Stannylene

Stannylenes (R₂Sn:) are a class of organotin(II) compounds that are analogues of carbene. Unlike carbene, which usually has a triplet ground state, stannylenes have a singlet ground state since valence orbitals of tin (Sn) have less tendency to form hybrid orbitals and thus the electrons in 5s orbital are still paired up. Free stannylenes are stabilized by steric protection. Adducts with Lewis bases are also known.

The first persistent stannylene, [(Me₃Si)₂CH]₂Sn, was reported by Michael F. Lappert in 1973. Lappert used the same synthetic approach to synthesize the first diamidostannylene [(Me₃Si)₂N]₂Sn in 1974.

The short-lived, transient stannylene Me₂Sn has been generated by thermolysis of cyclo-(Me₂Sn)₆.

Most alkyl stannylenes have been synthesized by alkylation of tin(II) dihalides with organolithium reagents. 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 a tin(IV) compound by KC₈

Amidostannylene can also be synthesized by using a tin(II) dihalide and the lithium amide.

The isolation of a 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 infrared 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.