Main Group Multiple Bonds for Bond Activations and Catalysis

Abstract Since the discovery that the so‐called “double‐bond” rule could be broken, the field of molecular main group multiple bonds has expanded rapidly. With the majority of homodiatomic double and triple bonds realised within the p‐block, along with many heterodiatomic combinations, this Minireview examines the reactivity of these compounds with a particular emphasis on small molecule activation. Furthermore, whilst their ability to act as transition metal mimics has been explored, their catalytic behaviour is somewhat limited. This Minireview aims to highlight the potential of these complexes towards catalytic application and their role as synthons in further functionalisations making them a versatile tool for the modern synthetic chemist.


Introduction
Molecular main group multiple bond chemistry has rapidly developeds ince the isolation of the first silicon-silicon double bond. West'sd isilene [1] broke the so called "double-bond" rule, in which it was thought that p-blocke lements with ap rincipal quantum number greater than two (i.e. aluminium onwards) would not form multiple bonds with themselves or other elements.S eminale xamples, all from 1981, reported by West (Si= Si), Yoshifuji (P=P), [2] Brook (Si=C), [3] and Becker( P C) [4] paved the way for this new field. Almost 40 years on, homodiatomic double bonds have now been isolated for all p-block elements in groups 13-15, rows 2-6. [5][6][7] Extension to homodiatomic triple bonds is complete for group 14, whilst only one clear example of ag roup 13 triple bond exists. [8] Further advances in heterodiatomic multiple bonding for p-block elements has yielded many new complexes, yet severalstill remain elusive.
Synthetic challenges in main group multiple bond chemistry have largely been overcome through choice of supporting ligand.C arefuld esign of sterically demanding ligands is required in order to provide sufficient kinetic stabilisation to the multiple bond ( Figure 1). If too small higher oligomers are obtained or if too large steric clash will prevent multiple bond formation.F or example, on increasing the steric demands of phenylt om esityl (mesityl = 2,4,6-trimethylphenyl) multiple bond formation is achieved (Compounds 1 [9] vs. 2, [1] Figure 1). Similarly,t he widely studied tetrameric pentamethylcyclopentadiene (Cp*) aluminium complex (3) [10] dissociates into its monomeric form at elevated temperatures. It was only recently,h owever,t hat am onomeric Cp-derived Al I species was isolated (4) [11] through increasing the steric demands of the substituents. It is of note, that no dimeric( i.e. multiple bond) structure has been observed for the aluminium Cp systems.
On descending the group the stabilityofthe lower oxidation state increases and thus its desire to partake in bond formation decreases.F or example in group 14, Sn II is more stable than Sn IV ,w hilst for the lightest congener C IV is more stable than C II .T his can also influence the complex formation in both the solutiona nd solid state as highlighted by Lappert's stannylene. The use of ab is(trimethylsilyl)methyl ligand (CH(SiMe 3 ) 2 )p rovides sufficient kinetic stabilisation to isolate a two-coordinate Sn II compound, however in the solid state this exists as ad imer yielding aS n =Sn multiple bond. [12,13] One of the reasons for the rapid development of main group multiple bonds is due to their ability to act as transition metal mimics. Owing to similarly energetically accessible frontier orbitals, main group multiple bonds have been shown to activate small molecules,s uch as dihydrogen,u nder ambient conditions ( Figure 2). [14,15] This often results in an oxidativea d-  dition reactiono ccurring at the main group centre. However, unlike transition metals,r eductive eliminationa tm ain group centres is more challenging due to the resulting high stability of the M(n + 2) oxidation state, particularly for the lighter,more earth abundant elements such as aluminium and silicon. This impedes the catalytic ability of main group metals in traditional redox based cycles as turnover is not possible. This also applies to main group chemistry in general, however alternative catalytic processes have been utilised which circumvent the change in oxidation state. These non-redox processes typically involveaseries of s-bond metathesis/insertion steps to enable turnover. [16] Multiple bonds offer an attractive toolf or the main group chemistd ue to the presence of am etal-metalb ond. Metalmetal bonds from across the periodict able bonds have enabled aseries of unique bond activations and catalysis, particularly in transition metal chemistry [17] and f-elements. [18] In transition metal chemistry,m etal-metal bonds have broad applications and have been found to play key roles in catalytic processes. [17] The use of transition metal multiple bonds in catalysis has allowed for retention of the dinuclear complex on addition of the substrate (Scheme 1A). They have also found roles as pre-catalysts,p roviding access to am onomeric low valent "active" species(Scheme1B).
