Organogermanium Analogues of Alkenes, Alkynes, 1,3-Dienes, Allenes, and Vinylidenes

In this review, the latest achievements in the field of multiply bonded organogermanium derivatives, mostly reported within the last two decades, are presented. The isolable Ge-containing analogues of alkenes, alkynes, 1,3-dienes, allenes, and vinylidenes are discussed, and for each class of unsaturated organogermanium compounds, the most representative examples are given. The synthetic approaches toward homonuclear multiply bonded combinations solely consisting of germanium atoms, and their heteronuclear variants containing germanium and other group 14 elements, both acyclic and cyclic, are discussed. The peculiar structural features and nonclassical bonding nature of the abovementioned compounds are discussed based on their spectroscopic and structural characteristics, in particular their crystallographic parameters (double bond length, trans-bending at the doubly bonded centers, and twisting about the double bond). The prospects for the practical use of the title compounds in synthetic and catalytic fields are also briefly discussed.


Introduction
One of the most fundamental topics in modern organogermanium chemistry is the study of low-coordinate species, and within this realm, the field of multiply bonded compounds is now one of the mainstreams. It therefore comes as no surprise that the literature covering the latter field is vast. In this review, we limit our discussion to unsaturated organogermanium compounds (i.e., heavier analogues of alkenes, alkynes, 1,3-dienes, allenes, and vinylidenes) reported in the literature, mostly from 2000 to the present date, considering only combinations between the group 14 elements of the types >Ge=E< and -Ge≡E-(E = group 14 element). Heteronuclear multiply bonded combinations of germanium with the Main Group elements of groups 13, 15, and 16, >Ge=E 13 -, >Ge=E 15 -, and >Ge=E 16 , respectively, are excluded from our consideration. Likewise, aromatic organogermanium compounds (such as germabenzene, germanaphthalene, etc.), as well as germylene and germylyne transition metal complexes with Ge=M and Ge≡M bonds (M = transition metal), are also outside of the scope of this review and are not discussed. Moreover, numerous compounds, in which the low-coordinate Ge center is intramolecularly (by n-donor substituents) or intermolecularly (through external donor ligands) coordinated, thus experiencing remarkable electronic perturbation, are also not considered, except for silagermenylidenes >Si=Ge(NHC): and digermanium(0) complexes :Ge 0 (NHC/or NHSi)=Ge 0 (NHC/or NHSi):, which otherwise cannot be stabilized for their isolation.
For each class of unsaturated organogermanium compounds, the discussion starts with a brief introduction of the first stable representatives going on to consider their most important examples reported from 2000 to date. For a more comprehensive reading on the topic of multiply bonded organogermanium compounds, we refer interested readers to the previously published reviews [1][2][3][4][5][6][7].
The peculiar structure and bonding nature of the doubly and triply bonded derivatives of the heavy group 14 elements that are distinctly different from those of their organic the topic of multiply bonded organogermanium compounds, we refer interested readers to the previously published reviews [1][2][3][4][5][6][7].
The peculiar structure and bonding nature of the doubly and triply bonded derivatives of the heavy group 14 elements that are distinctly different from those of their organic counterparts (that is, alkenes and alkynes) have been the subject of numerous comprehensive theoretical and experimental treatments described in several reviews [1][2][3][4][5][6][7]. Thus, in contrast to organic alkenes C=C and alkynes C≡C, which are typically planar and linear, respectively, their heavy tetrel analogues of the type E=E and E≡E (E = Si-Pb) are not planar and linear. Thus, for example, for the heavy alkene analogues E=E, there are two fundamental structural deformations leading to a departure from planarity: transbending at the doubly bonded E centers and twisting about the E=E bond. However, if twisting of the double bond generally results from the steric hindrance caused by the presence of exceptionally bulky substituents, the origin of the trans-bending and, thus, pyramidalization at E centers is best rationalized in terms of the MO interaction approach. Several qualitative bonding models have been proposed to account for the geometrical distortions in alkene analogues of the heavy group 14 elements. The first one, proposed by Lappert and coworkers, rationalized the trans-bending in E=E as a result of double donor-acceptor interaction of two singlet fragments [E↑↓], as opposed to the covalent interaction of the two triplet fragments [↑C↑], resulting in the formation of planar alkene C=C ( Figure 1) [8]. Alternatively, the geometrical deviation from planarity (in alkenes) to trans-bent structures (in heavy alkene analogues) can be treated as a manifestation of a second-order Jahn-Teller effect in the form of the interaction of MOs having the same symmetry [1,4,5,9]. Specifically, this is mixing of HOMO(πE=E) and LUMO+1(σ*E=E) orbitals, stabilizing the HOMO and destabilizing the LUMO+1 and thus generating nonbonding electron density at E, which finally results in the observed trans-bending caused by the electronic repulsion ( Figure 2). This π-σ* orbital interaction is facilitated for Ge-containing heavy alkene analogues, because the energy difference (inversely proportional to the extent of a second-order Jahn-Teller interaction) between the interacting orbitals is lower for the heavier tetrels due to their progressively decreasing double-bond strength. Route A (photolysis of hexaarylcyclotrigermanes) is mostly of a historical importance as the method employed by Masamune and coworkers for preparation of the first stable tetra(aryl)digermenes Ar 2 Ge=GeAr 2 [13]. However, because of the synthetic limitations of this approach, which requires cyclotrogermane precursors that are not readily available, currently this method is not commonly used. Since 2000, there was only one report from the Baines group on an improved synthetic procedure for the tetra(mesityl)digermene Mes 2 Ge=GeMes 2 2 that was generated by photolysis of hexa(mesityl)cyclotrigermane precursor in THF at −70 • C [14].
In comparison to organic alkenes >C=C<, structural deformations of the double bond in digermenes >Ge=Ge< (stretching, trans-bending, and twisting) are even more pronounced compared to those of the corresponding disilenes >Si=Si<. The extent of these structural distortions in digermenes follows some general tendencies: electronegative substituents provoke notable elongation and weakening of the Ge=Ge bond, as well as trans-bending at the doubly bonded Ge centers, whereas electropositive substituents cause shortening and strengthening of the Ge=Ge bond and planarization at the doubly bonded Ge centers. Accordingly, the shortest Ge=Ge bond [r Ge=Ge = 2.2576(5) Å] was found in the di(aryl)di(silyl)digermene 20, whereas the longest one [r Ge=Ge = 2.5087(7) Å] was detected in the di(aryl)di(bromo)digermene 6, with the exceptionally bulky Bbt substituents (Table 1). Moreover, in line with what was mentioned above, tetra(silyl)digermene 9 has planar (the least trans-bent) geometry at its sp 2 -Ge centers (θ = 0.3 • ), whereas the greatest transbending was observed in the di(aryl)digermene 12 (θ = 45.0 • ). Twisting in digermenes is controlled by the substituent effect, to range from non-twisted Ge=Ge double bonds (τ = 0.0 • in tetra(alkyl)digermene 1 and di(aryl)di(silyl)digermene 20) to extraordinarily twisted (τ = 52.8 • in tetra(silyl)digermene 14 with very bulky silyl substituents). Table 1. X-ray crystallographic parameters of the stable acyclic digermenes (reported after 2000). The"-" implies the lack of the relevant structural information.  Table 2). Unlike their acyclic congeners, all of these cyclic digermenes are synthesized by special methods which are not outlined in Scheme 2.

