Butadiene complexes bonding

A carbon-iron bond is also formed by the reaction of the cyclopropenium salt 185 with dicarbonyl(i/5-cyclopentadienyl)(trimethylsilyl)iron [92], (Scheme 69) In the reaction with benzocyclobutenylidene- 5-cyclopentadienyliron(II) hexafluorophosphate 186, CpFe(CO)2R (R=cyelo-C3H5, CH2-cyclo-C3H5) is converted to the allene and butadiene complexes, 187 and 188, respectively [93]. (Scheme 70)... 

If the unsaturated hydrocarbon is a diene, both double bonds may coordinate to palladium ). (Diene)palladium(II) complexes have been isolated and characterized. For example, 2 and 3 are stable complexes in which both double bonds are coordinated to the metal10. Conjugated dienes constitute a special case and although /j4-diene complexes, e.g. 4, are postulated as intermediates, they have not yet been isolated. The butadiene complex 4 is in equilibrium with the zr-allyl complex 5 in solution, and attempts to isolate the diene complex from this mixture lead to formation of a yellow crystalline complex 511. 

Some [MX]+ ions enter into reactions in which the ligand X and the reacting molecule become chemically bonded. Polymerization processes have been observed involving the [MC4H4]+ ions (147). The butadiene complex ions [MC4H4]+ of Co and Ni are unreactive to ethyne but the Fe, Ru, and Rh ions react to yield benzene and the bare metal ion. The [MC4H4]+ complex ions of Os+, Ir+, and Pt+ react with ethyne to form the MC4I I4 + ions that probably correspond to the benzyne complexes previously observed for platinum (126). 

These results suggest the presence of two competing pathways to products, which depend upon the location of protonation at the M=C carbyne bond. Charged controlled protonation at the carbyne carbon followed by nucleophilic attack of the CT leads to the butadiene complex. Frontier control of protonation results in attack at the metal center, leading ultimately to the hydride complex164. This has been verified by reaction of the... 

Molecular orbital calculations by Hofmann (162) indicated that an if-allyl anion complex with a noncomplexed butadiene group is more stable than an -butadiene complex, in which the allyl anion portion of the seven-carbon ring is not bonded to the metal. We have now been able to confirm this result by an X-ray structure analysis (see Fig. 8) on [Ph4As][C7H7Fe(CO)3] (163). 

Bu NC yields related imine complexes (calix[4]OMe)Ta() -Bu N=CR2). Although butadiene is -bonded to the metal in Ta / -Bu -calix[4]-(0Me)(0)3 () " -C4H6), it behaves as ifitwere bonded in the migratory insertion reaction... 

The molecular structure (362) of the complex (C4Hg)[Fe(CO)4]2 is similar to that of its manganese analog (45). It consists of a planar (tows-1,3-butadiene moiety bonded in such a way to two Fe(CO)4 groups that each of the double bonds occupies an equatorial position in a different trigonal bipyramid (70). 

At a temperature below —2()°C, a butadiene complex [(C4Ha)PdCl2]2, in which only one of the double bonds of each diene molecule is complexed can be obtained by ligand exchange with the 1-pentene complex 180). 

The general reaction model for the allylnickel complex-catalyzed 1,4-polymerization of butadiene is outlined in [26]. From the starting / -allylnickel(II) complex, which has a quasi-planar structure, two structurally different butadiene complexes are formed as the actual catalysts by successive ligand or anion substitution a monoligand allylnickel(II) complex, which may also contain the anion X instead of the neutral ligand L, with an coordinated butadiene, and a ligand-free complex with an t/ -cis coordinated butadiene. The concentration of these complexes, which is also limited by the double- bond coordination from the growing chain, and their reactivity determine the catalytic activity. 

For each of the butenylnickel(II) complexes, and for the butadiene complexes as well, an anti-syn equilibrium has to be assumed, with the syn configuration having the thermodynamically more stable structure (Xg/s 10 -10 ) [36, 60]. The rate of anti-syn isomerization has proved to be strongly dependent on stmcture. In the bis(ligand)-butenylnickel(II) complexes the isomerization rate is very low = 10 s ) [66], but in the ligand-free Ci2-allylnickel(II) complexes the anti-syn isomerization is accelerated considerably by the coordination of the next double bond, so that it is completed instantaneously even at -70 °C [60]. 

This means that the cis-trans selectivity depends solely on the difference between the free standard enthalpies AGg - AGf of the transition states for the insertion reaction with the anti and the syn form of the polybutadienyl cw-butadiene complex e and f, respectively, independently of their concentration ratio in the anti-syn equilibrium K. To explain the cis-trans selectivity of 93 % cis and 4 % trans attributed to the ligand-free allylnickel(II) cation, a stability difference between the transition states of about 1.9 kcal mol (8 kJ mof ) must be assumed, which can easily arise from the different steric conditions for the necessary coordination of the next double bond in the anti and the syn forms e and f of the catalyst complex. 

Using the parent zirconocene-butadiene complex as a representative example, a typical bonding situation in these types of molecules is presented in Scheme 48. For 297, equilibration between the s-trans and the s-cis isomers occurs with a barrier of 23 kcal mol 1 at 283 K. The 72-olefin complex is believed to be a high-energy intermediate on the interconversion reaction surface. Significantly, structural data indicates that the s-cis complexes are best described as Zr(iv) compounds with a er2, ir ligand.158,175 The dynamic NMR measurements have also been extended to ansa-zirconocene and hafnocene butadiene complexes.176 Moreover, photoelectron spectroscopy has been used to determine the relative energetics of the two isomers for // -metallocenes.177... 

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