Both above-mentioned steps within ac atalytic process could be envisioned for main group multiple bond chemistry, yet their use in catalysis is currently limited to just two examples. Ad igermyne, ag ermanium-germanium triple bond, which was used for the cyclotrimerisation of terminal alkynes [19] and ad ialumene, an aluminium-aluminium double bond, which was found to be an active pre-catalyst for the reductiono fc arbon dioxide. [20] These two examples showt he potentialf or this field to offer an alternative to expensive and often toxic transition metals that are currently used in industry. In addition to main group multiple bonds, low oxidation state and/or coordinate main group complexes have also undergone ar enaissance in recent years. With several new breakthroughs revealing new classes of compounds, such as nucleophilic aluminyls, [21,22] as well as advances in bond activations and catalysis. [23][24][25][26][27][28] The synthesis and bonding nature of main group mul-tiple bondsh ave been reviewed recently. [5,6,[29][30][31][32][33] Thism inireview,t herefore, focusses on the reactivity of these compounds highlighting the unique transformations that can be achieved due to molecular main group multiple bonds.

Group 13 Multiple Bonds E 13 -E 13 multiple bonds
Historically it wast hought that group 13 elements (E 13 )w ould preclude multiple bond formation.T he presence of only 3v alence electrons, as wella sw eak E 13 ÀE 13 bond energies, leads to ah igh tendency for decomposition and disproportionation reactions.O ne successful methodt oo vercome these challenges is to use Lewis bases to help stabilise the multiple bond, through donation of al one pair into the vacant E 13 p-orbital. Owingt ot heir easily tuneable steric and electronic properties, NHCs have ap rovent rack record in main group chemistry, [34] and have enabled the isolation of the first diborene (5), [35] diboryne (6) [8] andd ialumene (7) [7] (Figure 3). As expected for lighter elements these complexes exhibit planar geometries. On descending the group,t he stabilityo ft he lone pairi ncreases, and Lewis base stabilisation is no longer required. The three-coordinate trans-bent double bonds can be stabilised using sterically demanding terphenyl ligands( Compounds 8-10,F igure 3). [36][37][38] Recent reviewsh ave highlighted the different synthetic methodologies anda lternate ligand choices for the formationo ft hese electron precisem ultiple bonds, and as such will not be discussed herein. [29,39,40] In terms of their reactivity,h omonuclear E 13 bonds have found themselves to be efficient tools for small molecule activation. For the lighteste lement, the reactivity of diboryne compounds were found to be influenced by the p-acceptor ability of the supporting ligand. [41][42][43] CO coupling was observed for NHC-stabilised complex (6)h owever these were unable to activate dihydrogen (Scheme 2a). Switching to cyclic alkyl amino carbene( cAACs) ligands, which have increased pacceptora bilities relative to NHCs, enables diboryne (12)a ctivation of dihydrogen at room temperature (13)b ut only coordination of CO waso bserved (14,S cheme 2b). Additionally, NHC-stabilised diborenes are able to fixate CO 2 via [2+ +2]-cyclo-addition. The CO 2 fixated compound was found to be thermally unstable andr earranged at room temperature through CÀO cleavage and loss of the BÀBbond to form abridging carbonyl species. [44] In as imilarf ashion, the NHC-stabilised dialumene( 7)w as also found to fixate CO 2 . [20] In contrast to the diborene system, the CO 2 fixation product wasf ound to be stable and underwent further reactivity in the absence (carbonyl formation) or presence( carbonate formation)o fa dditional CO 2 .T he ability to access this carbonate species was found to be pivotali nt he catalytic reduction of CO 2 .C atalytic reduction could be achieved with the addition of pinacolb orane (HBpin). The mechanism for this was probedc omputationally (Scheme3). It was found that dialumene( 7)a cts as ap re-catalyst which forms carbonate 15 upon addition of CO 2 .R eduction by HBpin occurs at the exocyclic carbonyl to yield 15 a.C O 2 then inserts into the bottom side of the Al-Al line of centres.T he resultant eight-membered ring (15 b)c ollapses with releaseo ft he formic acide quivalent, regenerating 15 in the process. Whilst this catalytic cycle does not contain an Al-Al bond, the dinuclear complex remains intact due to bridging oxo and carbonate units and the ability of this system to alternate between re-ductiona nd insertion on the different sides of the Al-Al line of centres.