Three-Membered Ring Compounds
There are currently 14 heavy cyclopropene analogues incorporating a Ge=Ge double bond into the three-membered ring: 12 homonuclear cyclotrigermenes cyclo-[Ge3] and 2 heteronuclear 1H-siladigermirenes cyclo-[Si-Ge=Ge] ( Table 2). Unlike their acyclic congeners, all of these cyclic digermenes are synthesized by special methods which are not outlined in Scheme 2.
In due course, the solid-state thermolysis of the tetra(silyl)digermene ( t Bu 2 MeSi) 2 Ge=Ge(SiMe t Bu 2 ) 2 14 27 (170 • C, 1 h, evacuated sealed tube) was found to be an attractive alternative to the above-described preparation of 1H-trigermirene 25b, improving the isolated yield of the latter up to 48% [47].
The alkyl-substituted heavy cyclopropene analogues 26a,b (a: E = Si; b: E = Ge), representing the nearest homologues of the above-described 1H-siladigermirene 25a and 1Htrigermirene 25b, being distinguished from them by only one CH 2 -unit, were prepared by Lee, Sekiguchi, and coworkers by the reductive dehalogenation of 1,3-di(chloro)cyclobutane derivatives (Scheme 9) [48]. The overall process was proposed to proceed via the transient bicyclo[1.1.0]butane derivatives, rapidly isomerizing at room temperature to the more stable heavy cyclopropenes 26a,b.
In due course, the solid-state thermolysis of the tetra(silyl)digermene ( t Bu2MeSi)2Ge=Ge(SiMe t Bu2)2 14 27 (170 °C, 1 h, evacuated sealed tube) was found to be an attractive alternative to the above-described preparation of 1H-trigermirene 25b, improving the isolated yield of the latter up to 48% [47]. The alkyl-substituted heavy cyclopropene analogues 26a,b (a: E = Si; b: E = Ge), representing the nearest homologues of the above-described 1H-siladigermirene 25a and 1H-trigermirene 25b, being distinguished from them by only one CH2-unit, were prepared by Lee, Sekiguchi, and coworkers by the reductive dehalogenation of 1,3di(chloro)cyclobutane derivatives (Scheme 9) [48]. The overall process was proposed to proceed via the transient bicyclo[1.1.0]butane derivatives, rapidly isomerizing at room temperature to the more stable heavy cyclopropenes 26a,b. Scheme 9. Synthesis of the alkyl-substituted heavy cyclopropene analogues 26a,b. Scheme 9. Synthesis of the alkyl-substituted heavy cyclopropene analogues 26a,b.
The very first compound of this type, namely disiladigermetene 27, was reported by Lee, Sekiguchi, and coworkers, formed by the unexpected ring expansion of either 3H-or 1H-disilagermirenes [49] with GeCl 2 •diox (Scheme 10 and Table 2) [50]. The Ge=Ge double bond in 27 [r Ge=Ge = 2.2911(4) Å] is one of the longest reported for the cyclic digermenes, whereas the endocyclic Si-Ge bonds are sizably shortened, and the exocyclic Si-Cl bonds are elongated. This was explained by the important extent of π Ge=Ge -σ* Si-Cl negative hyperconjugation promoted by the presence of electronegative chlorine atoms and folding of the Si 2 Ge 2 -ring (folding angle = 28.3 • ). The geometry around the sp 2 -Ge atoms in 27 is only slightly pyramidal: Σ Ge = 357.2 • /358.6 • .
The very first compound of this type, namely disiladigermetene 27, was reported by Lee, Sekiguchi, and coworkers, formed by the unexpected ring expansion of either 3H-or 1H-disilagermirenes [49] with GeCl2•diox (Scheme 10 and Table 2) [50]. The Ge=Ge double bond in 27 [rGe=Ge = 2.2911(4) Å] is one of the longest reported for the cyclic digermenes, whereas the endocyclic Si-Ge bonds are sizably shortened, and the exocyclic Si-Cl bonds are elongated. This was explained by the important extent of πGe=Ge-σ*Si-Cl negative hyperconjugation promoted by the presence of electronegative chlorine atoms and folding of the Si2Ge2-ring (folding angle = 28.3°). The geometry around the sp 2 -Ge atoms in 27 is only slightly pyramidal: Σ Ge = 357.2°/358.6°. Likewise, Lee, Sekiguchi, and coworkers found that the similar reaction of 1Htrigermirene 25b with the GeCl2•diox yielded the first (and still the only known) homonuclear heavy cyclobutene analogue, tetragermetene 28 (Scheme 11 and Table 2) [51]. Structurally, tetragermetene 28 is similar to the above-described disiladigermetene  Weidenbruch and coworkers developed an alternative approach toward heavy cyclobutene analogues by reacting tetra(germa)buta-1,3-diene Tip2Ge=Ge(Tip)-Ge(Tip)=GeTip2 with 2-methoxyphenyl isocyanide to produce 1,2,3-trigermet-1-ene 29, featuring a germanium-germanium double bond within the Ge3C-skeleton (Scheme 12 and Table 2) [52]. The Ge=Ge bond in 29 was marginally shorter that those in bond in 27 [rGe=Ge = 2.2911(4) Å] is one of the longest reported for the cyclic