Germylenes are a class of germanium(II) compounds with the general formula :GeR₂. They are heavier carbene analogs.^[1] However, unlike carbenes, whose ground state can be either singlet or triplet depending on the substituents, germylenes have exclusively a singlet ground state.^[2][3] Unprotected carbene analogs, including germylenes, has a dimerization nature.^[4] Free germylenes can be isolated under the stabilization of steric hindrance or electron donation.^[5] The synthesis of first stable free dialkyl germylene was reported by Jutzi, et al in 1991.^[6]

Bonding situation for germylene is distinctively different from that for its light analog carbene. The carbon atom from carbene is sp² hybridized. Although germylenes still have some sp² hybridization character, the larger energy gap between s and p-orbitals for germanium permits the retainment of 4s²4p² electron configuration to some degree. The bond angle for H₂Ge and Me₂Ge was found to be: H-Ge-H 93° and C-Ge-C: 98°, which is smaller than 120°, the ideal bond angle for sp² hybridized structure and thus proves the 4s²4p² valence electron configuration nature of germylene.^[1][7][8] The lone pair of germylene tends to stay in the high-s-character orbital which is relatively inert, making germylene exclusively singlet.^[3]

Dimerization of germylenes lead to the formation of germylene dimers (R₂Ge=GeR₂).^[9] Digermylene dimers (as well as the higher-order digermynes) have a trans-bent structure quasi-tetrahedral at each germanium atom. Consequently the dimerization is believed to proceed through two donor-acceptor adducts instead of the triplet double-pairing found during carbene dimerization.^[3]

Dimerization of free germylenes does not have a noticeable energy barrier, which means that the dimerization reaction is almost spontaneous and diffusion limited, so the free germylene monomers without stabilization could dimerize or further polymerize once they form.^[4] Free germylenes have to be stabilized kinetically or thermodynamically due to their high reactivity originating from the vacant p-orbital. Thermodynamical stabilization of this p-orbital is usually realized by coordination of a pentamethylcyclopentadiene (Cp*) ligand or of nitrogen (N), oxygen (O) or phosphorus (P) containing ligands, which are able to donate electrons and thus deactivate the vacant p-orbitals.^[1] At the same time, stabilization can be accomplished by steric protection of bulky R groups like mesityl groups (Mes) to prevent nucleophiles from getting close to the germanium center.^[10]

Carbon substituents is different from other heteroatom N, O, P substituents which have lone pairs in that they provide less electronic perturbations.^[11] As a result, a stronger steric and electronic stabilization is required to guarantee a monomeric product. Carbon substituted germylenes can be synthesized using various methods: (1) reduction of dibromogermanes with reducing agents like lithium naphthalene (LiNp) or potassium graphite (KC₈), etc., (2) photolysis of strained cyclogermanes or Ge(IV) species, (3) substitution of a dihalo Ge(II) precursor species with nucleophiles like organometallic ligands (e.g. RLi/RMgBr).^[1]

The introduction of heteroatom in the ligand backbone enhances the stability of reactive Ge(II) center by electron donation from N lone pair to vacant p-orbitals of germanium center. Typically, the strategy for synthesizing five-membered N-heterocyclic tetrylene involves the reaction between N-substituted 1,4-diaza-1,3-butadiene, the alkali metal based reducing agents and group 14 halides.^[12] In the case of n-heterocyclic germylene (NHGe) synthesis, the method involves an initial reduction of N-substituted 1,4-diaza-1,3-butadiene by lithium. The following cyclization of the dianion with the corresponding Ge(II) halides gives the final product.^[12]

The cyclic(alkyl)(amino)carbenes (CAACs) has already been known as both a better donor and better acceptor than n-heterocyclic carbenes (NHCs) due to its higher highest occupied molecular orbital (HOMO) and lower lowest unoccupied molecular orbital (LUMO).^[13]

The synthetic strategy of CAAGe involves the synthesis of a α-β-unsaturated imine from a ketone and an amine via condensation followed by the treatment with GeCl₂·dioxane. The resulting product is then reduced with KC₈ to give CAAGe.^[14] Analogous to CAAC, the electrophilicity of the germanium center can be obviously enhanced by the substitution of a π-donating and σ-withdrawing amino group along with σ-donating trimethylsilyl groups.^[11]

In 2016, Muller et al reported the synthesis of a unique homoconjugation stabilized germylene in a relatively high yield by the reaction between hafnocene dichloride and dipotassium germacyclopentadienediide in THF at -80 °C.^[15][16] The product is stabilized by a remote interaction between a C=C double bond and vacant p-orbital of Ge center through homoconjugation.^[17] This stabilization strategy results in a special structural which possesses unusual reactivity.^[11]

