Ethylene metallation

Figure 2.33. Schematic picture of the frontier-orbital description of ethylene-metal bonding via -donation and Tr -backdonation interactions with metal electron states. From Ref. [3].

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. 

The olefin signal in the tris(ethylene)metal complexes experiences a coordination shift of approximately -60 ppm. Interestingly, the magnitude of the complexation shifts increases in the order Pd < Ni < Pt. This result has also been observed for the bis(r/3-allyl)metal complexes discussed in Section IV. 

In Table VII are recorded mean values for in a number of ethylene-metal carbonyl complexes and parent metal carbonyls as well as values for the double-bond infrared stretching frequency rc c the magnetic shielding parameter t for ethylene in those transition metal complexes for which data are available. Although with the metal carbonyl complexes, differences of geometry, oxidation state, etc., do not permit a correlation to be drawn between the absolute values of rco and for the various complexes, it is quite apparent from the tabulated data for the Mo, Mn, and Fe complexes that for a given metal. 

Cupric chloride. Ethane, butane, ethylene, metallic copper. Wanhlyn and Carius, AnnuUny 1861, 120, 69. 

In Section III, A, 2, thermodynamic and mechanistic factors were said to determine selectivity. The effectiveness of the thermodynamic factor depends partly upon the strength of ethylene adsorption. Heats of adsorption of ethylene on supported metals are not available, but it is known, however, (see Section IV, B, la) that the stability of ethylene-metal and other olefin-metal complexes increases with increasing atomic number down each triad of Group VIII. If it is assumed that the normal adsorbed state of olefin is that of a 7r-complex with the surface, it would... 

In a recent review Quinn and Tsai 2> tabulated data for a series of ethylene-metal carbonyl complexes and concluded that for a given metal or group in the... 

Table 1. Ethylene-Metal Atom and -Metal Dimer Complexes Prepared FOR Matrix Isolation Spectroscopy...

Consider a pseudo-octahedral d complex of the type [ML5( j -C2H4)] (3-38), where the five L ligands are supposed to have only a interactions with the metal centre. If this complex is decomposed into a 4 fragment MLs, with a square-base pyramidal (SBP) geometry, and an ethylene fragment, the interaction between the orbitals on the two fragments enables us to analyse the electronic factors that are at the origin of the ethylene-metal bond. 

Isodesmic hydride exchanges (48) show that the primary carbocations MeHgCHjCHf and Me3SnCH2CHf are kinetically more stable than trityl cation (49). These species can be considered as three-centered bound ethylene metal ions (50) in which a considerable amount of the positive charge is carried by the metal. Only the relatively soft metals can be expected to interact so strongly with the moderately soft carbenium center. 

Densothene Poly(ethylene) Metal Box, Great Britain... 

Another reaction which appears to be general for o-allyl complexes is their facile protonation with acids, thereby affording novel tr-ethylenic metal complexes,... 

Ziegler T, Rank A (1979) A theoretical study of the ethylene-metal bond in complexes between Cu", Ag, Au", Pt° or Pt and ethylene, based on the Hartree-Fock-Slater transitimi-state method. Inoig Chem 18 1558... 

Among the catalytic systems used for homo- and copolymerization of ethylene, metals supported on silica are Indeed the leading catalysts in the polyolefin industry (1,2). Therefore, the great amount of research works appearing in the technical and scientific literature is mainly concerned with this type of system (3). However, in spite of the efforts to understand how the Si02 features, such as surface area, pore volime, pore size distribution, etc., influence the polymerization activity and even resin properties, many questions are still open to discussion (4). This is particularly true in the case of catalytic systems based on titanium supported on Si02> because relatively fewer works, compared to Cr/Si02, have been pub-... 

The ethylene-metal system provides a useful model by which to describe the bonding in w-bonded metal-organic complexes. As we shall see, such bonding is difficult to treat in terms of a conventional classical valence bond approach. It is more meaningful to use descriptions based on molecular orbital theory. 

Figure 30. Valence bond representations of the ethylene-metal bond (except d, see below).

It may be envisaged that both the hydride abstraction and the protonation reactions proceed via the formation of a j3-carboninm ion intermediate which is stabilized by formation of an ethylene-metal bond and partial oxidation of the metal, e.g. 

In the earlier discussion of the ethylene-metal bond (Chapter S) the ethylene was compared, in extreme examples, to the carbon monoxide ligand or to two alkyl groups. In the former situation the ethylene behaves as an essentially monodentate ligand whilst in the latter it behaves as a bidentate ligand. Acetylenes are similar in their behaviour. 

Ethylenic complexes analogous to these are known and it seems reasonable to suppose that the acetylene metal bond should resemble the ethylene-metal bond in this case. This means that the acetylene, which has two orthogonal p7r-bonds at right angles to each other, would use one filled pir-bond to donate to the manganese and back-donation from the metal would take place mainly into the corresponding anti-bonding pir -mole-cular orbital of the acetylene. 

Fig. 2. Structure of the anion [CiH PtCiil [41] in Figure 2. The anion has Qv symmetry the directions of the x-, y- and z-axes are shown. The ethylene electrons which are involved in the a bonding of the H2C—CH2 system are considered not to be involved in the ethylene-metal bonding. If and

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