digermenes, whereas the endocyclic Si-Ge bonds are sizably shortened, and the exocyclic Si-Cl bonds are elongated. This was explained by the important extent of πGe=Ge-σ*Si-Cl negative hyperconjugation promoted by the presence of electronegative chlorine atoms and folding of the Si2Ge2-ring (folding angle = 28.3°). The geometry around the sp 2 -Ge atoms in 27 is only slightly pyramidal: Σ Ge = 357.2°/358.6°. Likewise, Lee, Sekiguchi, and coworkers found that the similar reaction of 1Htrigermirene 25b with the GeCl2•diox yielded the first (and still the only known) homonuclear heavy cyclobutene analogue, tetragermetene 28 (Scheme 11 and Table 2) [51]. Structurally, tetragermetene 28 is similar to the above-described disiladigermetene Weidenbruch and coworkers developed an alternative approach toward heavy cyclobutene analogues by reacting tetra(germa)buta-1,3-diene Tip2Ge=Ge(Tip)-Ge(Tip)=GeTip2 with 2-methoxyphenyl isocyanide to produce 1,2,3-trigermet-1-ene 29, featuring a germanium-germanium double bond within the Ge3C-skeleton (Scheme 12 and Table 2) [52]. The Ge=Ge bond in 29 was marginally shorter that those in Scheme 11. Synthesis of tetragermetene 28. The four-membered ring 1,2-digermet-1-ene 30 with an endocyclic Ge=Ge double bond was prepared by Sasamori, Tokitoh, and coworkers by [2 + 2]   Except for the above-described neutral heavy cyclobutene analogues 27-30, there are several four-membered ring compounds where the Ge=Ge double bond is a part of the tri(germa)allylic system (cationic, radical, or anionic). Thus, Weidenbruch and coworkers reported the tetra(germa)cyclobutenyl anion 31 unexpectedly produced by exhaustive reduction of Tip2Ge=GeTip2 with excess of lithium (Scheme 14) [54]. In 31, the Ge4-ring is practically planar (sum of the internal bond angles = 360°), and within the tri(germa)allylic system, the Ge-Ge bonds length of 2.3679(6) Å is just in-between those of the typical Ge-Ge single and Ge=Ge double bonds. The four-membered ring 1,2-digermet-1-ene 30 with an endocyclic Ge=Ge double bond was prepared by Sasamori, Tokitoh, and coworkers by [2 + 2]  The four-membered ring 1,2-digermet-1-ene 30 with an endocyclic Ge=Ge double bond was prepared by Sasamori, Tokitoh, and coworkers by [2 + 2]  Except for the above-described neutral heavy cyclobutene analogues 27-30, there are several four-membered ring compounds where the Ge=Ge double bond is a part of the tri(germa)allylic system (cationic, radical, or anionic). Thus, Weidenbruch and coworkers reported the tetra(germa)cyclobutenyl anion 31 unexpectedly produced by exhaustive reduction of Tip2Ge=GeTip2 with excess of lithium (Scheme 14) [54]. In 31, the Ge4-ring is practically planar (sum of the internal bond angles = 360°), and within the tri(germa)allylic system, the Ge-Ge bonds length of 2.3679(6) Å is just in-between those of the typical Ge-Ge single and Ge=Ge double bonds. Except for the above-described neutral heavy cyclobutene analogues 27-30, there are several four-membered ring compounds where the Ge=Ge double bond is a part of the tri(germa)allylic system (cationic, radical, or anionic). Thus, Weidenbruch and coworkers reported the tetra(germa)cyclobutenyl anion 31 unexpectedly produced by exhaustive reduction of Tip 2 Ge=GeTip 2 with excess of lithium (Scheme 14) [54]. In 31, the Ge 4 -ring is practically planar (sum of the internal bond angles = 360 • ), and within the tri(germa)allylic system, the Ge-Ge bonds length of 2.3679(6) Å is just in-between those of the typical Ge-Ge single and Ge=Ge double bonds. several four-membered ring compounds where the Ge=Ge double bond is a part of the tri(germa)allylic system (cationic, radical, or anionic). Thus, Weidenbruch and coworkers reported the tetra(germa)cyclobutenyl anion 31 unexpectedly produced by exhaustive reduction of Tip2Ge=GeTip2 with excess of lithium (Scheme 14) [54]. In 31, the Ge4-ring is practically planar (sum of the internal bond angles = 360°), and within the tri(germa)allylic system, the Ge-Ge bonds length of 2.3679(6) Å is just in-between those of the typical Ge-Ge single and Ge=Ge double bonds.  [56]. In 34, the SiGe3 fourmembered ring is strongly folded (folding angle 40.4°), thus enabling Ge2•••Ge3 throughspace orbital interaction manifested in the short transannular Ge2-Ge3 distance of only 2.9346(3) Å. Overall, the structural peculiarities of 34 (ring folding and short transannular distance) testify to the important extent of its homoaromaticity. Accordingly, 34 is to be classified as a germanium analogue of the cyclobutenylium ion, i.e., a homotri(germa)cyclopropenylium ion. The homoaromaticity of 34 was further confirmed by the calculation of the nucleus-independent chemical shift (NICS) at 1 Å above the Ge3-ring center, which was diagnostically negative (−17.3) as a manifestation of the diatropic ring current. The "homoaromatization energy" of 34, calculated as the barrier to inversion of the Ge3Si-ring (through the planar allylic-type cationic transition state lacking homoaromatic stabilization), was exceedingly low, i.e., only 3.7 kcal/mol [56]. In accordance with its homoaromaticity, 34 showed practically no alteration in the Ge-Ge bond lengths of its Ge3-fragment [rGe=Ge Upon the one-electron reduction of 34 with KC8, a free-radical species, namely silatri(germa)cyclobutenyl radical 35, was cleanly formed (Scheme 16) [56]. The Most recently, Lee, Sekiguchi, and coworkers synthesized silatri(germa)cyclobutenylium ion derivative 34 by the oxidative demethylation of the cyclotrigermene 25b [45] with [Et 3 Si] + ·[B(C 6 F 5 ) 4 ] − (Scheme 16) [56]. In 34, the SiGe 3 four-membered ring is strongly folded (folding angle 40.4 • ), thus enabling Ge2···Ge3 through-space orbital interaction manifested in the short transannular Ge2-Ge3 distance of only 2.9346(3) Å. Overall, the structural peculiarities of 34 (ring folding and short transannular distance) testify to the important extent of its homoaromaticity. Accordingly, 34 is to be classified as a germanium analogue of the cyclobutenylium ion, i.e., a homo-tri(germa)cyclopropenylium ion. The homoaromaticity of 34 was further confirmed by the calculation of the nucleus-independent chemical shift (NICS) at 1 Å above the Ge 3 -ring center, which was diagnostically negative (−17.3) as a manifestation of the diatropic ring current. The "homoaromatization energy" of 34, calculated as the barrier to inversion of the Ge 3 Si-ring (through the planar allylictype cationic transition state lacking homoaromatic stabilization), was exceedingly low, i.e., only 3.7 kcal/mol [56]. In accordance with its homoaromaticity, 34 showed practically no alteration in the Ge-Ge bond lengths of its Ge 3 -fragment [r Ge=Ge = 2.3327(3) Å (Ge1-Ge2) and 2.3400(3) Å (Ge1-Ge3)], and it showed essentially planar geometry at all skeletal Ge atoms [Σ Ge = 360.0 • (Ge1), 358.7 • (Ge2), and 357.5 Upon the one-electron reduction of 34 with KC 8 , a free-radical species, namely silatri(germa)cyclobutenyl radical 35, was cleanly formed (Scheme 16) [56]. The homoaromaticity of 34 is completely lost upon its reduction, which was seen in the remarkable flattening of the SiGe 3 -ring in 35 (folding angle was reduced from 40.4 • in the starting 34 to only 6.9 • in the resulting 35) and great elongation of the Ge2···Ge3 transannular distance  Table 2. X-ray crystallographic parameters of the stable cyclic digermenes. The "-" implies lack of the relevant structural information.  [56]. In 34, the SiGe3 fourmembered ring is strongly folded (folding angle 40.4°), thus enabling Ge2•••Ge3 throughspace orbital interaction manifested in the short transannular Ge2-Ge3 distance of only 2.9346(3) Å. Overall, the structural peculiarities of 34 (ring folding and short transannular distance) testify to the important extent of its homoaromaticity. Accordingly, 34 is to be classified as a germanium analogue of the cyclobutenylium ion, i.e., a homotri(germa)cyclopropenylium ion. The homoaromaticity of 34 was further confirmed by the calculation of the nucleus-independent chemical shift (NICS) at 1 Å above the Ge3-ring center, which was diagnostically negative (−17.3) as a manifestation of the diatropic ring current. The "homoaromatization energy" of 34, calculated as the barrier to inversion of the Ge3Si-ring (through the planar allylic-type cationic transition state lacking homoaromatic stabilization), was exceedingly low, i.e., only 3.7 kcal/mol [56]. In accordance with its homoaromaticity, 34 showed practically no alteration in the Ge-Ge bond lengths of its Ge3-fragment [rGe=Ge Upon the one-electron reduction of 34 with KC8, a free-radical species, namely silatri(germa)cyclobutenyl radical 35, was cleanly formed (Scheme 16) [56]. The homoaromaticity of 34 is completely lost upon its reduction, which was seen in the remarkable flattening of the SiGe3-ring in 35 (folding angle was reduced from 40.4° in the starting 34 to only 6.9° in the resulting 35) and great elongation of the Ge2•••Ge3 transannular distance [3.3315(4) Å in 35, or 14% elongation compared with 34]. Thus, 35 is to be classified as the allylic free radical, featuring the unpaired electron delocalized Scheme 16. Synthesis of silatri(germa)cyclobutenylium ion derivative 34.

