Ethylene complexes stability

Figure 4.79 displays the optimized structures of secondary-Cp (IIsec) and primary-Cp(IIPri) complexes, and Table 4.43 includes geometrical and charge parameters of these propylene complexes for comparison with those of the corresponding ethylene complex in Table 4.42. The IIsec complex can be seen to have smaller Ti—Cp metal-alkene separation (by 0.1 A) and other evidence of tighter metal-alkene binding than that in the IIpri complex, in accordance with the donor-acceptor stabilizations discussed above. 

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. 

A quantitative treatment of tt complex formation is, however, more complicated, since it is generally recognized that all three wave functions are necessary for an accurate description of the bond. For instance, it has been pointed out by Orgel (27) that n complex stability cannot solely be the result of n electron donation into empty metal d orbitals, since d and ions (Cu+, Ag+, Ni , Rh+, Pt , Pd++) form some of the strongest complexes with poor bases such as ethylene, tt Complex stability would thus appear to involve the significant back-donation of metal d electrons into vacant antibonding orbitals of the olefin. Because of the additional complication of back-donation plus the uncertainty of metal surface orbitals, it is only possible to give a qualitative treatment of this interaction at the present time. 

Ir(I), Ir(II) andlr(V) complexes stabilized by an O-donor ligand (e.g. [Ir(coe)(triso)] and [Ir(C2H4)2(triso)] (triso = tridentatetris(diphenyloxosphosphoranyl)methanides) are effective catalysts for the dehydrogenative silylation and hydrosilylation of ethylene [16-18]. 

Copper(I) and silver(I) complexes are exceptions of the general trend in stability constants with electron-donating or attracting substituents. Thus most known 7i-complexes of silver and copper are less stable than their respective ethylene complexes (154 156). The steric hindrance introduced by the substituents seems to have a major effect in those systems. 

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). 

Braunstein et al. recently reported an interesting reaction of a base-stabilized mononuclear silylene complex with platinum-ethylene complexes [Eq. (35)]. In this reaction, the Pt-bound ethylene ligand is displaced by the base-free silylene complex, but the products can be also regarded as a dinuclear complex where two metals are bridged by a silylene unit.67... 

Metallocene complexes of Ti, Zr, and Hf have attracted considerable attention in recent years because of their high activity and since their ligand framework can be tailored to a wide variety of polymerization requirements.102 The active species is the 14-electron cationic alkyl [Cp2M—R]+, with a pseudo-tetrahedral structure and a vacant site suitable for forming a weak complex with ethylene. Calculations show that the alkyl transfer to ethylene is stabilized by an a-agostic interaction with the metal, with a very low (ca. 2 kJ mol-1) activation barrier 103... 

The stability of a number of rhodium(I)-olefin complexes relative to that of the ethylene complex has been established by spectrophoto-metric determination of the extent of displacement by another olefin of C2H4 from (C2H4)2Rh(acac) in a closed system according to Eq. (4). 

Cryptands are macro-bi- or -poly-cycles able to encapsulate an ion by providing it higher protection because of their cagelike structures, as in (147) and (148). For these ligands the correspondence between cavity size and complex stability is more pronounced than for simple crown ethers. Recent approaches to improve the metal-ion selectivity of cryptands, as for example by replacement of ethylene units between each donor atoms with propylene units, or by insertion of several substituents into the macrocycles, have been reviewed.245 A new, interesting family of cryptands is constituted by borocryptands (149), which are useful receptors for chiral substrates, where enantiomeric differentiation can be achieved by using NMR spectroscopy.246... 

Dibenzyl complexes stabilized by tridentate dianionic ligands containing hard and soft pendant donors have been described. Reactions of Ti(CH2Ph)4 with the corresponding aminophenols give the mononuclear pentacoordinate dibenzyl Ti derivatives (Scheme 68). Activated with MAO, these complexes have been used as catalysts for ethylene polymerization, showing marked activity enhancements for the compounds containing soft donor substituents.141... 

The phosphine-stabilized ethylene complex of zirconocene(n) reacts with 1 equiv. of B(G6F5)3 to form the girdle-type zwitterionic complex 754 (Equation (49)).574 Both the solution and solid-state structures of 754 feature a strong f3-CH agostic interaction. The zwitterion 754 is a single-component catalyst for the polymerization of ethylene under ambient conditions, although for optimal activity an additional equivalent of B(G6F5)3 is needed. 

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