ML3-Containing Metallacycles

In Section 11.2.B, we showed that the concerted or least-motion dimerization of two olefins requires excessively high activation energies. This is the classic case of a symmetry-forbidden reaction. A two-step reaction mechanism, or at least a different reaction path, has to be followed. In this section, a somewhat analogous reaction, the dimerization of two olefins in the presence of Fe(CO)s and CO [21 ], is investigated. An initial sequence In this reaction is the photosubstitution of CO by two olefins on Fe(CO)s which gives the 18-electron intermediate, 18.26. It rearranges to the 16-electron metallacyclopentane, 18.27, wherein one C—C and two Fe—C bonds 

We start our analysis by building up the valence orbitals of 18.26 in terms of a C2v Fe(CO)3 fragment and two ethylenes. Notice that the olefins lie in the equatorial plane. This, recallfrom 17.31 and 17.32, is the electronically preferred way to orient olefins in a d trigonal bipyramidal complex. Symmetry-adapted linear combinations oftheTrandu levels of the ethylenes are shown in 18.30 to 18.33 from atop view. 

Two major geometrical parameters can be used to describe a reaction path for the oxidative coupling of 18.26 to the ferracyclopentane tricarbonyl, 18.27. They are illustrated from a top view of the complex in 18.34. Decreasing f causes 

An idealized plot of the orbital energies for the oxidative coupling reaction, 18.26 to 18.27. 

A number of routes can be envisioned which are symmetry-allowed [22]. The most plausible one involves twisting the equatorial carbonyl off from the y axis as the cyclization proceeds to form 18.35. 18.35 is ready to coordinate an additional CO 

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