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). Double bonds for these heavier elements, 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 dimerize or even polymerize. Moreover, the multiple bonds of the elements with n=2 are much stronger than usual, because lone pair repulsion weakens their sigma bonding but not their pi bonding. An example is the rapid polymerization that occurs upon condensation of disulfur, the heavy analogue of . Numerous exceptions to the rule exist.
+ Double bonds for carbon and nearest neighbours Bboron(n=2) Ccarbon(n=2) Nnitrogen(n=2) Ooxygen(n=2) Sisilicon(n=3) Pphosphorus(n=3) Ssulfur(n=3) B diborenes alkylideneboranes aminoboranylidenes, rare Some research has been done on isomerization of B=NH2 to triple-bonded iminoborane HBNH oxoboranes, rare, rapid oligomerization borasilenes (rare) boranylidenephosphanes, rare, stable compounds are knownExample: Ar*P=B(TMP)2, where Ar* is 2,6-dimesityl-phenyl and TMP is 2,2,6,6-tetramethylpiperidine; see thioxoboranes, rareC alkenes imines carbonyls silenes phosphaalkenes thioketonesN azo compounds nitroso compounds silanimines, rare, easy oligomerization, observed only at low tempphosphazene (P=N) sulfilimines O Singlet oxygen silanones, Si=O bonds extremely reactive, oligomerization to siloxanes numerous, e.g. phosphine oxides, phosphonates, phosphinates,phosphates numerous, e.g. sulfuric acid, sulfates, sulfoxides (R-S(=O)-R′, compounds with a sulfinyl group), and sulfones (R-S(=O)2-R′, the sulfonyl group)Si disilenes silylidenephosphanes a.k.a. phosphasilenes, rareExample: Ar*tBuSi=PAr*, where Ar* is 2,4,6-trisiopropylphenyl and tBu is tert-butyl; see silanethiones, rare, easy oligomerizationP diphosphenes common compounds such as thiophosphates and phosphine sulfides, for example, triphenylphosphine sulfide and certain dithiadiphosphetanesS disulfur, thiosulfoxides
Triple bonds + Triple bonds for carbon and nearest neighbours Bboron(n=2) Ccarbon(n=2) Nnitrogen(n=2) Ooxygen(n=2) Sisilicon(n=3) Pphosphorus(n=3) Ssulfur(n=3) Gegermanium(n=4) Asarsenic(n=4)B diborynes Borataalkynes have been observed Observed in (t-Bu)BN(t-Bu) (an iminoborane) C alkynes cyanides Carbon monoxide (C≡O) silynes phosphaalkynes Carbon monosulfide (C≡S) arsaalkynesN Dinitrogen, Diazonium Phosphorus mononitride (P≡N) Arsa-diazoniumO Silicon monoxide has some triple-bond character Si disilynes P Diphosphorus S Observed in Ge Digermyne As Arsenic monophosphide (As≡P)
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