Hydride ligand, four coordinate

Both five-coordinate and four-coordinate pathways have been proposed for these reactions. The associative (five-coordinate) mechanism involves the formation of a trigonal bipyramidal or square pyramidal intermediate, which can revert back to tetracoordination by alkene insertion into the Pt—H bond.151 The dissociative (four-coordinate) mechanism involves initial substitution of a ligand other than hydride by alkene, followed by insertion to form the alkyl product. The ligand which is substituted is usually the anionic ligand, and if this group is trans to hydride an isomerization will need to occur prior to insertion of the coordinated alkene into the Pt—H bond. 

Homogeneous hydrogenation catalyzed by the four-coordinated rhodium complex, Rh[(C6H5)3P]3Cl, has been particularly well investigated. With this catalyst, the first step is formation of the six-coordinated rhodium hydride of known configuration, 16, in which we abbreviate the ligand, triphenylphosphine, (C6H5)3P, as L ... 

In coordinatively saturated metal hydrides, such as the HM(CO)s (M => Cr, Mo, W) derivatives, formation of the four-centered transition state for C02 insertion (Scheme 1) may proceed with or without CO loss and concomitant coordination ofC02 at the metal center. That is, C02 insertion may occur by means of dissociative (D) or dissociative interchange (Id) processes, or an associative interchange (Ia) process (47, 48). In either instance an acid-base interaction between the anionic hydride ligand and the electrophilic carbon center of carbon dioxide as represented in 6 may occur prior to formation of the four-centered transition state depicted in Scheme 1. An interaction of this type has been observed for these HM(CO)j derivatives with Lewis acids such as BH3 (49). 

Previous calculations showed that the propene prefers to be equatorial, whereas the H ligand prefers to be axial in the trigonal bipyramidal coordination complex. The authors considered six possible propene adducts as intermediates, two with both phosphines in equatorial positions, and four with one phosphine equatorial and the other axial. The two most stable intermediates account for 90% of the Boltzmann population considering all of the possible intermediates. The most stable intermediate has phosphines in equatorial-axial positions, whereas the other one has the phosphines in equatorial-equatorial positions. Both intermediates have the methyl group of propene close to the hydride ligand. 

The high asymmetric inductions that can be attained in these reductions are above all to be attributed to steric interaetions of the substrate with the phosphorus-bound phenyl groups of the [Ru-f/fj-BINAP] complex R)-S (Scheme 5). It can be assumed that hydride, being the smallest ligand, coordinates cis to both phosphine ligands. The P-bound phenyl groups occupy equatorial (parallel to the substrate coordination plane) and axial positions (perpendicular to the substrate coordination plane) with respect to the metal-ligand cycle. The skewed BINAP skeleton requires the equatorial phenyl substituents to protrude directly into two of four coordination quadrants that are accessible to the substrate. 

The existence of four-coordinate hydrogen completed the series of high-connectivity hydride ligands (e.g.,... 

The mechanisms of alkene insertions are known for some square-planar Pt(II) complexes , but it is difficult to distinguish between pathways in which a four-coordinated alkene/hydride species collapses by migratory insertion to a three-coordinated (perhaps solvated) species, and pathways in which a five-coordinated alkene/hydride collapses to a four-coordinated product. Both pathways occur, depending on the ligands. The rates of insertion of ethylene into the neutral and cationic hydrides trans-PtHX(PEt3)2 and trans-[PtHL(PEt3)2] + (X = Cl , NOj", CN L = acetone, CO, PEtj, AsPha, P(OMe)3 and P(OPh)3) leads to Scheme 1 °. 

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