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.^[1] (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.)^[2] An example is the rapid polymerization that occurs upon condensation of disulfur, the heavy analogue of O₂. Numerous exceptions to the rule exist.^[3]

Double bonds for carbon and nearest neighbours
	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^[4]	oxoboranes, rare, rapid oligomerization^[5]	borasilenes (rare)^[6]	boranylidenephosphanes, rare, stable compounds are known^[7]	thioxoboranes, rare^[8]
C		alkenes	imines	carbonyls	silenes	phosphaalkenes	thioketones
N			azo compounds	nitroso compounds	silanimines, rare, easy oligomerization, observed only at low temp^[9]	phosphazene (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, rare^[10]	silanethiones, rare, easy oligomerization^[11]
P						diphosphenes	common compounds such as thiophosphates and phosphine sulfides, for example, triphenylphosphine sulfide and certain dithiadiphosphetanes
S							disulfur, thiosulfoxides

^ Jutzi, Peter (1975). "New Element‐Carbon (p‐p)π Bonds". Angewandte Chemie International Edition in English. 14 (4): 232–245. doi:10.1002/anie.197502321.
^ Kutzelnigg, Werner (1984). "Chemical Bonding in Higher Main Group Elements". Angewandte Chemie International Edition. 23 (4): 272–295. doi:10.1002/anie.198402721.
^ West, Robert (2002). "Multiple bonds to silicon: 20 years later". Polyhedron. 21 (5–6): 467–472. doi:10.1016/S0277-5387(01)01017-8.
^ Some research has been done on isomerization of B=NH₂ to triple-bonded iminoborane HBNH Rosas-Garcia, Victor M.; Crawford, T. Daniel (2003). "The aminoboranylidene–iminoborane isomerization". The Journal of Chemical Physics. 119 (20): 10647–10652. Bibcode:2003JChPh.11910647R. doi:10.1063/1.1620498.
^ Vidovic, Dragoslav; Moore, Jennifer A.; Jones, Jamie N.; Cowley, Alan H. (2005). "Synthesis and Characterization of a Coordinated Oxoborane: Lewis Acid Stabilization of a Boron−Oxygen Double Bond". Journal of the American Chemical Society. 127 (13): 4566–4567. doi:10.1021/ja0507564. PMID 15796509.
^ Franz, Daniel; Szilvási, Tibor; Pöthig, Alexander; Inoue, Shigeyoshi (2019). "Isolation of an N‐Heterocyclic Carbene Complex of a Borasilene". Chemistry – A European Journal. 25 (47): 11036–11041. doi:10.1002/chem.201902877. PMID 31241215. S2CID 195660396.
^ Example: Ar*P=B(TMP)₂, where Ar* is 2,6-dimesityl-phenyl and TMP is 2,2,6,6-tetramethylpiperidine; see Rivard, Eric; Merrill, W. Alexander; Fettinger, James C.; Wolf, Robert; Spikes, Geoffrey H.; Power, Philip P. (2007). "Boron−Pnictogen Multiple Bonds: Donor-Stabilized PB and AsB Bonds and a Hindered Iminoborane with a B−N Triple Bond". Inorganic Chemistry. 46 (8): 2971–2978. doi:10.1021/ic062076n. PMID 17338516.
^ Tokitoh, Norihiro; Ito, Mitsuhiro; Okazaki, Renji (1996). "Formation and reactions of a thioxoborane, a novel boron-sulfur double-bond compound". Tetrahedron Letters. 37 (29): 5145–5148. doi:10.1016/0040-4039(96)01039-8.
^ Zigler, Steven S.; West, Robert; Michl, Josef (1986). "Observation of a Silanimine in an Inert Matrix and in Solution at Low Temperature". Chemistry Letters. 15 (6): 1025–1028. doi:10.1246/cl.1986.1025.
^ Example: Ar*tBuSi=PAr*, where Ar* is 2,4,6-trisiopropylphenyl and tBu is tert-butyl; see Driess, M.; Rell, S.; Merz, K. (1999). "Ungewöhnliche Reaktivität der Silicium-Phosphor-Doppelbindung in einem Silyliden(fluorsilyl)phosphan: Intramolekulare C,H-Inserierung und seine Umwandlung in ein neues Silyliden(silyl)phosphan". Zeitschrift für Anorganische und Allgemeine Chemie. 625 (7): 1119–1123. doi:10.1002/(SICI)1521-3749(199907)625:7<1119::AID-ZAAC1119>3.0.CO;2-1.
^ Suzuki, Hiroyuki; Tokitoh, Norihiro; Nagase, Shigeru; Okazaki, Renji (1994). "The First Genuine Silicon-Sulfur Double-Bond Compound: Synthesis and Crystal Structure of a Kinetically Stabilized Silanethione". Journal of the American Chemical Society. 116 (25): 11578–11579. doi:10.1021/ja00104a052.