The pincer based germylene is of great importance not only for their ability to stabilize transition metal species via chelation effects in homogeneous catalysis, but also for its serving as a good luminescence source.^[18][19] A PNHNHP ligand was used to synthesize the PGeP pincer stabilized germylene by treatment with two equivalents of potassium hexamethyldisilazide (KHMDS) and GeCl₂·dioxane, which finally leads to the formation of the PGeP pincer compound.^[20]

Dimerization of carbon substituted germylenes gives R₂Ge=GeR₂ dimers which could further polymerize to form polygermanes (R₂Ge)_n compounds.^[5] The dimer could show a certain stability if prepared in an independent way.^[21] Bulkier substituents are able to reduce the polymerization rate by steric effect.^[22] More steric hindrance could even stop the polymerization or dimerization reactions and renders a germylene thermodynamically stable.^[23][24][25]

R₂Ge insertion into C-C bonds has not been reported so far.^[5] However, going down the group 14, C-E (E = Si, Ge, Sn, Pb) bonds become more accessible for R₂Ge insertion.^[5] The strained C-Ge bonds allow insertion of germylene to 7,7-dialkyl-7-germanorbornadienes in the melt, forming digermabicy-clooctadienes.^[26]

C-H bonds are generally unreactive toward germylene insertion.^[27] However, strain release may help to overcome the activation energy barrier.^[25]

Insertion to carbon-halide bonds is common for germylene. The mechanism for insertion of free Me₂Ge into the C-Br bond of benzyl bromide was reported to be a two-step, radical abstraction-recombination process under thermal and photolytical conditions.^[28][29] An identical mechanism through a caged singlet radical pair was proposed for C-Cl bond insertion.^[29] However, the interaction between halogen electrons and empty p-orbital of the germylene may result in the formation of a donor-acceptor complex before occurrence of any of the insertion mechanisms.^[5]

The insertion into the C-Hal bond in alkyne compounds go by a one-step mechanism under thermal or photolytical conditions.^[30]

For C-O, the R₂Ge insertion product could only remain stable at a very low temperature.^[31]

Addition reaction of Me₂Ge to unsaturated systems is well studied. As mentioned above, dimerization and polymerization of Me₂Ge does not have a noticeable activation energy barrier and is only controlled by diffusion. As a result, addition reactions should be rapid enough complete before getting polygermanes as products.

There is no reaction between simple alkenes and free germylenes.^[5] However, styrenes and α-substituted styrenes are able to react with Me₂Ge. The resulting product is a 1:1 mixture of the syn and anti-isomers of 3,4-diphenyl-3,4-R-1,1-dimethyl-1-germacyclopentane.^[32]

A variety of 1,2-substituted-vinylgermyl compounds can be synthesized in both high yield and high regioselectivity by addition of germylene to alkynes.^[33]

1,4-Cycloaddition of conjugated (hetero-)dienes by free germylenes gives the corresponding 5-membered ring.^[34][35][36]

Germylenes reacts only with one of the strained double bonds in cumulated systems like allenes (C=C=C).^[5] Germylenes prefer to react with more electron-deficient allenes.^[5]

During complexation with donors, the germylenes stay in the singlet ground state, where the lone pair is placed in the high-s-character orbital, while the heteroatom-containing donors like R₂O, ROH, R₂S, R₃P, R₃N and RCl interact with the vacant p-orbital at germanium center, which could stabilize the singlet germylene and prevent further polymerization. Most of the complexes are stable in room temperature.^[37] The absorption bands of adducts commonly exhibits shorter wavelengths in comparison to those of the free germylenes due to substituent-influenced n-p transitions at the Ge center.^[31]

Germylenes could also act as catalysts as transition metals do.^[11] Oxidative addition and reductive elimination, along with the related Mⁿ⁺/M⁽ⁿ⁺²⁾⁺ redox couples are of great significance to the transition metal catalysis.^[38] Due to the electronic structure and chemical properties of germylenes, including the empty p-orbital, germylenes are able to activated small molecules and give the corresponding Ge(IV) complexes, which raised researchers' interests in germylenes' acting as spectator ligands in certain catalytic cycles.^[39] However, subsequent regeneration of Ge(II) compound through reductive elimination is not thermodynamically favored for germylenes.^[11] The key of germylene catalysis chemistry is to maintain a balance between oxidative addition and reductive elimination.^[40] One example of germylene catalyzed reaction is hydroboration of CO₂, where a preliminary hydrogermylation of CO₂ step is followed by the formation of methanol derivatives with 3 equivalent of catecholborane to regenerate the germylene compound.^[41]

• Carbene analogs
• Silylene
• Stannylene