Organometallic Compounds with tt Bonding

The bonding in tt complexes of alkenes is the result of two major contributions. The filled tt orbital acts as an electron donor to the metal ion. There is also a contribution to bonding, called back-bonding, from a filled metal orbital interacting with the alkene tt orbital. These two types of bonding are represented in Fig. 8.1. 

These same general bonding concepts apply to all the other tt organometallics. The details of structure and reactivity of an individual compound depend on such factors as (a) how many electrons can be accommodated by the metal orbitals (b) the oxidation level of the metal and (c) the electronic character of other ligands on the metal. 

Alkene-metal complexes are usually prepared by a process in which some other ligand is first dissociated from the metal. Both thermal and photochemical reactions are used. 

TT-Allyl complexes of nickel can be prepared either by oxidative addition to Ni(0) or by ligation of a Ni(II) salt. 

Many related compounds have been prepared. The total number of electrons contributed by the ligands (six for each cyclopentadiene ion) plus the number of valence electrons on the metal atom or ion usually equals 18, to satisfy the effective atomic number rule.  

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Organometallic Compounds with tt Bonding General References... 

Compounds of these metals involving either a- or tt-bonds to carbon are generally much more reactive to both air and water than those of the d-block metals. Thus there is no lanthanide equivalent of ferrocene, an unreactive air- and heat-stable compound. They are often thermally stable to 100 °C or more, but are usually decomposed immediately by air (and are not infrequently pyrophoric). Within these limitations, lanthanide organometallic compounds have their own special features, often linked with the large size of these metals. 

Carbon monoxide and isonitxile, which have an isoelectronic structure and serve as TT-acid ligand on transition metals, form various stable transition metal complexes. One of the most fundamental and important reactions with carbon monoxide and isonitrile in organometallic chemistry is their insertion into the carbon-metal bond of organometallic compounds (Scheme 1). 

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