metal-ligand species. The pH dependence of the rate was used to evaluate the rate constants for the individualexchange steps. From comparison of the rate constants in these and other systems, mechanisms of the second-order exchange reactions are proposed, and the structure of the protonated metal-ligand species is concluded to be such that only one iminodiacetate segment of ethylenediaminetetraacetate is metal coordinated.
espite the importance of the alkaline earth metal ions and the extensive thermodynamic data available on their complexes, only meager kinetic information on such complexes is available. This circumstance results from the rapid reaction rates involving these ions and eliminates the possibility of employing conventional techniques for rate studies. In an earlier paper, the ligand-exchange kinetics in the calcium-ethylenediaminetetraacetic acid (EDTA) system were studied using proton nuclear magnetic resonance (nmr) line-broadening techniques. In the present paper we have studied the exchange kinetics of two related systems in an attempt to elucidate the mechanisms of aminopolycarboxylic acid ligand exchange with the alkaline earth ions. One of these systems is strontium and EDTA, which can be described by the same pH-
D
(1) (a) This work was supported by a grant (GP 4423) from the National Science Foundation and by a National Institute of Health Predoctoral Fellowship (D. L. R.); (b) presented at the 152nd National Meeting of the American Chemical Society, New York, N. Y., Sept 1966. (2) R. J. Kula and G. H. Reed, A n d . Chem., 38, 697 (1966).
Journal of the American Chemical Society / 89:3
dependent rate expression as calcium and EDTA, suggesting that exchange occurs by similiar mechanisms in both systems. The second system is calcium and N’(2-hydroxyethyl)ethylenediamine-N,N,N’-triaceticacid (HEEDTA). The latter ligand, while structurally related to EDTA, is capable of only pentadentate coordination to metal ions. Although the rates in the present investigation are generally greater than the corresponding rates found in the CaEDTA study, they are still within the time limits accessible to nmr linebroadening measurements. Advantage has been taken of the extensive p H range available with the alkaline earth ions to isolate the various pH-dependent rate processes. The lower p H limit is approximately 4 because of dissociation of the chelates, but there is n o upper p H limit because metal-(hydroxy)-ligaiid species d o not exist. (3) In this paper La- is used to designate the trianion of HEEDTA and HL2- the monoprotonated anion; Y 4- represents the EDTA tetraanion and HY3- the monoprotonated anion; M2+ represents either Ca2’ or Sr2+.
/ February I , 1967
