Double bond rule

The double bond rule states that chemical elements with a principal quantum number greater than 2 (e.g. Period 3 elements and lower) should not form multiple bonds (e.g. double bonds and triple bonds) with themselves or with other elements.^[1] The double bonds, when they exist, are often weak due to poor orbital overlap. Although such compounds are not intrinsically unstable, they instead tend to polymerize. This effect is manifested by the rapid polymerization that occurs upon condensation of disulfur, the heavy analogue of O₂.

This rule was challenged and ultimately disproven starting from 1981 with the isolation of crystalline samples of compounds with silicon–silicon and phosphorus–phosphorus double bonds.^[2] Double bonds that would ordinarily not form can often be stabilized with proper functional groups either electronically or sterically.

Double bonds for carbon and nearest neighbours
	boron	carbon	nitrogen	oxygen	silicon	phosphorus	sulfur
boron	diborenes	alkylideneboranes	aminoboranylidenes rare [3]	oxoboranes rare, rapid oligomerization [4]	not known	boranylidenephosphanes, rare, stable compounds are known [5]	thioxoboranes, rare [6]
carbon		alkenes	imines	carbonyls, oxonium ions	silenes	phosphaalkenes	thioketones
nitrogen			Azo compounds	nitroso compounds	silanimines, rare easy oligomerization, only at low temp. [7]	phosphazene	sulfilimines
oxygen				dioxygen	Silanones Si=O bonds do not form oligomerisation to siloxane, only fleeting evidence	Numerous, e.g. phosphine oxides, phosphonates, Phosphinates, phosphates	sulfinyls
silicon					disilenes	Silylidenephosphanes a.k.a. phosphasilenes rare [8]	silanethiones, rare easy oligomerization [9]
phosphorus						diphosphenes	Common, for example in thiophosphates and in phosphine sulfides for example triphenylphosphine sulfide, certain dithiadiphosphetanes
sulfur							disulfur, thiosulfoxides

Other meanings

Another unrelated double bond rule exists that relates to the enhanced reactivity of sigma bonds once removed from a double bond. In bromoalkenes the C-Br bond is very stable but in an allyl bromide this bond is very reactive. Likewise bromobenzenes are generally inert whereas benzylic bromides are reactive. The first to observe the phenomenon was Conrad Laar in 1885. The name for the rule was coined by Otto Schmidt (1874–1943) in 1932 ^{[10] [11]}

References

1. Peter Jutzi New Element-Carbon (p-p) Bonds Angewandte Chemie International Edition in English, 1975, Volume 14, pp. 232 - 245. doi:10.1002/anie.197502321
2. Robert West Multiple bonds to silicon: 20 years later Polyhedron 2002, Volume 21, pp. 467-472. doi:10.1016/S0277-5387(01)01017-8
3. some research efforts exists in isomerization of B=NH₂ to triple bonded iminoborane HBNH The aminoboranylidene–iminoborane isomerization Victor M. Rosas-Garcia and T. Daniel Crawford J. Chem. Phys., 119, 10647 2003.
4. Synthesis and Characterization of a Coordinated Oxoborane: Lewis Acid Stabilization of a Boron-Oxygen Double Bond Dragoslav Vidovic, Jennifer A. Moore, Jamie N. Jones, and Alan H. Cowley J. Am. Chem. Soc., 2005, 127 (13), 4566-4567 doi:10.1021/ja0507564.
5. For instance Ar*P=B(TMP)2 with TMP = 2,2,6,6-Tetramethylpiperidine and Ar* = 2,6-dimesityl-phenyl Boron-Pnictogen Multiple Bonds: Donor-Stabilized PdB and AsdB Bonds and a Hindered Iminoborane with a B-N Triple Bond Eric Rivard, W. Alexander Merrill, James C. Fettinger, Robert Wolf, Geoffrey H. Spikes, and Philip P. Power Inorg. Chem. 2007, 46, 2971-2978 doi:10.1021/ic062076n
6. Formation and Reactions of a Thioxoborane, a Novel Boron--Sulfur Double-bond Compound Norihiro Tokitoh, Mitsuhiro Ito, and Renji Okazaki Tetrahedron Letters, Vol. 37, No. 29, pp. 5145-5148, 1996
7. Observation of a silanimine in an inert matrix and in solution at low temperature Steven S. Zigler, Robert West and Josef Michl Chemistry Letters Vol.15 , No.6(1986)pp.1025-1028 doi:10.1246/cl.1986.1025
8. Example Ar*tBuSi=PAr* with Ar* 2,4,6-trisiopropylphenyl and tBu tert-butyl in Ungewohnliche Reaktivitat der Silicium-Phosphor-Doppelbindung in einem Silyliden(fluorsilyl)phosphan: intramolekulare C,H-Inserierung und seine Umwandlung in ein neues Silyliden(silyl)phosphan M. Driess, S. Rell und K. Merz Z. Anorg. Allg. Chem. 1999, 625, 1119±1123
9. The First Genuine Silicon-Sulfur Double-Bond Compound: Synthesis and Crystal Structure of a Kinetically Stabilized Silanethione Hiroyuki Suzuki, Norihiro Tokitoh, Shigeru Nagase, Renji Okazaki J. Am. Chem. Soc., 1994, 116 (25), pp 11578–11579 doi:10.1021/ja00104a052
10. A History of the Double-Bond Rule Bernard E. Hoogenboom Journal of Chemical Education 1998 75 (5), 596 doi:10.1021/ed075p596
11. Schmidt, O. Z. Phys. Chem. 1932, A159, 337–356.