Double bonds, heavier main group elements

Double bonding between the heavier main-group elements from reactive intermediates to isolable molecules. A. H. Cowley, Polyhedron, 1984, 3, 389-432 (116). 

Compounds of multiple bond systems involving heavier main group elements were long considered to be unstable and synthetically inaccessible. In particular, the so-called double bond rule, which forbade the formation of (pn-pn) multiple bonds between silicon and other elements, hindered the development of the chemistry of low-coordinate silicon compounds containing Si=X (X = C, N, Si, P) double bonds for some years. 

Double-bond compounds containing heavier main group elements are highly reactive to undergo ready addition reactions with various reagents. 

Molecules of heavier main-group elements show significant differences to compounds of elements from the first-octal row of the periodic system. The coordination number of the lighter elements is rarely larger than 4 while many stable molecules of the heavier elements with coordination number 5 and 6 are known. Another difference concerns the stability of molecules with multiple bonds. Many stable molecules with double or triple bonds between the lighter elements are well known, particularly compounds that have C=C, C=N, C=0,... 

Until the early 1980s, the predominant view was expressed by the so-called double bond rule , which stated that elements with a principal quantum number greater than 2 do not form mnltiple bonds with themselves or with other elements. Exciting events in 1981 saw this rule overturned, with the annonncement of the first sUene Si=C and disilene Si=Si donble bonds, followed not long after by a P=P donble bond in Mes P=PMes . In the 25 years since that time, a phenomenal advance in experimental results has taken place in this area, accompanied by a parallel advance, which has not been without controversy, in the understanding of the phenomenon of multiple bonding in the heavier main group elements. 

Scheme 2.9.1 Double-bond compounds of heavier main group elements...

The Pb=Pb double bond distance of 3.0515(3) A is easily compared to the Pb-Pb single bond distances of 2.83 A for PhaPb-PbPha (Ph = phenyl) or 2.8697(10) A for BpsPb-PbBpa (Bp = biphenyl).Here again is an example of a heavier main group element engaging in multiple bond formation. However, the heavier main group atoms display a resistance to hybridize in the same fashion as carbon. Rather, the Pb=Pb double bond is depicted (6) showing the orbital overlap in two donor-acceptor interactions. 

It is well-known that multiple bonds involving heavier main group elements are unstable, and thus, some stabilizing techniques are needed to prepare compounds with heavy unsaturated skeletons. Kinetic stabilization utilizing steri-cally crowded substituents to stabilize a reactive species is a method to obtain such multiple bonding, and many kinds of kinetically stabilized heavy multiple bonds have been derived so far [Ij. As for phosphorus compounds, in 1978 Bickelhaupt and coworkers reported the first kinetically stabilized phosphorus-carbon double bond I (phosphaethene) [2], and in 1981 Yoshifuji and coworkers reported the first stable phosphorus-phosphorus double bond II (diphosphene)... 

Cowley, A. H., Stable Compounds with Double Bonding between the Heavier Main-Group Elements, Accounts Chem. Res. 17 [1984] 386/92. 

Only a few years ago, students were taught that silicon and germanium formed no multiple bonds to anything. Textbooks emphasized that these elements, as well as other, heavier main group elements, were simply not able to self-link through double bonds to yield stable compounds analogous to those of alkenes, the important family of unsaturated hydrocarbons with C=C bonds. Now the facile synthesis of multiple bonds in heavier main group elements is routinely reported, and doubtless the detailed thermochemistry will keep apace. 

A.H. Cowley - Stable Compounds with Double Bonds Between the Heavier Main-Group Elements, Acc. Chem. Res. 17,386,1984 From Multiple Bonds to Materials Chemistry, J. Organomet. 400, 71,1990 Some Past Vignettes and Future Prospects for Main Group Chemistry, J. Organomet. Chem. 600,168,2000 From Group 13-Donor-acceptor Bonds to Triple-decker Cations, Chem. Commun. (Feature Article), 2369, 2004. 

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