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Germylenes are compounds containing a divalent germanium atom (Fig. 1). Early studies of the synthesis and properties of germylenes were primarily concerned with dihalogermylenes GeX2 (X=Cl, Br, I). More recently, the chemistry of divalent species bearing Ge-H, Ge-O, Ge-S, Ge-P, Ge-N, etc., bonds has been developed. Germylenes can be regarded as heavy carbene analog ues, however their singlet-triplet gap has been detected to be much larger (14 kcal/mol (SCF) for Me2Ge), leading to almost all of their reactions to occur in the singlet state. Akin to carbenes, insertions of germylenes into σ-bonds are very common but more selective, while intramolecular insertions with the formation of germenes (R2Ge=CR2) are usually not observed. Germylenes undergo addition to many unsaturated systems with a preference of 1,4-addition in reactions with conjugated multiple bonds, unlike carbenes or silylenes where 1,2-additions dominate.

The diverse chemical behaviour of germylenes is attributed to the ambiphilic character of Ge. Due to the lone pair of valence electrons on the Ge atom, germylenes possess a strong reducing ability, while the vacant 4p orbital on Ge accounts for their intrinsically high electrophilicity. Although these species are weaker Lewis acids than their silylene homologues, GeMe2 as a Lewis acid is both stronger and softer than trimethylborane.

From 7,7-disubstituted-7-germanorbornadienes
7,7-disubstituted-7-germabenzonorbornadienes (1) are stable crystalline compounds which can be prepared from the corresponding tetraphenyl germoles (synthesized as shown on Scheme 1a ) and dehydrobenzene (Scheme 1b). 7-dimethylgerma-1,2,3,4-tetraphenyl-5,6-benzo-norbornadiene was first synthesized in 1980 and since then the method has been expanded to R = Et, Bu, Ph, etc. R2Ge is generated by a first-order cycloreversion of 1, triggered by UV light or heating.

By photolytic cleavage of Ge-Si, Ge-N, and Ge-Ge bonds
Diaryl bissilylgermanium compounds have been used to generate diaryl germylenes since 1985. The reaction occurs under UV radiation (λmax = 320 nm for Ar = Ph) and is thermodynamically driven by the formation of the Si-Si bond, which is more stable than the Ge-Si bond.

Aromatic groups proved to be essential for the efficient absorption of UV radiation to initiate the process, and no reaction was observed upon radiation of the dimethyl derivative of bissilylgermanium.

Formation of Me2Ge requires a much greater release of Gibbs free energy which can be achieved upon photolytic cleavage of dimethylgermanium diazide:

Germylenes can also be generated by photolytic splitting of Ge-Ge bonds from cyclotrigermane (4) or digermirane (5):



Dehalogenation - cleavage of Ge-Hal bonds
Traditionally, germylenes have been synthesized by dehalogenation of a germanium precursor upon addition of alkali metals or alloys (Scheme 3), or aryllithium species. Similarly, α-dehydrohalogenation of R2GeHHal promoted by amines (Et3N, pyridine) giving R2Ge has also been reported. Mechanistically, the participation of germyl anions as reactive intermediates is often assumed in such processes, and a stepwise mechanism of germylene generation tends to be favoured over concerted pathways when both leaving groups have a strong tendency of ion formation.



Oligomerization, polymerization, and co-polymerization
Under standard conditions, polymerization of most germylenes is very rapid and effectively diffusion-controlled ($$k\approx10^9$$M-1s-1), yielding polygermanes (R2Ge)n. Digermene (R2Ge=GeR2) and cyclogermanes have been found amongst other products in certain cases. Bulky substituents hinder the polymerization process: while R = 2,6-Et2-Ph stops the reaction at the germene dimer R2Ge=GeR2, monomeric germylene R2Ge where R = 2,4,6-tBu3-Ph has been isolated in solid state and is stable at -10 °C.

The reducing power of germylenes can promote their reactions with various oxidative co-monomers in redox co-polymerizations. Reaction of ((TMS)2N)2Ge with p-benzoquinone completes within 1 hour at -78 °C giving a 1:1 alternating copolymer with MW > 106. This reaction proceeds via a biradical mechanism, as shown by ESR spectroscopy and trapping experiments. Co-polymerizations of germylenes with various partners, including α,β-unsaturated ketones and alkynes, have been developed and described.

Insertion into σ-bonds
Examples of insertions into H-H, C-H, C-Hal, O-H, and M-C (where M = Al, Ge, etc.) bonds are known, however insertions into C-C bonds have not been reported.

The first examples of hydrogen activation with germylenes was reported by Power in 2009, where (2,6-Mes2C6H3)2Ge reacted with H2 at 60-70 °C. Curiously, the same reaction did not proceed with the corresponding stannylene (2,6-Mes2C6H3)2Sn. Such a difference in reactivity could be rationalized by analyzing the the proposed frontier molecular orbital interactions behind this process, as depicted on Scheme 4. The approach of H2 to Ge results in a two-way interaction, where the H-H bond is weakened via both the interaction of the bonding σ-orbital of H2 with the empty 4p orbital on Ge and the lone pair on Ge donating electron density into the σ*(H-H). Consequently, the lack of reactivity of the stannylene could be attributed to the larger energy separation between the lone-pair orbital and the empty p orbital of the stannylene.

C-H bonds are generally stable towards germylenes. These reactions can be promoted by strain release, and one of the very few examples of such a process is shown on Scheme 5.