Structures metal olefin complexes

Synthesis, structure and reactions of chelate metal-olefin complexes... 

None of the theories proposed before 1951 to explain the nature of the bonding in metal-olefin complexes was entirely satisfactory (35). Chatt (S3) suggested that, in addition to the ordinary coordinate bond, some sort of bond involving the filled d-orbitals of the metal atom was essential for coordination of the olefin, but such a bond was difficult to formulate until Dewar (64) described it in terms of molecular orbitals. The structure which he proposed for the silver-olefin complexes, and that subsequently proposed for the platinum-olefin complexes by Chatt and Duncanson (35) are shown schematically in structures (I) and (II). The type bond, which has also been called a ji-bond (64, 4), is formed by the overlap of the filled bonding... 

Accurate structural analyses on metal-olefin complexes during the last 10 years (14, 106, 147, 162, 195) have suggested the following generalizations. 

The cationic Jt-allyl complex is often isolable and has been the subject of considerable study. X-ray structures of the 7t-allyl complexes with chiral ligands have been a primary source of structural information from which the design and predictive model of chiral catalysts derive. Chiral-metal-olefin complexes, which constitute another important class of intermediates have also been isolated, albeit few in number [31]. These static studies have been complemented by a growing number of NMR studies taking advantage of modem heteronuclear correlation and NOE techniques, which offer opportunities to monitor solution structures of the catalytic species [32-34],... 

There have been too many crystallographic studies of transition metal-olefin complexes to present a comprehensive survey in this limited space. Therefore, only representative structures of major classes of compounds will be discussed, drawing on pertinent structural determinations as they are needed. Many of the important features of olefin bonding can be illustrated in the d10 system whore most of the complexes are approximately trigonal-planar (Fig. 4). 

From the earliest days of metal-catalyzed olefin polymerization, the olefin complexation step, eq 1, has been thought to be important for catalysis. For Ni(II) and Pd(II) catalysts, " 4, the olefin binding event is well precedented with numerous crystal structures of olefin complexes. In fact, the resting state of the catalyst is thought to be an olefin complex. " The binding for these d complexes is properly explained in the terms of the classic Dewar— Chatt—Duncanson model. Figure 4. In this model, bonding consists of a donation of electron density from the olefin ji orbital into an empty o orbital on the metal (forward coordination), and simultaneous donation from a filled metal dji orbital into the empty 71 orbital of the olefin (back-donation). 

Theory and importance of oxygen-bridge bonding (B. Jezowska-Trzebiatowska) p. 89. Synthesis, structure, and reactions of chelate metal olefin complexes (R. Nyholm) p. 127. 

The changes in structure during this attack are depicted in Equation 11.21. Eisenstein and Hoffmann concluded many years ago that a symmetric metal-olefin complex is deactivated toward nucleophilic attack, but slippage of the olefin creates a LUMO that can interact with the incoming nucleophile. Thus, in the transition state for external attack, "the organometallic olefinic complex cannot be near its equilibrium structure." ... 

Table 6.2. Structural Data for Metal-Olefin Complexes...

Salomon, R.G. and Kochi, J.K., Cationic olefin complexes of copper(l). Structure and bonding in group lb metal-olefin complexes,/. Am. Chem. Soc., 95, 1889, 1973. 

In the case of r)2-coordination of the exocyclic C=C bond, it becomes substantially elongated compared with the double bond of free alkenes, as a result of back donation from the metal to the 7t orbitals of the double bond. For instance, in complex 17b the coordinated bond length is 1.437 A (see Fig. 3.2).18 This is also reflected in the loss of planarity around the quaternary exocyclic carbon, the methylenic carbon being bent out of the ring plane by 10.78°.18 Similar structural features were also observed with other P2Pd conjugated olefin complexes.39... 

In contrast, methyl cyclopropenone is reported283) to react with the Pt-olefin complex 455 at low temperature with replacement of the olefin ligand. In the resulting complex 456 the cyclopropenone interacts with the central atom via the C /C2 double bond according to spectroscopic evidence284). At elevated temperatures a metal insertion to the C1<2)/C3 bond occurs giving rise to 457. Pt complexes of a similiar type were obtained from dimethyl and diphenyl cyclopropenone on reaction with 455 and their structures were established by X-ray analysis285). 

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