Six-Membered Ring Compounds
Only one "heavy cyclohexene" derivative has been reported to date. Marschner and coworkers prepared this tetracyclic digermene 38 with an endocyclic Ge=Ge double bond bridging polycyclic scaffold (Scheme 19) [58]. Digermene 38 was available via the synthetic strategy applied for the preparation of the above-described bicyclic digermene Scheme 17. Synthesis of heavy cyclopentene analogues 36a,b with endocyclic Ge=Ge double bonds.
central Ge nucleus: a( 73 Ge2) [or a( 73 Ge3)] = 1.54 mT [or 1.44 mT] vs. a( 73 Ge1) = 0.59 mT. This observation agrees well with the allylic radical formulation of 35, in which the odd electron is mostly localized at the Ge2 and Ge3 termini. Given that the small values of the 73 Ge hfcc in 35 imply the location of its unpaired electron in the orbital of π-symmetry, 35 should be categorized as a π-radical. The allylic free radical 35 can be compared with the cyclotrigermenyl radical 33 prepared by Power and coworkers by the stoichiometric reduction of the aryl(chloro)germylene Ar(Cl)Ge: [Ar = 2,6-(2,4,6-Me3-C6H2)2-C6H3] with KC8 (Scheme 15) [55]. In the cyclotrigermenyl radical 33, the average Ge-Ge bond distance within the Ge3-ring of 2.35 (7)