Very recently,asecond neutral NHC-stabilised dialumene (16)w as isolated. [45] This differed to compound 7,d ue to the use of an aryl supporting ligand,w hichr esulted in a trans-bent and twisted geometry.T he influence of the different ligands on the dialumene geometry was largelyt he result of the steric demands of the ligand.I nt erms of the electronics, the silyl ligand resultsinanalmost neutral Al 2 core whilst the aryl dialumene core is highly polarised, which can be attributed to the relative differences in electronegativities. Notably, this results in am uch more reactive dialumene and reactivity towards ste- Scheme2.Ligand controlled small molecule activationwith diborynes.
Scheme3.Proposed catalytic cycle for CO 2 reduction by dialumene (7). rically more demanding substrates is now possible due to the increased flexibility in the trans-bent and twisted structure. Furthermore, facile activation of dihydrogen is now achievable (previously 7 showed no reactivity), yielding both the cis and trans-isomers of a1 ,2-dihydro-dialumane (17,S cheme 4). The influence of ligand choice is further implicated in two different catalytic reactions, namely hydroboration of CO 2 and amineborane dehydrocoupling. The aryl-stabilised dialumene( 16)i s more catalytically active and results in different product distributions.T hus, indicating the likelihood of alternate mechanisms simply through change of supporting ligand.
Reactivity of heavier E 13 multiple bonds( Ga-Tl) [36][37][38] are limited to af ew examples. Digallene (8)i sc apable of activating dihydrogen and ammonia [46] whilst dithallene (10)readily dissociates to its monomeric form in hydrocarbon solutionsa nd therefore acts as aL ewis base in the formation of donor-acceptor complexes. [38] Further reactivity studies of digallene show that whilst it can dissociate to its monomeric speciesi ti s in fact the double bond that is responsible for the observed reactivity. [47][48][49] The chemistry of homonuclear E 13 multiple bonds is still in its infancy,w ith the potentialf or these systems far from fully realised. Boron is capable of forming stable triple bonds with itself, whilst aluminium [50] and gallium [51] have been isolated as anionic species, thus making their true bond order challenging to define. Extension to heteronuclear multiple bondingi sa lso limited within E 13 .C urrently,t here is one example of aE 13 -E 13' multiple bond, af ew examples of E 13 -E 14 and E 13 -E 15 multiple bondsa nd several E 13 -E 16 multiple bonds. Again, this is due to difficulties within the synthesis and stabilisation of these inherently reactive species.

E 13 -E 13' multiple bonds
The first, and currently only,g roup E 13 -E 13' complex which contains multiple bond character,w as reported by Braunschweig and co-workers( Compound 18,S cheme 5). [52] The resultant cAAC stabilised B-Al bond is best described as a3 -centre-2electron p-bond as DFT analysisf ound the HOMO to contain conjugation across the Al-B-cAAC unit. This bondings ituation had been previously observed in relatedc AAC stabilised BÀCO or BÀN 2 complexes. [53,54] 18 was also found to be efficient in small molecule activation as reaction of 18 with CO 2 resulted in CÀOa nd BÀAl cleavage to form ab oryleneC Oc omplex and an aluminoxane,c ompounds 19 and 20,r espectively (Scheme 5).

E 13 -E 14 multiple bonds
Examples of group E 13 -E 14 multiple bonds are also limited to a handfulo fe xamples and as such the reactivity of these compounds is largely unknown. Attempts to isolateb orasilenes, that is compoundsc ontaining B=Si doubleb onds, have been achieved through matrix isolation techniques. [55] To date, only one neutral borasilene (21) [56] ando ne Lewis base stabilised borasilene (25) [57] exist in the condensed phase. Reactivity studies of 21 towards chalcogens revealed the formationo f three-membered rings with sulfur (22)a nd selenium ( 23), whilst with oxygen (24)afour-membered ring, with loss of the BÀSi bond, was found (Scheme6a). [58] The bondings ituation in the Lewis base stabilised borasilene (25), based on experimental solid-state structuralf eatures and DFT calculations, suggest 25 is best described as az witterionic double bond in contrast to borasilene 21.Aseries of resonance structures can be drawn (Scheme 6b)w ith 25 A ,wherein the positive charge is located on the borona tom, representing the major resonance form. Attempts to use compound 25 for small molecule activation revealed no reactivity towards dihydrogen and an ill-defined mixturew ith CO 2 .H owever,B ÀSi cleavage was observed on addition of HBpin, to yield BH 2 and Si(Bpin) 2 containing species. [57] E 13 -E 15 multiple bonds E 13 -E 15 (pnictogen) multiple bonds are of interest to both academia and industry due to their interesting materials properties. Boron nitrides are widely used in the ceramics industry due to their high thermal and chemical stability, [59] whilst AlN, GaN, InN have interesting electronic properties. [60] Attempts to isolated iscrete M=NR complexes has found limited success.
