Abstract
Publisher Summary This chapter discusses the π complexes of all main-group elements, including their synthesis, structure, and bonding and the literature up to 1985. Compounds described have been characterized by X-ray or electron diffraction or at least by conclusive infrared (IR) or nuclear magnetic resonance (NMR) data. The term “π complex” is connected with compounds in which π ligands are polyhapto bonded to a transition metal. Many examples of the interaction between lithium and diverse π systems are documented in the literature. There are many examples known of π bonding between the heavier alkali metals and carbanionic fragments. There is no example in the literature of a stable alkene (or alkyne) complex with magnesium as central atom. Many compounds of the heavier alkaline–earth metals with organic ligands are described in the literature, for which from spectroscopic data π-interactions can be anticipated. π complexation is believed to be involved in the first step of the hydroboration of alkenes and alkynes; rearrangement reactions of organoboranes most likely involve the intermediates of a π-complex type. Alkene or alkyne complexes are mechanistically important intermediates in hydroalumination and carboalumination reactions. Using the isolobal relationship between CH or CH 2 units and corresponding transition metal or main-group element fragments, the majority of the nonclassical carbocations can be theoretically treated as π complexes.
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