Organometallics, sigma bonding

D. B. Malpass, L. W. Fannin and J. J. Ligi, Organometallics-Sigma Bonded Alkyls and Aryls, Kirk Othmer Encyclopedia of Chemical Technology, Third Edition, 1981, Volume 16, pp. 573-579. 

V. A New General Synthesis for Trifluoromethyl Organometallic Compounds and Other Sigma-Bonded Metal Compounds Based on Metal Vapor as a Reagent... 

The purpose of this article is to review recent results on the carbonylation chemistry of actinide-to-carbon sigma bonds, bearing in mind the unique properties of 5f-organometallics cited above. We focus our attention on the properties of bis(pentamethylcyclopentadienyl) actinide acyls. Just as transition metal acyls (A) occupy a pivotal role in classical carbonylation chemistry, it will be seen that many of the unusual... 

Organometallics such as Grignard and lithium reagents give rise to deinsertion in hydrido complexes (cf Sect. 3.1). However, their reactivity is quite different toward normal sigma bonded transition metal-group IVg metal complexes. 

A common motif in organometallic chemistry is the agostic interaction, which can act to stabilize low-coordination low-e-count complexes. The requirement is an alkyl group with a / - or a y-C—H bond attached to the metal within reach of (i.e., cis to) an empty coordination site. An attractive interaction occurs with the C—H bond acting as a 2e donor into the low-lying metal valence orbital that occupies that site. In the case of a / -C—H bond, hydride transfer may occur with little activation, resulting in an M—H sigma bond and complex with an alkene as discussed above. 

Marks has reviewed this area. Basset has bound organometallic species to a silica surface and seen interesting alkane reactions that go via initial CH bond activation this step probably proceeds by a sigma bond metathesis pathway since the metals involved are d . Snbseqnent reactions inclnde CC bond cleavage and carbon skeletal rearrangements. 

W. T. Reichle Preparation, physical properties, and reactions of sigma-bonded organometallic compounds, pp. 653-826 (696). Main Group elements only. 

In the case of two-center bonds, a pi bond has a nodal plane that includes the internuclear bond axis, whereas a sigma bond has no such nodal plane. (A delta bond in organometallic or inorganic molecular species has two nodes.) Radicals are classified by analogy into sigma and pi radicals. 

We can consider a partially ionic sigma bond as an already partially broken bond. An organometallic bond, for example, has electrons that are very available. Conversely, a carbon-carbon covalent bond is completely covalent its electrons reside in a very stable sigma bonding orbital and are not available. The main exception to this is the C-C bonds of cyclopropane, which are destabilized by ring strain, and thus are more reactive. 

G.J. Kubas, Metal-dihydrogen and sigma-bond coordination the consummate extension of the Dewar-Chatt-Duncanson model for metal-olefin pi bonding . Journal of Organometallic Chemistry, 635, 37 (2001). 

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