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Stannylene (R2Sn:) is a class of compound considered as the tin analogue of carbene and a member of organotin(II) compounds. Unlike carbene, which usually has a triplet ground state, stannylene has a singlet ground state since valence orbitals of Sn have less tendency to form hybrid orbital and thus the electrons in 5s orbital is still paired up. To be able to isolate stannylene, either thermodynamic stabilization from electron donating group (e.g. amine, imine), or kinetic stabilization from bulky group, is needed.

History
The first stannylene with bulky substituents (i.e. persistent stannylene) was synthesized by Michael F. Lappert in 1973. It was isolated as a dialkylstannylene with the formula [(Me3Si)2CH]2Sn. Lappert used the same synthetic approach to synthesize the first diaminostannylene [(Me3Si)2N]2Sn in 1974.

In 1982, Wilhelm P. Neumann found the first convincing evidence of short-lived, smaller stannylene Me2Sn (i.e. transient stannylene) by investigating the thermolysis of cyclic (Me2Sn)6.

Persistent stannylene
Most alkyl stannylenes have been synthesized using tin(II)dihalide and organolithium reagent. For example, the first stannylene, [(Me3Si)2CH]2Sn, was synthesized using (Me3Si)2CHLi and SnCl2.

In some cases, stannylenes have been prepared by reduction of tin(IV) compound by KC8



Aminostannylene can also be synthesized by using tin(II)dihalide and organolithium reagent

Short-lived stannylene
The isolation of transient alkyl stannylene is more difficult. The first isolation of dimethylstannylene was believed to be done by thermolysing cyclostannane (Me2Sn)6, which was the product of the condensation of Me2Sn(NEt2)2 and Me2SnH2. The evidence came from the vibrational frequencies of dimethylstannylene identified by IR spectroscopy, which is consistent with the calculated value. The existence of this elusive SnMe2 was further confirmed by the discovery of visible light absorption matching the calculated electronic transition of SnMe2 in gas phase.

Another method to prepare short-lived stannylene is laser flash photolysis using tetraalkyltin(IV) compound (e.g. SnMe4) as a precursor. The generation of stannylene can be monitored by transient UV-VIS spectroscopy.

Structure and bonding
Stannylenes can be viewed as sp2-hybridized with vacant 5p orbital and a lone pair. This gives rise to their red color from n to p transition.

With specific type of ligands, the electron deficiency of monomeric stannylene is reduced by the agostic interaction from B-H bond. This concept was proved by Keith Izod and coworkers in 2006 when they synthesized the cyclic dialkylstannylene [{n-Pr2P(BH3)}(Me3Si)CCH2]2Sn. The crystal structure of the synthesized compound showed the arrangement of one B-H bond toward the Sn atom with the B—H--Sn bond distance of 2.03 Å. The mitigation of Sn electron deficiency was proved by the spectroscopic data, especially the 119Sn NMR spectra which showed the drastically low chemical shift (587 and 787 ppm comparing to 2323 ppm in analogous dialkylstannylene) indicating more electron density around Sn in this case.

Oligomerization
Small, unstable stannylenes (e.g. dimethylstannylene) undergo self-oligomerization yielding cyclic oligostannanes, which can be used as stannylene sources.

More bulky stannylenes (e.g. Lappert's stannylene), on the other hand, tend to form a dimer. The nature of the Sn-Sn bond in stannylene dimer is rather different from C-C bond in carbene dimer (i.e. alkene). As alkene develops a typical double bond character and the molecule has a planar geometry, stannylene dimer has a trans-bent geometry. The double bond in stannylene dimer can be considered as two donor-acceptor interactions. The electron localization function (ELF) analysis of stannylene dimer shows a disynaptic basin (electrons in bonding orbitals) on both Sn atom, indicating that the interaction between two Sn atom is two unusual bent dative bonds. Apart from that, the stability of stannylene dimer is also affected by the steric repulsion and dispersion attraction between bulky substituents.

Insertion reaction
Alkylstannylenes can react with various reagents (e.g. alkyl halides, enones, dienes) in an oxidative addition (or insertion) fashion. Moreover, By using EPR spectroscopy, the insertion reaction between stannylene and 9,10-phenanthrolinedione produce an EPR signal that was identified to be 9,10-phenanthrenedione radical anion, indicating that this reaction proceeds via radical mechanism.



Cycloaddition
Although stannylene is more stable than its lighter carbene analogs, it readily undergoes [2+4] cycloaddition reaction with Z-alkene. The seminal work was done by Wilhelm Neumann in which he showed that free stannylenes, such as (CH(SiMe3)2)2Sn, react with 2,7-diphenylocta-2,3,5,6-tetraene in a cheletropic fashion allowed by Woodward-Hoffmann rules.

Metal center for oxidative addition and reductive elimination
The seminal work by Andrey Protchenko and Simon Aldridge showed that, with the intrinsic redox property of Sn center (SnII/SnIV), stannylene with proper ligand can serve as a catalytic center like transition metals. They tuned the singlet-triplet energy separation, which is considered to have a strong effect on oxidative addition reactivity, by utilizing a very strong σ-donor boryl (-BX2) ligand. The results demonstrated that not only molecular hydrogen but also E-H bond (E = B, Si, O, N) can be oxidative added on Sn. In ammonia and water cases, the oxidative added product could also undergo reductive elimination, yielding O- or N-B bond formation.