Metal-alkyne orbital interactions

Figure 16. Metal-alkyne orbital bonding interactions. Alkyne ligand a and k donation to metal da and dn orbitals (a), and corresponding metal-to-n back-donation (b).

Orbital interaction diagram and EHT calculations show that the 1,2 intramolecular shift of hydrogen is symmetry disfavored [10]. In presence of a transition metal fragment to which the alkyne coordinates, the activation energy is considerably lower. This has been attributed to the tendency of H to shift as a proton rather than as a hydride. 

Alkynes have two v and two it orbitals that can potentially interact with metal orbitals, and in some instances, it is thought that all of these are involved at the same time in a mononuclear complex. An extended Hiickel calculation on Mofmexo-tetra-p-tolylporphyrinXHC=CH) supports this view (Fig. I5.25).8" Thus both bonding orbitals of the alkyne (6, and at) can donate electron density to molybdenum to form (he 16, and lo, MOs. and both anti bonding orbitals (b2 and a2) can accept electron density to form the lf>2 and Ui2 MOs. Notice that both it bonding orbitals (a, and bt) of acetylene interact significantly with metal d orbitals of the same symmetry. 

Steric factors probably prohibit simultaneous rotation of the olefin and alkyne C2 units which would crowd all four metal-bound carbons into the same plane. Separate rotation of each unsaturated ligand was explored theoretically using the EHMO method. Rotation of the olefin destroys the one-to-one correspondence of metal-ligand tt interactions. Overlap of the filled dxz orbital with olefin n is turned off as the alkene rotates 90°, creating a large calculated barrier for olefin rotation (75 kcal/mol). Alkyne rotation quickly reveals an important point the absence of three-center bonds involving dir orbitals allows the alkyne to effectively define the linear combinations of dxy and dyz which serve as dn donor and dir acceptor orbitals for 7T and ttx, respectively. Thus there should be a small electronic barrier to alkyne rotation (the Huckel calculation with fixed metal... 

Dynamic NMR studies of W(CO)(HC=CH)(S2CNEt2)2 and related complexes (58) indicate that the barrier to alkyne rotation is around 11-12 kcal/mol (Table IV). Discordant metal-ligand 77 interactions anticipated as the alkyne rotates away from the ground state orientation are evident in EHMO calculations (Fig. 23). When the alkyne is orthogonal to the M—CO axis the dyz orbital is stabilized by CO it but destabilized by alkyne ir while dxy becomes the lowest lying dir orbital due to... 

An alkyne can also behave as a formal four-electron donor to one metal atom. In addition to the type of interaction just discussed, the other pair of it electrons may be partially donated to a metal dn orbital lying perpendicular to the plane of... 

The bonding in monometal alkyne complexes is usually interpreted in terms of the Dewar-Chatt-Duncanson model (293), since the alkyne molecule has a pair of n and n molecular orbitals which lie in the plane of the metal and the two carbon atoms. These two orbitals are denoted n and n, and are analogous to those in jr-bonded alkene complexes (394). There is also a pair of n and n molecular orbitals which lie perpendicular to the metal-carbon plane, denoted nL and n . These orbitals are illustrated in Fig. 14. Both sets of n and n orbitals have the correct symmetry to interact with metal d orbitals. The interaction... 

Consideration of molecular orbital interactions suggests that it would be ideal to use a transition metal complex having at least two empty and one filled nonbonding metal valence shell orbital, i.e. a 14-elec-tion species, for observing facile bicyclization reactions shown in equations (2)-(4). This is based on assumptions that effective ir-complexation of an alkyne or an alkene with a metal complex requires the... 

Transition metal-alkyne 7t-bonding interactions are similar to their alkene counterparts in that both can be considered as donation from the alkyne Jt-to-metal d orbitals, with concomitant back-donation from the metal d-to-alkyne tt orbitals leading to bent alkyne structures (Fig. 16) (304, 305) Commonly, the donor interactions are primarily ct type while the back-donation occurs mainly... 

Metal-alkyne bonding generally is considered to be dominated by two orbital interactions (Figs, la and lb) which act synergistically (Dewar, 1951 Chatt and Duncanson, 1953 Maitlis, 1971a,c Nelson and Jonassen, 1971). 

The vacant orbital in 16e -zirconocene(IV) complexes allows a Ji-interaction with an incoming alkene or aUcyne. However no metal— alkene/alkyne backbonding is possible with the d°-Zr-metal center. As a consequence, the metal-olefin interaction is not stabilized, and formation of the thermodynamically favored o-bound organozirconocene complex (>10 kcal/mol) is then observed [36]. The product is the result of an overall cis addition of the zirconocene metal fragment and the hydrogen across the carbon-carbon multiple bonds. 

Following the proposals of Rooney et al. [85—87], it has generally been assumed that, as with monoolefins, the adsorbed state of an alkyne active in hydrogenation is a 7r-complex formed by the interaction of the 7r-orbitals of the acetylenic bond with two metal atoms. The 7r-complexed alkyne may be represented as structure L. 

In the idealized ethylene-acetylene model complex the HOMOl is the olefin stabilized dxz while the HOM02 orbital, dxy, reflects alkyne w overlap. The M—C alkyne distances employed in the calculation increase overlap responsible for the alkyne-metal v interactions relative to the olefin which is further from the metal and overlaps less (60). The dir bonding contribution of the single-faced 7r-acid olefin is to stabilize the lone filled d tr orbital which is independent of the alkyne. This role is compatible with the successful incorporation of electron-poor olefins cis to the alkyne in these d4 monomers. It may well be that the HOMOl and H0M02 orbitals in isolated complexes are reversed relative to the model complex as a result of electron-withdrawing substituents present on the olefins. 

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