Six-Membered Ring Compounds
Only one "heavy cyclohexene" derivative has been reported to date. Marschner and coworkers prepared this tetracyclic digermene 38 with an endocyclic Ge=Ge double bond bridging polycyclic scaffold (Scheme 19) [58]. Digermene 38 was available via the synthetic strategy applied for the preparation of the above-described bicyclic digermene Scheme 18. Synthesis of bicyclic digermene 37.

Six-Membered Ring Compounds
Only one "heavy cyclohexene" derivative has been reported to date. Marschner and coworkers prepared this tetracyclic digermene 38 with an endocyclic Ge=Ge double bond bridging polycyclic scaffold (Scheme 19) [58]. Digermene 38 was available via the synthetic strategy applied for the preparation of the above-described bicyclic digermene 37, namely by the reaction of 1,4-di(potassio)cyclohexasilane with GeBr 2 ·dioxane in the presence of

Germastannenes >Ge=Sn<
The first reported transient germastannene, [Mes 2 Ge=SnTip 2 ], was generated via the dehydrofluorination of Mes 2 (H)Ge-Sn(F)Tip 2 by t BuLi by Escudié and coworkers [80]. Decomposing at room temperature, this germastannene was proved to be as such on the basis of its low-field 119  The only currently known cyclic germastannene, 3 Δ-1,2,3,4-disilagermastannetene 64, featuring an endocyclic Ge=Sn double bond incorporated into the four-membered ring skeleton, was synthesized by Lee, Sekiguchi and coworkers in 2004 [83]. Moreover, 64 was readily available by the ring expansion of either 3H-or 1H-disilagermirenes [49] with SnCl2•diox (Scheme 30). As is typical for germastannenes, the doubly bonded Sn atom was diagnostically deshielded [δ( 119 Sn) = 439.3 ppm]. In a sharp contrast to the acyclic tetra(aryl)germastannenes Mes2Ge=SnTip2 [80] and Tip2Ge=SnTip2 61 [81], cyclic tetra(silyl)germastannene 64 was indefinitely stable both in the solid state and in solution, showing no signs of dissociation of its >Ge=Sn< double bond into the germylene >Ge: and stannylene >Sn:. The unexpected high thermal stability of 64 was assigned to the influence of its σ-donating silyl substituents, further enhanced by the proposed πGe=Sn→σ*Si-Cl orbital mixing lowering the πGe=Sn-orbital energy level and thus stabilizing the HOMO of the molecule [83]. Since 2004, no stable germastannenes >Ge=Sn< were reported.

Homonuclear Derivatives
Digermynes -Ge≡Ge- The first digermyne Ar′-Ge≡Ge-Ar′ [Ar′ = 2,6-(2,6i Pr2-C6H3)2-C6H3] 65 was synthesized and structurally characterized in 2002 by Power and coworkers via the reduction of the (chloro)germylene Ar′(Cl)Ge: with potassium [84]. The CAr′-Ge-Ge-CAr′ core in digermyne 65 was planar and trans-bent with the Ge-Ge-CAr′ bond angle of 128.7°. The Ge-Ge bond was rather short [rGe≡Ge = 2.2850(6) Å], which is indicative of its considerable multiply bonded character. On the other hand, accumulation of the lone pair electron density at the Ge atoms results in the trans-bending of their substituents, and consequently in the decrease (compared to the ideal triple bonding) of the Ge-Ge bond order and bond strength.

Homonuclear Derivatives
Digermynes -Ge≡Ge- The first digermyne Ar′-Ge≡Ge-Ar′ [Ar′ = 2,6-(2,6i Pr2-C6H3)2-C6H3] 65 was synthesized and structurally characterized in 2002 by Power and coworkers via the reduction of the (chloro)germylene Ar′(Cl)Ge: with potassium [84]. The CAr′-Ge-Ge-CAr′ core in digermyne 65 was planar and trans-bent with the Ge-Ge-CAr′ bond angle of 128.7°. The Ge-Ge bond was rather short [rGe≡Ge = 2.2850(6) Å], which is indicative of its considerable multiply bonded character. On the other hand, accumulation of the lone pair electron density at the Ge atoms results in the trans-bending of their substituents, and consequently in the decrease (compared to the ideal triple bonding) of the Ge-Ge bond order and bond strength.

Homonuclear Derivatives
Digermynes -Ge≡Ge- The first digermyne Ar -Ge≡Ge-Ar [Ar = 2,6-(2,6-i Pr 2 -C 6 H 3 ) 2 -C 6 H 3 ] 65 was synthesized and structurally characterized in 2002 by Power and coworkers via the reduction of the (chloro)germylene Ar (Cl)Ge: with potassium [84]. The C Ar -Ge-Ge-C Ar core in digermyne 65 was planar and trans-bent with the Ge-Ge-C Ar bond angle of 128.7 • . The Ge-Ge bond was rather short [r Ge≡Ge = 2.2850(6) Å], which is indicative of its considerable multiply bonded character. On the other hand, accumulation of the lone pair electron density at the Ge atoms results in the trans-bending of their substituents, and consequently in the decrease (compared to the ideal triple bonding) of the Ge-Ge bond order and bond strength.

