Ethylene complexes bonding

Fig. 3. Important valence orbitals of some metal fragments. The energy scale markings are eV. (Reprinted, with permission, from Ethylene Complexes, Bonding, Rotational Barriers, and Conformational Preferences, Albright, T. A. et al. J. Am. Chem. Soc. 101, 3802, Fig. 1, copyright, 1979, by the American Chemical Society)...

Fig. 2. Time-evolution of the methyl/ethyl C-C distances for both the zirconocene and the corresponding titanocene catalyst. The two curves starting at around 3.2 A represent the distance between the methyl carbon atom and the nearest-by ethylene carbon atom in the zirconocene-ethylene and the titanocene-ethylene complex, respectively. The two curves starting at around 1.35 A reflect the ethylene internal C-C bond lengths in the two complexes.

Fig. 3. Time evolution of the distance between the Zr atom and each of the three hydrogen atoms belonging to the methyl group (the original methyl group bonded to the Zr) in the zirconocene-ethylene complex. The time-evolution of one of the hydrogen atoms depicted by the dotted curve shows the development of an a-agostic interaction. Later on in the simulation (after about 450 fs) one of the other protons (broken curve) takes over the agostic interaction (which is then a 7-agostic interaction).

Figure 4.76 Side and end views of the 90°-twisted ethylene complex (0.5 kcal mol-1 above the untwisted equilibrium complex, Fig. 4.74(b)), showing the interaction of the unsymmetric pi bond with Ti.

The leading NBO Lewis structure of the less strongly bound Au(HCCH)+ complex does indeed correspond to separated Au+ HCCH reactants. Figure 4.89 illustrates the principal NBO donor-acceptor interactions for the Au(HC=CH)+ complex, which are seen to be rather similar to those for the long-range Ti(H2C=CH2) complex (Fig. 4.72). Thus, for a transition metal with only one vacant valence orbital, acetylene and ethylene 7tCc bonds function rather similarly as two-electron donors, and the p2, two-electron complex description is apt. 

The first isolable alkenetitanium complex, the bis(pentamethylcyclopentadienyl)-titanium—ethylene complex 5, was prepared by Bercaw et al. by reduction of bis(penta-methylcyclopentadienyl)titanium dichloride in toluene with sodium amalgam under an atmosphere of ethylene (ca. 700 Torr) or from ( (n-C5Mc5)2Ti 2(fJ-N2)2 by treatment with ethylene [42], X-ray crystal structure analyses of 5 and of the ethylenebis(aryloxy)trimethyl-phosphanyltitanium complex 6 [53] revealed that the coordination of ethylene causes a substantial increase in the carbon—carbon double bond length from 1.337(2) A in free ethylene to 1.438(5) A and 1.425(3) A, respectively. Considerable bending of the hydrogen atoms out of the plane of the ethylene molecule is also observed. By comparison with structural data for other ethylene complexes and three-membered heterocyclic compounds, the structures of 5 and 6 would appear to be intermediate along the continuum between a Ti(11)-ethylene (4A) and a Ti(IV)-metallacyclopropane (4B) (Scheme 11.1) as... 

Substituted cyclopropanols were also obtained, albeit in moderate yields, upon reaction of esters such as methyl pentanoate with l,4-bis(bromomagnesium)butane (38) in the presence of titanium tetraisopropoxide. This corroborates the formation of a titanacy-clopropane—ethylene complex 40 from an initially formed titanacyclopentane derivative 39 (Scheme 11.12) [103], Apparently, an ester molecule readily displaces the ethylene ligand from 40, and a subsequent insertion of the carbonyl group into the Ti—C bond, a formal [2S + 2J cycloaddition, leads to the oxatitanacyclopentane 42, the precursor to 1-butylcyclopropanol (43). 

The first step is coordination of the ethylene through its n orbital. The ethylene is trans to Cl with the C=C bond in the Cl-Ru-H plane. Facile migratory insertion (AE = 7.6 kcal.mol 1) of the coordinated ethylene in the Ru-H bond leads to an alkyl intermediate 6.2 kcal.mol 1 less stable than the n ethylene complex. The alkyl intermediate has a strong P C-H agostic interaction as illustrated by the unusually long agostic C-H bond (1.221 A) which helps to stabilize the unsaturation in the formally 14-electron alkyl intermediate. 

The first organometallic compound of the transition metals to be characterized (1827) was Zeise s salt, K[(C2H4)PtCl3]-H20 (Fig. 18.1). It forms when K2[PtCl4] in aqueous ethanol is exposed to ethylene (ethene) a dimeric Pt—C2H4 complex with Cl bridges is also formed. In both species, the ethylene is bonded sideways to the platinum(II) center so that the two carbon atoms are equidistant from the metal. This is called the dihapto-or T]2 mode. A ligand such as an allyl radical with three adjacent carbons directly bonded to a metal atom would be trihapto- or t 3, and so on. 

The stability of the olefin complexes seems to be determined by the steric and electronic characters of both the phosphorus ligand and the olefin (22). For example, ethylene complexes have only been isolated for the cases with sterically large ligands such as P(0-o-tolyl)3 and PPh3 however, maleic anhydride forms a stable isolable complex with the smaller P(0-p-tolyl)3 ligand. The nickel-ethylene bond strength is estimated to be 39 kcal/mol based on values of 36 kcal/mol for 1-hexene and 42 kcal/mol for acrylonitrile [when L = P(0-o-tolyl)3] (22). 

The same ethylidene ruthenium complex, as well as its iron congener, is alternatively obtained through direct protonation of the dimetallacycles 64a (M = Fe) and 64b (M = Ru) (64). In this case, the carbonyl alkyne carbon-carbon bond is broken irreversibly to give the cationic /x, 17s-vinyl complexes 65a and 65b, which undergo nucleophilic attack by hydride (NaBFLi) to produce complexes of methylcarbene (63a,b) (Scheme 21a). Deuterium-labeling experiments prove that the final compounds arise from initial hydride addition to the /3-vinylic carbon of 65. However, isolation of small amounts of the 7j2-ethylene complex 66 indicates that hydride attack can also occur at the a-vinylic carbon (64). 

In contrast, when the diethyl complexes mr-[Pd(C2H5)2L2] react with C02 at - 20 to - 40° for 15 hours (L = PEt3, hexane solution L = PMePh2, toluene solution), ethane is evolved and the products are formulated as zero-valent [Pd(C2H4)(C02)L2] (123). These complexes are much less stable, decomposing at room temperature and 50°, respectively, with approximately stoichiometric evolution of C02 and ethylene. The bonding mode of C02 in these complexes does not appear to be firmly established, but C02 may well function as a discrete ligand, as in the known Ni(0) complexes (115, 117). 

T. Ziegler and A. Rauk, Inorg. Chem., 18,1558 (1979). A Theoretical Study of the Ethylene-Metal Bond in Complexes Between Cu+, Ag+, Au+, Pt°, or Pt2+ and Ethylene, Based on the Hartree-Fock-Slater Transition-State Method. 

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