Bonds to Main-Group Metals

Couret, J. Escudie, J. Satge, and G. Redoules, Angew. Chem. Intemat. Edn., 1976, 15, 429. 

The reaction of oligomeric dimethyltin with (Me2SnS)3 leads to a five-membered heterocycle, 2,2,4,4,5,5-hexamethyl-l,3-dithia-2,4,5-tristannolan (12), 

Kozima, K. Kobayashi, and M. Kawanisi, Bull. Chem. Soc. Japan, 1976, 49, 2873. 

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Haaland A (1989) Covalent versus dative bonds to main group metals. Angew Chem Int Ed 28 992-1007... 

There is a large number of 6( Se) of selenium-metal compounds which, according to Duddeck, can be classified into three types (a) selenium with covalent bonds to main group metals. Deshielding is... 

Compounds containing transition metals bonded to main-group elements, such as Hg and Sn, react with Au derivatives to form metal-metal bonds with the elimination of R3MCI ... 

Addition of hydride bonds of main group metals such as B—H, Mg—H, Al—H, Si—H and Sn—H to alkenes and alkynes to give 513 and 514 is called hydrometallation and is an important synthetic route to compounds of the main group metals. Further transformation of the addition product of alkenes 513 and alkynes 514 to 515,516 and 517 is possible. Addition of B—H, Mg—H, Al—H and Sn—H bonds proceeds without catalysis, but their hydrometallations are accelerated or proceed with higher stereoselectivity in the presence of transition metal catalysts. Hydrometallation with some hydrides proceeds only in the presence of transition metal catalysts. Hydrometallation starts by the oxidative addition of metal hydride to the transition metal to generate transition metal hydrides 510. Subsequent insertion of alkene or alkyne to the M—H bonds gives 511 or 512. The final step is reductive elimination. Only catalysed hydrometallations are treated in this section. 

The transition metals form complex ions. A complex ion consists of a central metal ion with molecules or ions attached to the metal ion by coordinate bonding. Some main group metals, such as Al, can form complex ions, but the transition metals form a much greater variety of complexes. Transition metal complexes are described in more detail in the next section. 

Multiple bonding between main group metals is a vigorous research area, with many seminal synthetic achievements pnblished since the turn of the century. The diborene and gallyne complexes are provided here as a brief introduction to this exciting area helpful reviews are available for more details. ... 

As discussed in the previous chapter, with respect to main group metal elements, the verification of the intramolecular-coordination bonds had already been published with IR spectra data in a Japanese acadanic journal in 1966. 

The reaction of the ionic cyclopentadienides with metal compounds is by far the most general method for synthesis of cyclopentadienyl metal compounds. This method is applicable both to transition metals, which form 7T-bonded cyclopentadienyl compounds principally, and to main group metals which form a-bonded cyclopentadienyl compounds. 

In general, carbon-metal bonds involving transition elements are not as polar as those of the elements in Groups lA and 2A and exhibit less carbanionic character. The availability of d orbitals of transition elements, however, provides opportunities for novel and useful types of reactivity not available to main-group metals. The first part of this chapter deals with Group lA and 2A organometallics, the second part with the transition-metal organometallics. Both emphasize synthetic applications. 

Palladation of aromatic compounds with Pd(OAc)2 gives the arylpalladium acetate 25 as an unstable intermediate (see Chapter 3, Section 5). A similar complex 26 is formed by the transmetallation of PdX2 with arylmetal compounds of main group metals such as Hg Those intermediates which have the Pd—C cr-bonds react with nucleophiles or undergo alkene insertion to give oxidized products and Pd(0) as shown below. Hence, these reactions proceed by consuming stoichiometric amounts of Pd(II) compounds, which are reduced to the Pd(0) state. Sometimes, but not always, the reduced Pd(0) is reoxidized in situ to the Pd(II) state. In such a case, the whole oxidation process becomes a catalytic cycle with regard to the Pd(II) compounds. This catalytic reaction is different mechanistically, however, from the Pd(0)-catalyzed reactions described in the next section. These stoichiometric and catalytic reactions are treated in Chapter 3. 

Organometallic compounds which have main group metal-metal bonds, such as S—B, Si—Mg,- Si—Al, Si—Zn, Si—Sn, Si—Si, Sn—Al, and Sn—Sn bonds, undergo 1,2-dimetallation of alkynes. Pd complexes are good catalysts for the addition of these compounds to alkynes. The 1,2-dimetallation products still have reactive metal-carbon bonds and are used for further transformations. 

The lobes of electron density outside the C-O vector thus offer cr-donor lone-pair character. Surprisingly, carbon monoxide does not form particularly stable complexes with BF3 or with main group metals such as potassium or magnesium. Yet transition-metal complexes with carbon monoxide are known by the thousand. In all cases, the CO ligands are bound to the metal through the carbon atom and the complexes are called carbonyls. Furthermore, the metals occur most usually in low formal oxidation states. Dewar, Chatt and Duncanson have described a bonding scheme for the metal - CO interaction that successfully accounts for the formation and properties of these transition-metal carbonyls. 

In summary, one can state that s-p-hybridization on the heavier main group metals is not responsible for the stereochemical activity of a lone pair. Instead, the general conclusion can be drawn that anti-bonding metal ns-ligand np interactions lead to structural distortions in order to minimize these unfavorable interactions. 

In contrast to these adducts in which the boratabenzene ring is bound ti to the main-group metal, reaction of [C5H5B-Me]Li with PbCl2 affords a bent-sandwich complex, Pbfi/ -CsI LBMeh.31 This report provided the first structural characterization of an r 6-bonding mode to a p-block metal. Reaction of Pb(Ti6-C5H5BMe)2 with a Lewis base such as bipyridine leads to a complex wherein the bipyridine is bound in the pseudoequatorial plane. 

This section is limited to complexes which have a group 1 metal in conjunction with another, different main group metal, but also includes Cu and Cd since they exhibit properties akin to their main group analogs. It is also limited mainly to those complexes in which the metals find themselves attached to different atoms and there is a particular emphasis on compounds with alkali metal-carbon bonds of various types, except where the evolution of inverse crown complexes is discussed. There are many more heterobimetallic-heteroatom complexes (e.g., mixed metal amides), but these lie outside the scope of this current review though references may be found to them in the references for the complexes described herein. 

Monodentate (monometallic monoconnective) phosphor-1,1-dithiolato ligands are rare. Bidentate (monometallic biconnective) form chelate rings and three sub-types can be distinguished according to the degree of asymmetry (Scheme 2). The most asymmetric type (anisobidentate) occurs when a covalent bond is associated with a secondary bond this takes place mostly in main-group metal complexes. The second type is rare and is the result of the association between a covalent and a dative coordinate bond. The symmetric bidentate bonding (isobidentate) is found mainly in transition metal complexes. 

Distibines react as monodentate (type 5) or bridging bidentate (type 6) ligands through donation of the lone pairs of electrons to main group or transition metal centers. Fission of the Sb-Sb bond leads to complexes with bridging R2Sb ligands (type 7, 8 and 9) (Scheme 3). 

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