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A Complex Exhibiting Low-Lying Methylidene-Like and Carbene-Like States'

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A Complex Exhibiting Low-Lying Methylidene-Like and Carbene-Like States' 2180 J. Am. Chem. SOC.1986, 108, 2180-2191 Bonding in Transition-Metal-Methylene Complexes. 2. (RuCH2)+, a Complex Exhibiting Low-Lying Methylidene-like and Carbene-like States' Emily A. Cartert and William A. Goddard III* Contribution No. 7266 from the Arthur Amos Noyes Laboratory of Chemical Physics, California Institute of Technology, Pasadena, California 91 125. Received August 15, I985 Abstract: The electronic structure for a representative late-transition-metal-methylene complex, Ru=€H2+, has been studied by ab initio methods (generalized valence bond/configuration interaction). The electronic-state spectrum reveals five states close in energy (spread of 12.9 kcal/mol) that partition into two groups in terms of energy separation and mode of metal-carbon bonding. The ground state has *A2symmetry and contains covalent M-C u and a bonds ('metal-methylidene"); a 2A, state of the same bond character is only 1.2 kcal/mol higher. A cluster of three degenerate excited states (4A2,4B1, and 4B2) 12.9 kcal/mol above the ground state exhibits completely different bonding character, namely, a-donor/a-acceptor M-C bonds are formed ("metal-carbene"). We conclude that for highly unsaturated, late-transition-metal systems, metal-carbene bonding may be competitive with metal-alkylidene bonding, leading to donor/acceptor bonds comparable in strength to that of covalent double bonds! I. Introduction states of the system are formed by combining the various low-lying Metal complexes containing CH2 ligands have been postulated metal atomic configurations with 10 and 11 to form various as intermediates for numerous catalytic reactions (e.g., Fischer- bonding states. Tropsch reductive polymerization of CO and olefin metathesis) Metal-methylene complexes involving 10 have covalent met- and have been isolated in a number of cases includingk,b al-carbon double bonds and are termed metal methylidenes to emphasize the double-bond character. Examples include the Me, ,OMe 1. C so-called Schrock complexes Metal-methylene complexes oc ',, I( ..co involving 11 require empty du or s orbitals on the metal (that can o/ u" accommodate the CH2 u pair) and prefer a doubly occupied ds orbital on the metal that can overlap the empty orbital of u2 and OC c s "to CH2. This leads to a metal-carbon bond best described in terms 0 of donor-acceptor or Lewis acid-Lewis base concepts (as in 1 2 "Fischer"-type carbenes such as 2, or, in general, as 12). We In the simple oxidation-state formalism, the CH2 is thought of as (CH#, with the metal oxidized by two units; however, the chemistry of these systems tends to fall into one of two distinct classes, one of which is nucleophilic and the other electrophilic. A series of generalized valence bond (GVB) studies on high- oxidation-state metal complexes such as3 C12Ti=CH2 (3), C14- 12 Cr=CH2 (4), and C14Mo=CH2 (5) showed that these systems will refer to such systems as metal carbenes. Supporting evidence all have the form 6 with a covalent metal-carbon double bond for such differences in the metal-carbon bond character is the drastic contrast in chemical reactivity of 6 with 12. Metal me- thylidenes such as 3-5 are precatalysts for metathesis6 and po- lymerization reactions with olefins,' whereas metal carbenes such as 2 generally exhibit stoichiometric reactivity with olefins, leading to the formation of cyclopropanes.* 6 involving a s bond composed of one electron in a metal dsorbital (1) Paper 1 in this series: Carter, E. A,; Gdard, W. A,, 111. J. Phys. spin-paired with one electron in a C pa orbital, and a u bond Chem. 1984,88, 1485. consisting of one electron in a metal do orbital spin-paired with (2) (a) Schrock, R. R. J. Am. Chem. SOC.1975,97,6577. Guggenberger, u L. J.; Schrock, R. R. Ibid. 1975,97,6578. (b) Fischer, E. 0. Ado. Organomet. one electron in a C sp2 orbital. Similar studieslAon (Cr=CH2)+ Chem. 1976,14, 1. For an extensive review of both Fischer- and Schrock-type (7), (Mn=CH2)+ (8), and (Fe=CH2)+ (9) lead also to a double carbenes, DBtz, K. H.; Fischer, H.; Hofmann, P.; Kreissl, F. R.; Schubert, U.; bond with a similar covalent M ds-C p~ bond but a u bond having Weiss, K. Transition Metal Carbene Complexes; Verlag Chemie: Deer field varying amounts of du and su character on the metal. Beach, FL, 1984. (3) (a) RappE, A. K.; Gcddard, W. A,, 111. In Potential Energy Surfaces Such studies suggest the following valence bond view of met- and Dynamics Calculations; Truhlar, D. G., Ed.; Plenum: New York, 1981; al-methylene bonds. The metal