Ligands ethylene

With electrons flowing from ethylene to zirconium the Zr—CH3 bond weakens the carbons of ethylene become positively polarized and the methyl group migrates from zirconium to one of the carbons of ethylene Cleavage of the Zr—CH3 bond is accom panied by formation of a ct bond between zirconium and one of the carbons of ethylene m Step 3 The product of this step is a chain extended form of the active catalyst ready to accept another ethylene ligand and repeat the chain extending steps... 

Step 3 The methyl group migrates from zirconium to one of the carbons of the ethylene ligand. At the sane time, the tt electrons of the ethylene ligand are used to fonn a a bond between the other carbon and zirconium. 

Pyridine 1-oxide, like pyridine, can act as a ligand in transition metal complexes, but unfortunately good stability constants are not known. However, Shupack and Orchin have found that the C===C stretching frequency of the ethylene ligand in trans-ethylene pyridine 1-oxide dichloroplatinum(II) varies linearly with the pA and hence with the C7-value (ct+ or a, respectively) of substituents in the pyridine oxide. The data for the above reaction series are included in Table V. 

Dimethyltitanium complex 25, bearing an ethylene and methyl ligands, catalyzed the dimerization of ethylene via a metallacyclopentane intermediate 26 (Eq. 1) [30]. During the dimerization, no insertion of ethylene into the Ti-Me bond was observed due to the perpendicular orientation between methyl and ethylene ligands. This inertness could be attributed to the low oxidation state of 25, i.e. Ti(II). 

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 platinum complex Pt(PPh3)2C2H is inserted into Si-H and Ge-H bonds with the elimination of ethylene ligands ... 

It was found that the stability of the mixed complexes increased considerably with the number of ethylene ligands. More exactly, the Li(C2H4)2(N2) complex was stable up to the temperature at which the rare-gas matrix sublimed. It was identified and described as a Li(C2H4)2 complex with two equivalent ethylene ligands, slightly perturbed by the addition of a N2 molecule (Scheme The comparison of the IR spectrum... 

Fig. 15.23 Representation of it coordination of an ethylene ligand und a transition metal.

The reaction of OFCOT with the 18-electron Rh(I) complex [Rh(>/5-C5FI5)( 72-C2H4)2] is slow at room temperature and yields a product 71 resulting from displacement of only one ethylene ligand (150). This is formulated as a l,2- /2-OFCOT complex of Rh(I), rather than a l,4- /2-OFCOT complex of Rh(III) (see Section VIII,F,l), based on similarities between the... 

The study of species in which ethylene is coordinated to transition metal centres holds great interest in areas of catalytic and polymerization chemistry (7). The bonding of the ethylene ligand to the metal centre in such species has been compared to that of the dihydrogen complexes described above (14,15,22). Photolysis of chromium hexacarbonyl, Cr(CO)6, in conventional solvents in the presence of dissolved ethylene gas is known to lead initially to a highly labile species in which one CO ligand is replaced by ethylene. Further photolysis leads to a more stable compound which contains two ethylene ligands trans to each other across the metal centre (25), equation 3. The conventional synthesis is experimentally difficult the two photochemical... 

Introduction of one or two bulky trimethylsilyl groups accelerates the polymerization and leads to a modest increase in molecular weight four Me3Si groups lead to deceleration. The 1,2-bis(indenyl)ethylene ligand (12) appears to increase reaction rate, molecular weight and tendency for monomer [2,1] insertion. 

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... 

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