Double bond rule

A brief history of (3p-2p)7i bonds between phosphorus and carbon followed by an introduction to the methods of phosphaalkene synthesis that are pertinent to this review will be provided. The earliest stable compound exhibiting (3p-2p)7x bonding between phosphorus and carbon was the phosphamethine cyanine cation (1) [33]. An isolable substituted phosphabenzene (2) appeared just two years later [34]. The parent phosphabenzene (3) was later reported in 1971 [35]. These were remarkable achievements and, collectively, they played an important role in the downfall of the long held double bond rule . The electronic delocalization of the phosphorus-carbon multiple bond in 1-3, which gives rise to their stability, unfortunately prevented a thorough study of the chemistry and reactivity of the P=C bond. 

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 chemistry of unsaturated silicon compounds, i.e. silylenes and molecules having (p-p)ic-sili-con element multiple bonds >Si=E (E = C, Si, Ge, Sn, N, P, As, O, S), is an interesting field of research for the theoretician as well as for the preparative chemist because of the unexpected and fascinating results which can be obtained. Yet 30 years ago, such compounds were considered "non existent" because of the classical "double bond rule", established by Pitzer and Mulliken in the early fifties. Since then, the chemistry of unsaturated silicon compounds proceeded from the investigation of small" species in the gas phase to the synthesis and isolation of stable species with bulky substituents at the > Si =E moiety, and to the determination of their structural features. 

Early calculations (Pitzer, K. S. J. Am. Chem. Soc. 1948, 70, 2140) are usually mentioned in connection with the double-bond rule, predicting a small overlap due to the diffuse orbitals of the heavy atoms from the second or lower long row of the periodic table. 

Edilor. Angew. Chen. bn. Ed. Engl. 1991.30. A-69. The double bond rule uui be visited os fellows Elements having a principal quant inn number greater than two are not likely to form pw-p bends. 

The successful isolation of all of these compounds is more a tribute to the persistence with which they were pursued than to any inherent stability of the bonds themselves. To invert George Leigh Mallory s remark about Mt. Everest, the extraordinary efforts expended on this class of compounds stemmed from the fact that they were not there These efforts and their corresponding successes have caused one observer to comment Finding exceptions to the double-bond rule has become a... 

Following the classical double-bond rule, multiple bonds between silicon and sulfur, both elements of the third period, should be very difficult to obtain. Except for SiS119,120,... 

It should be remembered that the heteroatom under scrutiny belongs to the second full period of the periodic table of the elements. Therefore, the double bond rule also applies to its double bonds. Thus, the double bond of an ylene form is not a p, but a d p-n double bond. Because of the presence of this (albeit not very stable) dou-... 

The double bond rule that limited (p,p)-n-bonds to elements of the first 8-atom period was violated for the first time in the 1960s. The generation of phos-phaacetylenes l,5 diphosphene 2,6 and phosphacyanine 37 initiated dramatic developments, an end of which is not yet in sight (Figure 8.1). 

In addition to the periodic system of the elements, in chemistry two rules have proved particularly useful with respect to attempts to classify and systemize the variety of elements and their compounds the octet rule and the double-bond rule. 

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. 

As mentioned before, carbon and silicon mostly differ in their ability to from multiple p,-p, bonds E=Y with suitable partners (E = C, Si Y = element of group 14 to 16 ). While the p orbital overlap in compounds >C=Y is sufficient to yield stable multiple bonded species, this overlap is strongly reduced in the case of silicon (classical double bond rule of Pitzer and Mulliken). Consequently, under comparable conditions the equivalents of many unsaturated monomeric compounds of carbon, such as H2C=CH2, R2C=0 or CO2 are silicon single bonded polymeric products, e g. polysilanes (-H2Si-SiH2-)n, silicones (-R2Si-0-) and silicon dioxide (Si02)n... 

Transition metal phosphinidene complexes were originally prepared in order to access what was expected to be a rich chemistry of phosphorus(I)T However, terminal phosphinidenes have been found to be difficult to prepare, partly because they tend to either catenate or bridge. This is a manifestation of the double-bond rule for the main group elements. However, breakthroughs in syntheses of these complexes came about through the use of devious synthetic techniques in combination with sterically encumbered ligands. 

As we did in Example 27-6(b), we first find the longest chain that includes the double bond, and then number it beginning at the end nearer the double bond (Rules 1 and 2). Then we specify the identities and positions of substituents in the same way we did for alkanes. 

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