Sigma-bonded complexes

These observations show that despite the bulkiness of the benzyl groups in Zr (benzyl) 4, sufficient room is available for coordination of the monomer without displacing the benzyl group. This can be readily seen from the structure of Zr (benzyl) 4 represented by Fig. 2. We therefore write the initiation step for sigma-bonded complexes as follows... 

For an overview of complexation of molecular dihydrogen to metals, see G. J. Kubas, Metal Dihydrogen and Sigma-Bond Complexes Structure, Theory, and Reactivity (New York, Kluwer Academic, 2001). 

Synthesis and Reactions of Palladium-Carbon Sigma-Bonded Complexes... 

Palladium (II)-Nucleophile Addition across Olefins. Adding palladium complexes to olefins, either in the presence of an external nucleophile or a ligand which is attached to palladium, produces a palladium-carbon sigma-bonded complex which is not usually isolated in the case of monoolefins. Instead it decomposes and in doing so oxidizes the olefin to an organic carbonyl compound or a vinyl compound, exchanges a substituent group on the olefin, isomerizes the double bond, arylates (alkylates) the olefin, or carboxylates the olefin (2, 3). 

In the case of certain diolefins, the palladium-carbon sigma-bonded complexes can be isolated and the stereochemistry of the addition with a variety of nucleophiles is trans (4, 5, 6). The stereochemistry of the addition-elimination reactions in the case of the monoolefins, because of the instability of the intermediate sigma-bonded complex, is not clear. It has been argued (7, 8, 9) that the chelating diolefins are atypical, and the stereochemical results cannot be extended to monoolefins since approach of an external nucleophile from the cis side presents steric problems. The trans stereochemistry has also been attributed either to the inability of the chelating diolefins to rotate 90° from the position perpendicular to the square plane of the metal complex to a position which would favor cis addition by metal and a ligand attached to it (10), or to the fact that methanol (nucleophile) does not coordinate to the metal prior to addition (11). In the Wacker Process, the kinetics of oxidation of olefins suggest, but do not require, the cis hydroxypalladation of olefins (12,13,14). The acetoxypalladation of a simple monoolefin, cyclohexene, proceeds by trans addition (15, 16). 

Recent studies have established the following generalizations regarding a (sigma-bond) complexes ... 

The X-H (X = B, C, Si) bonds in some ligand molecules, like the H-H bond in H2, provide their a bonding electron pairs to bind the ligands to metal centers. They form intermolecular coordination compounds (i.e., sigma-bond complexes) or intramolecular coordination compounds (i.e., agostic-bond complexes). All such compounds involve non-classical 3c-2e bonding and are collectively termed a complexes. 

G. J. Kubas, Metal Dihydrogen and Sigma Bond Complexes , Kluwer, Amsterdam, 2001. 

Kubas GJ (2001) Metal-dihydrogen and sigma-bond complexes structure, theory, and reactivity. Kluwer, Academic, New York... 

Kubas, G. J. Metal Dihydrogen and Sigma-bond Complexes Structure, Theory and Reactivity, Kluwer New York, 2001. 

The CT-bonded Fe and Co complexes are generally unstable upon reduction at the metal center and this electrode reaction may be followed by a cleavage of the metal-carbon bond [12]. This is not the case for the osmium sigma-bonded complex (OEP)Os(Ph)2, which undergoes both oxidations and reductions at the metal center without loss of the axial ligand [79]. 

Canty, A. J. Palladum-Carbon Sigma-bonded Complexes. In Comprehensive OrganometalUc Chemistry // Abel, E. W., Stone, F, G. A, Wilkinson, G., Eds. Elsevier Oxford, 1995 Vol. 9, p 226. 

G. J. Kubas Metal Dihydmgen and Sigma Bond Complexes, Kluwer, New York, 2001 R. H. Crabtree, Angew. Chem., Int. Ed. 32, 789, 1993. 

---