Heteronuclear Derivatives
Germynes -Ge≡C-Heteronuclear alkyne analogues of the type -Ge≡E-(E = C, Si, Sn, Pb), featuring a triple bond between germanium and different group 14 element, were always among the top challenges for experimental pursuits. However, despite numerous research efforts, such compounds (as stable derivatives) have not been isolated to date, although germynes R-Ge≡C-R were most closely approached, both computationally and experimentally.
Theoretical studies by Su and a coworker predicted that the germynes can be synthetically accessible, if appropriately substituted with bulky groups to prevent isomerization and oligomerization of highly reactive germynes [89].

Germaallenes >Ge=C=C<
Only a couple of isolable 1-germaallenes >Ge=C=C< are currently known. The first one, Tip 2 Ge=C=C( t Bu)Ph 83, was reported by West and coworkers synthesized by the reaction of Tip 2 Ge(F)-C≡C-Ph with t BuLi via the transient [Tip 2 Ge(F)-C(Li)=C( t Bu)Ph], which eliminated LiF to finally yield 5 [107]. In solution, 83 decomposed at room temperature within 15 h. In 83, as in other heteroallenes, the Ge=C and C=C bonds were short [r Ge=C = 1.783(2) Å and r C=C = 1.314(2) Å], the Ge=C=C fragment was bent (159.2 • ), the geometry at the Ge atom was pyramidal (Σ Ge = 348.4 • ), and the central C atom was greatly deshielded [δ( 13 C) = +235.1 ppm].
The second 1-germaallene, Tbt(Mes)Ge=C=CR 2 (CR 2 = fluorenylidene) 84, was prepared by the reductive dechlorination of Tbt(Mes)ClGe-C(Cl)=CR 2 with t BuLi [108]. Like in 83, the central allenic carbon of 84 resonated in the diagnostic low-field at +243.6 ppm. Without trapping reagents, 84 underwent slow intramolecular cyclization via C-H activation of one of the (Me 3 Si) 2 CH groups by the Ge=C bond to form a benzogermacyclobutene derivative.

Heavy Analogues of Vinylidenes
Vinylidenes R 2 C=C:, with substituent-free terminal carbon atoms, are the valence isomers of alkynes RC≡CR. They are exceptionally reactive, and for their stabilization, external bases, such as NHC, are particularly effective. Accordingly, all-except for Aldridge's di(germa)vinylidene (vide infra)-isolable heavy group 14 analogues of vinylidenes R 2 E=E : (E, E = heavy group 14 elements) were stabilized by NHC-coordination.

Prospects for the Use of Unsaturated Organogermanium Compounds
Organogermanium compounds featuring multiple bonds to germanium are highly promising candidates for their potential use in synthetic organic and organometallic chemistry, as well as in the materials science field, due to the presence of highly reactive germanium-containing double and triple bonds. Although there is still much to be achieved in this direction, some prominent examples of the synthetic utility of unsaturated organogermanium compounds have been reported to date, mostly for digermenes and digermynes, as the most abundant representatives of the class.
Among the most important examples of digermynes reactivity, one can mention their activation of small molecules (H 2 , NH 3 , CO, CO 2 , N 2 O, etc.), which typically can be activated under the transition metal catalysis conditions. Thus, in 2005, Power and coworkers reported facile room temperature/atmospheric pressure activation of dihydrogen by digermyne Ar Ge≡GeAr [Ar = 2,6-(2,6-i Pr 2 -C 6 H 3 ) 2 -C 6 H 3 ] 65 as the first example of H 2 addition to an unsaturated main group compound under mild non-catalytic conditions (Scheme 42) [25]. This reaction, producing a mixture of doubly bonded digermene Ar HGe=GeHAr 12, singly bonded digermane Ar H 2 Ge-GeH 2 Ar , and primary germane Ar GeH 3 , depending on the amount of H 2 gas used, is likely enabled by the highly reactive singlet biradical contribution of the germanium-germanium bonding.

