Double bond rule
{{Short description|Chemistry rule of thumb}}
{{for|reactivity at allylic and benzylic positions|Schmidt 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). 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.{{Cite journal|doi=10.1002/anie.197502321|title=New Element-Carbon (p-p)π Bonds|year=1975|last1=Jutzi|first1=Peter|journal=Angewandte Chemie International Edition in English|volume=14|issue=4|pages=232–245}} 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.{{cite journal |last1=Kutzelnigg |first1=Werner |date=1984 |title=Chemical Bonding in Higher Main Group Elements |url= |journal=Angewandte Chemie International Edition |volume=23 |issue=4 |pages=272–295 |doi=10.1002/anie.198402721 |access-date=}} An example is the rapid polymerization that occurs upon condensation of disulfur, the heavy analogue of {{chem2|O2}}. Numerous exceptions to the rule exist.{{Cite journal|doi = 10.1016/S0277-5387(01)01017-8|title = Multiple bonds to silicon: 20 years later|year = 2002|last1 = West|first1 = Robert|journal = Polyhedron|volume = 21|issue = 5–6|pages = 467–472}} Several exceptions of this rule has been already made.{{Cite journal |last=Power |first=Philip P. |date=2020-12-14 |title=An Update on Multiple Bonding between Heavier Main Group Elements: The Importance of Pauli Repulsion, Charge-Shift Character, and London Dispersion Force Effects |journal=Organometallics |volume=39 |issue=23 |pages=4127–4138 |doi=10.1021/acs.organomet.0c00200 |issn=0276-7333}}
Triple bonds
| class="wikitable" align="center"
|+ Triple bonds for carbon and nearest neighbours |
| || B boron (n=2) || C carbon (n=2) || N nitrogen (n=2) || O oxygen (n=2) || width="220" | Si silicon (n=3) || width="220" | P phosphorus (n=3) || width="220" |S sulfur (n=3) || width=220|Ge germanium (n=4) || width=220|As arsenic (n=4) |
|---|
| B
| diborynes || Borataalkynes have been observed{{cite journal |title= Compounds with a Boron–Carbon Triple Bond |first1= Jürgen |last1= Allwohn |first2= Monika |last2= Pilz |first3= Ralf |last3= Hunold |first4= Werner |last4= Massa |first5= Armin |last5= Berndt |journal= Angew. Chem. Int. Ed. Engl. |volume= 29 |issue= 9 |year= 1990 |pages= 1032–1033 |doi= 10.1002/anie.199010321 }} || Observed in (t-Bu)BN(t-Bu) (an iminoborane) || colspan="6" style="background: silver;" | |
| C
| style="background: silver;" | || alkynes|| cyanides|| Carbon monoxide (C≡O) || silynes || phosphaalkynes || Carbon monosulfide (C≡S) || style="background: silver;" | || arsaalkynes |
| N
| colspan="2" style="background: silver;" | || Dinitrogen, Diazonium || colspan="2" style="background: silver;" | ||Phosphorus mononitride (P≡N) || colspan="2" style="background: silver;" | || Arsa-diazonium{{cite journal |title= Arsa-Diazonium Salts With an Arsenic–Nitrogen Triple Bond |doi= 10.1002/anie.201302725 |first1= Marcus |last1= Kuprat |first2= Axel |last2= Schulz |journal= Angew. Chem. Int. Ed. Engl. |volume= 52 |issue= 28 |year= 2013 |pages= 7126–7130 |pmid= 23740867 }} |
| O
|colspan="4" style="background: silver;" | || Silicon monoxide has some triple-bond character || colspan="4" style="background: silver;" | |
| Si
| colspan="4" style="background: silver;" | || disilynes || colspan="4" style="background: silver;" | |
| P
| colspan="5" style="background: silver;" | || Diphosphorus || colspan="3" style="background: silver;" | |
| S
|colspan="6" style="background: silver;" | |Observed in {{chem2|(I2)2S2(2+)}}{{cite journal |url= https://cen.acs.org/articles/83/i12/SULFURS-TURN-MULTIPLE-BONDS.html |title= Sulfur's Turn for Multiple Bonds |first= Stephen K. |last= Ritter |date= March 21, 2005 |journal= Chemical & Engineering News }} |colspan="2" style="background: silver;" | |
| Ge
| colspan="7" style="background: silver;" | || Digermyne || style="background: silver;" | |
| As
|colspan="5" style="background: silver;" | || Arsenic monophosphide (As≡P) || colspan="3" style="background: silver;" | |
References
{{Reflist}}