is considered to be in the atomic pp 661-684. (b) J. Am. Chem. Soc. 1982, 104, 297. (c) Ibid. 1982, 104,448. configuration (s1dw1,d", etc.) appropriate for its charge and (d) Ibid. 1980, 102, 5114. environment (no formal charge transfer to the CH2),and the CH2 (4) MnCH2+and FeCH2+work: Brusich, M. J.; Gcddard, W. A., 111, to is considered to be neutral and in one of its two most stable forms, be published. For another recent paper on CrCH2+concurring with our earlier results (ref I), see Alvarado-Swaisgood, A. E.; Allison, J.; Harrison, J. F. J. the triplet ua ground state 10 or the singlet u2 excited state (9 Phys. Chem. 1985,89, 2517. kcal/mol higher5) 11. The ground state and low-lying excited (5) Leopold, D. G.; Murray, K. K.; Lineberger, W. C. J. Chem. Phys. 1984, 81, 1048. (6) (a) Lee, I. B.; Ott, K. C.; Grubbs, R. H. J. Am. Chem. Soc. 1982, 104, 7491. (b) Wengrovius, I. A.; Schrock, R. R.; Churchill, M. R.; Missert, I. R.; Youngs, W. I. Ibid. 1980, 102, 4515. (c) Gilet, M.; Mortreux, A,; Folest, J.-C.; Petit, F. Ibid. 1983, 105, 3876. (d) Kress, J.; Osborn, J. A. Ibid. 1983, 105,6346. (e) Katz, T. J.; Han, C.-C. Organometallics 1982, I, 1093. (0 Howard, T. R.; Lee, J. B.; Grubbs, R. H. J. Am. Chem. Soc. 1980,102,6876. 10 11 (7) (a) Turner, H. W.; Schrock, R. R. J. Am. Chem. SOC.1982,104,2331. (b) Levisalles, J.; Rose-Munch, F.; Rudler, H.; Daran, J.-C.; DromzCe, Y.; 'National Science Foundation Predoctoral Fellow, 1982-1985. Jeannin, Y.J. Chem. Soc., Chem. Commun. 1981, 152. 0002-7863/86/1508-2180$01.50/0 0 1986 American Chemical Society Bonding in Transition- Metal-Methylene Complexes J. Am. Chem. SOC.,Vol. 108, No. 9, 1986 2181 This gestalt view of bonding in terms of combining complete overlaps the CH bonds, is singly occupied (less electron-electron many-electron states is a characteristic distinguishing the valence repulsion in the molecular plane than for the other two states). bond viewpoint from the molecular orbital viewpoint in which However, this state is 20.0 kcal/mol above the ground state. In one-electron orbitals are constructed (from the same atomic or- order to consistently predict such ordering of states in the bound bitals), but where distinctions between atomic configurations such complex, it is useful to examine the energies for the corresponding as 2 vs. UT methylene or s1d4vs. d5Cr' become blurred. Although atomic configurations of Ru'. As shown in Table I, the three this valence bond view of bonding has been implicit in several configurations in (1) lead to the following atomic energies: 12) papers, no examples of the metal-carbene bonding (as in have 2B2 (du) (dr)1(d6)2(d8)2(dii)I (20.1 kcal /mol above 'A2) been examined with GVB techniques. In this paper we report all-electron ab initio GVB calculations on a system (RuCH2)+ 2A (du) '(dx)I( d6)2(d8) (dii)2 (degenerate with 2A2) (2) (13) that exhibits both methylidene- and carbene-like states having comparable bond strengths. Indeed, the lowest carbene-like state 2Az (dg) (dx) ' (d6)' (d8)2(d7i)2 Although all three configurations are d7 Ru', they have different electron repulsion energies (even when the orbital shapes are +r" identical), and we see by examination of (2) that it is this atom- Ru= / H ic-electron repulsion energy that determines the relative energies in (1). For example, 2B2has four electrons in the xy plane (6*a2), 13 whereas 2Al and 2A2have the doubly occupied orbitals in different (4A2)is only 12.9 kcal/mol above the lowest methylidene-like state planes (z2ii2 or 62+2), leading to lower electron repulsion. Thus, (2A2). The results for the 2A2ground state of 13 (methylidene) in predicting the ground configuration of RuCH2' we need only are examined in section 11, while the wave function for the 4A2 consider two factors: excited state of 13 (carbene) is described in section 111. A sum- (i) which states of Ru' can form two covalent bonds, and mary of our conclusions is presented in section IV, while further (ii) of the states satisfying (i), which occupation of the non- details of the calculations are outlined in section V. bonding d orbitals has the lowest atomic energy (lowest electron 11. The Ground State of RuCH2+: Methylidene Bonding repulsion). B. Bonding in the Ground State, RuCH,' (2A2). The gener- A. Low-Lying Covalent States. Using the coordinate system alized valence bond (GVB) one-electron orbitals for the Ru-C Y u and x bonds are shown in Figure 1 where we see that both bonds are quite covalent. The Ru-C u bond pair has an overlap of 0.68, with 1.04 electrons ascribed to Ru+ and 0.96 electron associated + with CH2.9 The Ru-C x bond pair has an overlap of 0.48, with Ru=C /H 1.16 electrons localized on Ru+ and the other 0.84 electron on CH2.
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