Prospects for the Use of Unsaturated Organogermanium Compounds
Organogermanium compounds featuring multiple bonds to germanium are highly promising candidates for their potential use in synthetic organic and organometallic chemistry, as well as in the materials science field, due to the presence of highly reactive germanium-containing double and triple bonds. Although there is still much to be achieved in this direction, some prominent examples of the synthetic utility of unsaturated organogermanium compounds have been reported to date, mostly for digermenes and digermynes, as the most abundant representatives of the class.
An example of the catalytic application of a digermyne was recently reported by Sasamori and coworkers. By applying catalytic amounts (4 mol%) of their di(aryl)digermyne Tbb-Ge≡Ge-Tbb (Tbb = 2,6-[(Me 3 Si) 2 CH] 2 -4-t Bu-C 6 H 2 ) 72 [19], the authors achieved a high-yield regioselective cyclotrimerization of terminal arylacetylenes Ar-C≡CH, exclusively forming 1,2,4-triarylbenzenes (Scheme 43) [117]. Digermyne 72 acts as a highly efficient pre-catalyst of the process [turnover number (TON) = 15 -35], which can be regarded as the transition metal-free variation of the Reppe reaction that is catalyzed by the main group element derivative instead of by transition metals (Mo, Co). The mechanism of the whole catalytic transformation was backed by control experiments and accompanying computations, which revealed initial reaction of digermyne 72 with two equivalents of arylacetylene generating 3,5-diaryl-1,2-digermabenzene intermediate, followed by its reaction with another equivalent of arylacetylene forming 1,6-digermatricyclo[3.3.0.0 2,6 ]octa-3,7diene as a key resting state for the true catalytic species (Scheme 43). Readily switching between Ge II and Ge IV oxidation states, mimicking the fundamentally important switching between oxidation states in the transition metal catalytic processes, finally enables smooth cyclotrimerization of arylacetylenes.
Molecules 2023, 28, x FOR PEER REVIEW 30 of 36 important switching between oxidation states in the transition metal catalytic processes, finally enables smooth cyclotrimerization of arylacetylenes.

Conclusions
The field of compounds with multiple bonds to germanium, i.e., organogermanium analogues of alkenes, alkynes, 1,3-dienes, allenes, and vinylidenes, established by the synthesis of the first stable digermene, Dis2Ge=GeDis2, by Lappert and coworkers in 1976, is thriving. Currently, a huge variety of the above-described unsaturated organogermanium derivatives have been prepared and, in the majority of cases, structurally characterized. These compounds are no longer short-lived reaction intermediates; instead, they are commonly available preparative-scale reagents indispensable for the synthesis of organogermanium derivatives unavailable by any other synthetic route. The fundamentally important issue of the particular structure and nature of Ge=E double and Ge≡E triple bonds (E = group 14 element), as compared with those in the prototypical organic alkenes C=C and alkynes C≡C, was comprehensively addressed (both experimentally and theoretically) to show the principal distinctions in their bonding types: classical covalent in the planar alkenes C=C and alkynes C≡C vs. nonclassical donor-acceptor in the trans-bent organogermanium analogues Ge=E and Ge≡E. For future developments in the field, one can expect the successful design of still unknown types of multiple (both double and triple) bonding between Ge and the Main Group elements, such as R-Ge≡E-R′ (E = Si, Sn, Pb), which are among the most desirable candidates for experimental pursuits. Among the practical applications of unsaturated organogermanium compounds in material science, one can mention the potential use of germaalkenes >Ge=E< and germaalkynes -Ge≡E-(E = group 14 element) as novel ligands for transition metal complexes of the next Scheme 44. The acyclic diene metathesis (ADMET) polymerization of p-phenylene-bridged tetragermadiene precursor 21 forming ADMET-poly(digermene) 91.

Conclusions
The field of compounds with multiple bonds to germanium, i.e., organogermanium analogues of alkenes, alkynes, 1,3-dienes, allenes, and vinylidenes, established by the synthesis of the first stable digermene, Dis 2 Ge=GeDis 2 , by Lappert and coworkers in 1976, is thriving. Currently, a huge variety of the above-described unsaturated organogermanium derivatives have been prepared and, in the majority of cases, structurally characterized. These compounds are no longer short-lived reaction intermediates; instead, they are commonly available preparative-scale reagents indispensable for the synthesis of organogermanium derivatives unavailable by any other synthetic route. The fundamentally important issue of the particular structure and nature of Ge=E double and Ge≡E triple bonds (E = group 14 element), as compared with those in the prototypical organic alkenes C=C and alkynes C≡C, was comprehensively addressed (both experimentally and theoretically) to show the principal distinctions in their bonding types: classical covalent in the planar alkenes C=C and alkynes C≡C vs. nonclassical donor-acceptor in the trans-bent organogermanium analogues Ge=E and Ge≡E. For future developments in the field, one can expect the successful design of still unknown types of multiple (both double and triple) bonding between Ge and the Main Group elements, such as R-Ge≡E-R (E = Si, Sn, Pb), which are among the most desirable candidates for experimental pursuits. Among the practical applications of unsaturated organogermanium compounds in material science, one can mention the potential use of germaalkenes >Ge=E< and germaalkynes -Ge≡E-(E = group 14 element) as novel ligands for transition metal complexes of the next generation, as well as the development of the germanium variation of alkene metathesis for the fabrication of Ge-containing polymers with predesigned properties.

Conflicts of Interest:
The authors declare no conflict of interest.