Metal-alkene bonding

As a typical case, olefin-metal complexation is described first. Alkene complexes of d° transition metals or ions have no d-electron available for the 7i-back donation, and thus their metal-alkene bonding is too weak for them to be isolated and characterized. One exception is CpfYCH2CH2C(CH3)2CH=CH2 (1), in which an intramolecular bonding interaction between a terminal olefinic moiety and a metal center is observed. However, this complex is thermally unstable above — 50 °C [11]. The MO calculation proves the presence of the weak metal-alkene bonding during the propagation step of the olefin polymerization [12,13]. 

When an unsaturated ligand such as an alkene approaches the metal sideways to form a n complex, similar interactions lead to bonding. The filled 7t orbitals of the ligand bond to empty d orbitals of the metal, while filled d orbitals on the metal bond to the empty n orbitals of the ligand. The result is a % complex with the metal-alkene bond perpendicular to the plane of the alkene. The bond has both a and 7t character. 

The stereochemistry of nncleophilic attack is consistently trans to the metal-alkene bond (Scheme 39, eqnation 19) H >i49 regiochemistry of attack is normally at... 

The classical Dewar-Chatt-Duncanson model for metal-alkene bonding has been revisited with a combination of X-ray structural data (see Diffraction Methods in Inorganic Chemistry) and DFT calculations (see Molecular Orbital Theory), particularly on complexes of the type (acac)Rh(alkene)2. These indicate the existence of distortions from idealized geometry involving a twist (127), where the axis of the double bond is no longer perpendicular to the molecular plane and a roll (128), where the line... 

The most effective approach to interpreting the barriers for a wide range of compounds lies in the consideration of the relative interactions within the Dewar, Chatt, Ducanson model of metal alkene bonding. An extended Hiickel MO approach has explored the interactions of the valence orbitals and examined the important interactions. A comprehensive extended Hiickel MO treatment of ethylene bonding and rotational barriers by Albright, Hoffmann et a/. presents an excellent analysis and the reader is referred to their paper for further discussiou. We have found that the following approach, which considers oifly three orbitals on the metal and the n and y orbitals of the alkene, provides the essential elements for understanding the barriers to rotation. Naturally, steric effects and secondary interactions with other orbitals modulate these primary iuteractious. 

The douor compoueut of the metal-alkene bonding is a symmetric interaction and to a first approximation is... 

Alkenes and alkynes can also act as Lewis bases toward metals by using the two electrons in their 77 bonds. Such an interaction is called a it complex. The metal-alkene bond is a a bond due to its spherical symmetry around the axis formed by the metal and the midpoint of the C=C 77 bond. Even cr bonds such as the HU I and C-H bonds can act as two-electron donors to metals in this manner, and these compounds are called cr complexes. When a cr complex is formed in an intramolecular fashion, the bond is called an agostic bond or agostic interaction. Both 77 and cr complexes can be thought of as two-electron, three-center bonds in which three contributing orbitals comprise the occupied, bonding orbital. 

Fig. 23.5 Components of metal-alkene bonding (a) donation of electrons from the alkene tt-MO to a suitable metal d orbital or hybrid, and (b) back-donation of electrons from metal to alkene tt MO.

The stereochemistry of nucleophilic attack is consistently trans to the metal-alkene bond (Scheme 39, equation 19). Theregiochemistry of attack is normally at the alkene carbon bearing the most electron-donating (or least electron-withdrawing) group. For weakly electron-donating substituents, such as alkyl, the observed regioselection is modest for carbon-based nucleophiles. For amine and... 

When the metal is bound to the //2-alkene, there is free rotation around the metal-alkene bond as long as the substituent groups on the alkene are not too bulky. Thus, the dual processes of 1,2-insertion and / -elimination can be used to scramble the positions of the H atoms on metal alkyl groups, as shown in Figure 19.16. 

Fig. 6.4 Conventional representation of the metal—alkene bond, (a) Alkene->M a-donation (b) M lkene 7r-back donation.

The luminescence decay occurs as a single exponential with a triplet state lifetime of 0.53 0.02 //s. The transient difference absorption spectrum of Pd2(dba)3 shows an intense peak at 600 nm, which decays by a double exponential process. The observed lifetimes of the two processes are 1.0 0.1 ns and 0.50 0.03 The long-lived decay corresponds to an MLCT triplet deactivation process, whereas the short-lived pathway is likely associated with an isomerization of the complexed dba, rather than with a fluorescence decay from the excited singlet state. These excited state lifetimes in the nanosecond region are much shorter than that found for the cis-trans isomerization in free dibenzylideneacetone (5.2 //s). This isomerization must therefore be occurring at a coordinated and not a free dba. For Pt2(dba)3 only a single exponential decay is observed with a lifetime of 0.26 ps. This decay is due to an MLCT triplet deactivation process. In this case no short-lived decay process is observed. This difference may be due to the increased strength or to the decreased lability of the metal-alkene bond in the zerovalent platinum complex. 

It has been shown [41, 42] that partial donation of the 7r-electrons from alkene to an empty a-orbital of the metal weakens the 7r-bond of the unsaturated hydrocarbon and therefore lowers it energy, which makes electrons easier accepted from a back-donating d-orbital of the metal atom. Transfer of electrons from the metal d-orbital to the antibonding tt orbital increases the energy of the latter. As the consequence the degree of forming of metal-alkene bond should be reflected in HOMO-LUMO... 

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