Hydride complexes olefins

Heck and Breslow (62) have postulated equilibria between acylcobalt carbonyls and olefin-hydride complexes. 

BArF]. This cation is more reactive than Cp Ir(PMe3)(CH3)OTf and reacts with terminal C-H bonds of alkanes such as pentane and methylcyclohexane to eliminate methane and give olefin hydride complexes, similar to the reaction with ethane shown in Scheme 7 [67]. 

Bercaw has extended these studies of alkyl formation from niobium olefin-hydride complexes to include additional examples of meta-substituted styrene complexes of permethylniobocene. As noted in earlier work , electron-donating substituents on the j8-carbon accelerate this insertion reaction, whereas electron-withdrawing substituents retard the rate based on a correlation with a p of —1.2... 

Kinetic studies have also been carried out with the sterically less constrained endo-( ACp)2Nb(CH2=CHR)H system. Again, a correlation of insertion rate with (7pj is seen p= -1.1). However, unlike the ( -Cp)2Nb(CH2=CHR)H systems studied earlier, the ground-state energies of the niobium olefin-hydride complexes also vary with the electronic nature of the substituent. 

Protonation of an olefin-hydride complex to generate M(olefin)(H2) and observation of transfer of protons from q2-H2 to the olefin could prove direct reaction of H2 ligands, as in Eq. (9.55), where NBD is seen to be hydrogenated to nortricyclene.2,95 A dinuclear hydride with a vinylcyclopentadiene ligand is also... 

The addition of neighboring hydrogen results in formation of paraffin, whereas the reversible process leads to formation of an olefin-hydride complex and subsequently... 

Protonation of an olefin-hydride complex to generate M(olefin)(H2) and observation of transfer of protons from to the olefin could prove direct... 

Five-coordinate alkyl Pt(rv) complexes have been proposed as short-lived intermediates in platinum-catalyzed alkane functionalization cycles. Hence, interest in preparing suitable complexes to enable their chemistry to be studied has arisen.Novel five-coordinate platinum(iv) alkyl complexes with a variety of /3-diiminate ligands have been prepared and have been shown to be useful precursors to unsaturated Pt(ll) species for alkane. Stoichiometric alkane dehydrogenation was observed using either a five-coordinate Pt(iv) species or an olefin hydride complex. Mechanistic proposals were based on NMR spectroscopic measurements and, in one case, on X-ray crystallographic characterization of a product. Kinetic details were not reported in this communication, but the systems hold potential promise, and conversion to a catalytic system may be feasible upon further investigation. 

A study of a platinum /3-diiminate (nacnac) complex provides evidence for oxidative addition to C-H bonds. Starting with a stable Pt(rv) trimethyl adduct, loss of ethane produces a reactive Pt(ll) intermediate that metallates the ligand (Pt(ll) — Pt(iv)) and reductively eliminates methane(Pt(lv) —> Pt(ll)). / -Elimination then generates an observable pendant olefin hydride complex. This species serves as a reversible source of the unsaturated Pt(ll) intermediate, which can reversibly add to alkane solvent C-H or C-D bonds, permitting deuteration of the isopropyl groups of the ligand (Equation (14)). ... 

The intramolecular migration of a hydride to an olefin is fast enough that few complexes contain an olefin and a hydride ligand cis to each other. Some olefin hydride complexes, such as the iridium and platinum examples below, are stable because they contain hydride and olefin Hgands that are mutually trans. These complexes must isomerize before insertion can take place. 

Through ii-o rearrangements, olefin hydride complexes may act as catalysts for olefin isomerization. A general mechanism is as follows. Scheme 11.19 ... 

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