Double bond rule
In chemistry, the double bond rule states that elements with a principal quantum number (n) greater than 2 for their valence electrons (period 3 elements and higher) tend not to form multiple bonds (e.g. double bonds and triple bonds). The double bonds, when they exist, are often weak due to poor orbital overlap between the n>2 orbitals of the two atoms. Although such compounds are not intrinsically unstable, they instead tend to polymerize. An example is the rapid polymerization that occurs upon condensation of disulfur, the heavy analogue of O2. Numerous exceptions to the rule exist.
| B boron (n=2) | C carbon (n=2) | N nitrogen (n=2) | O oxygen (n=2) | Si silicon (n=3) | P phosphorus (n=3) | S sulfur (n=3) | |
|---|---|---|---|---|---|---|---|
| B | diborenes | alkylideneboranes | aminoboranylidenes, rare | oxoboranes, rare, rapid oligomerization | borasilenes (rare) | boranylidenephosphanes, rare, stable compounds are known | thioxoboranes, rare |
| C | alkenes | imines | carbonyls | silenes | phosphaalkenes | thioketones | |
| N | azo compounds | nitroso compounds | silanimines, rare, easy oligomerization, observed only at low temp | phosphazene (P=N) | sulfilimines | ||
| O | Singlet oxygen | silanones, Si=O bonds extremely reactive, oligomerization to siloxanes | numerous, e.g. phosphine oxides, phosphonates, phosphinates, phosphates | sulfinyls | |||
| Si | disilenes | silylidenephosphanes a.k.a. phosphasilenes, rare | silanethiones, rare, easy oligomerization | ||||
| P | diphosphenes | common compounds such as thiophosphates and phosphine sulfides, for example, triphenylphosphine sulfide and certain dithiadiphosphetanes | |||||
| S | disulfur, thiosulfoxides | ||||||
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