[1] Jutzi, Peter (1975). "New Element‐Carbon (p‐p)π Bonds". Angewandte Chemie International Edition in English. 14 (4): 232–245. doi:10.1002/anie.197502321.

[2] Kutzelnigg, Werner (1984). "Chemical Bonding in Higher Main Group Elements". Angewandte Chemie International Edition. 23 (4): 272–295. doi:10.1002/anie.198402721.

[3] West, Robert (2002). "Multiple bonds to silicon: 20 years later". Polyhedron. 21 (5–6): 467–472. doi:10.1016/S0277-5387(01)01017-8.

[:0-4] Some research has been done on isomerization of B=NH₂ to triple-bonded iminoborane HBNH Rosas-Garcia, Victor M.; Crawford, T. Daniel (2003). "The aminoboranylidene–iminoborane isomerization". The Journal of Chemical Physics. 119 (20): 10647–10652. Bibcode:2003JChPh.11910647R. doi:10.1063/1.1620498.

[:1-5] Vidovic, Dragoslav; Moore, Jennifer A.; Jones, Jamie N.; Cowley, Alan H. (2005). "Synthesis and Characterization of a Coordinated Oxoborane: Lewis Acid Stabilization of a Boron−Oxygen Double Bond". Journal of the American Chemical Society. 127 (13): 4566–4567. doi:10.1021/ja0507564. PMID 15796509.

[:2-6] Franz, Daniel; Szilvási, Tibor; Pöthig, Alexander; Inoue, Shigeyoshi (2019). "Isolation of an N‐Heterocyclic Carbene Complex of a Borasilene". Chemistry – A European Journal. 25 (47): 11036–11041. doi:10.1002/chem.201902877. PMID 31241215. S2CID 195660396.

[:3-7] Example: Ar*P=B(TMP)₂, where Ar* is 2,6-dimesityl-phenyl and TMP is 2,2,6,6-tetramethylpiperidine; see Rivard, Eric; Merrill, W. Alexander; Fettinger, James C.; Wolf, Robert; Spikes, Geoffrey H.; Power, Philip P. (2007). "Boron−Pnictogen Multiple Bonds: Donor-Stabilized PB and AsB Bonds and a Hindered Iminoborane with a B−N Triple Bond". Inorganic Chemistry. 46 (8): 2971–2978. doi:10.1021/ic062076n. PMID 17338516.

[:4-8] Tokitoh, Norihiro; Ito, Mitsuhiro; Okazaki, Renji (1996). "Formation and reactions of a thioxoborane, a novel boron-sulfur double-bond compound". Tetrahedron Letters. 37 (29): 5145–5148. doi:10.1016/0040-4039(96)01039-8.

[:5-9] Zigler, Steven S.; West, Robert; Michl, Josef (1986). "Observation of a Silanimine in an Inert Matrix and in Solution at Low Temperature". Chemistry Letters. 15 (6): 1025–1028. doi:10.1246/cl.1986.1025.

[:6-10] Example: Ar*tBuSi=PAr*, where Ar* is 2,4,6-trisiopropylphenyl and tBu is tert-butyl; see Driess, M.; Rell, S.; Merz, K. (1999). "Ungewöhnliche Reaktivität der Silicium-Phosphor-Doppelbindung in einem Silyliden(fluorsilyl)phosphan: Intramolekulare C,H-Inserierung und seine Umwandlung in ein neues Silyliden(silyl)phosphan". Zeitschrift für Anorganische und Allgemeine Chemie. 625 (7): 1119–1123. doi:10.1002/(SICI)1521-3749(199907)625:7<1119::AID-ZAAC1119>3.0.CO;2-1.

[:7-11] Suzuki, Hiroyuki; Tokitoh, Norihiro; Nagase, Shigeru; Okazaki, Renji (1994). "The First Genuine Silicon-Sulfur Double-Bond Compound: Synthesis and Crystal Structure of a Kinetically Stabilized Silanethione". Journal of the American Chemical Society. 116 (25): 11578–11579. doi:10.1021/ja00104a052.

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