Group 15 elements multiple bond formation

Multiple Bond Formation by 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. 

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. 

The synthesis and chemistry of compounds where germanium is linked to a group 16 element has undergone a number of developments since the field was last surveyed,1 and a review on the subject has been published.162 New developments include the formation of new heterocyclic compounds and formation of multiple bonds to the heavier group 16 elements. A review has also appeared on this subject.163... 

The discovery that the iron-group elements can form bonds which have in part the character of multiple bonds by making use of the orbitals and electrons of the 3d subshell, whilq surprising, need not be greeted with skepticism the natural formula for a compound ECO is that with a double bond from R to C, and the existence of the metal carbonyls might well have been interpreted years ago as evidence for double-bond formation by metals. The double-bond structure for nickel tetracarbonyl (structure E) was in fact first proposed by Langmuir62 in 1921, on the basis of the electroneutrality principle, but at that time there was little support for the new idea. 

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