Due to the necessity of sterically demanding ligands, the M= NR moiety is kinetically protected and therefore the reactivity of these compounds is somewhat impeded. To date, amongst the structurallyc haracterisede xamples of E 13 imides (E 13 =NR, E 13 =Al, [61][62][63] Ga, [64][65][66] and In [66,67] )o nly af ew have reported further reactivity.B oth Al and In imides (compounds 28 and 29, respectively) were synthesised from the correspondingE 13 (I) nucleophiles (Al 26;I n27)o nr eaction with MesN 3 (Mes = 2,4,6-tri-methylphenyl) (Scheme 7). In terms of their reactivity, the indium analogue undergoes further reactivity with organic azides to yield four membered rings (Scheme7,c ompounds 30, 31). [67] Whilst the aluminium imide shows further reactivity with CO 2 via a[ 2 + +2]-cycloaddition to yield ac arbamate dianion (Scheme 7, compound 32). [62] In related work Aldridge, Goicoechea and co-workers also obtaineda nA li mide complex (34)f rom reaction of their aluminyl ion (33)w ith DippN 3 (Scheme 8). [63] The highly polar nature of the AlÀNb ond was highlighted through itsr eactivity with small molecules. Dihydrogen was found to add in a1 ,2fashion across the AlÀNb ond at elevated temperatures (80 8C) to yield an amido aluminium hydride complex (35). Whilst two molecules of CO were found to react with 34 to yield com-pound 36 which is the result of CÀOc leavage and CÀCb ond formation. [63] Descending furtherd own the pnictogen series,afew examples of Lewis acid or based stabilised B=Pb onds exist, as well as B=As bonds. [68][69][70][71] Reactivity of thesem ultiple bondsi s scarce, but they have shown that they can be used as reagents to access CÀC/PÀBi soteres (Scheme 9). [72] Compound 37 was found to dissociate at elevated temperatures to providet he phosphaborene (38)i ns olution,t his then undergoes [2+ +2]-cycloaddition with phenylacetylene to yield compound 39.T he ring opening reactionc an be promoted through use of Lewis acids and bases to yield compounds 40 and 41,r espectively. [72] Further reactivity of 38 showed that it could be used to access mixed main group element rings (Compounds 42 and 43, Scheme9). [73] E 13 -E 16 multiple bonds E 13 -E 16 (chalcogen) bonds are also of high interest due to their materials properties. For example, alumina has found widespreadu se in industry from heterogenous supportst om aterials and even cosmetics. [74] The inert natureo fa lumina arises from the large differences in electronegativities (Al 1.61, O3.44) which results in at hermally stable material with high electricalr esistance. The highly polarised bonds, however,a lso increaset he difficulty of isolating ad iscrete E 13 =E 16 multiple bond.A ss uch, additional Lewisa cid and base stabilisation is often required to stabiliset he terminalE 13 =E 16 bond (Figure 4).
The synthesis and isolation of E 13 -E 16 multiple bonds have been highlighted recently, [40,75] and as such only recent progress in terms of their reactivity will be discussed herein. There have been significantly more reports of boron multiple bonds to chalcogenst han any other E 13 elements, including the only example of E 13 E 16 triple bond. [76] Aldridge and co-workers, recently reported the isolation of anionic oxoborane (44), [77] which is stabilised akin to type E in Figure 4. This compound can undergo p-bond metathesis with CS 2 to yield the related anionic thioxoborane. Furthermore, 44 was shown to act as an oxygen transfer agent (Scheme 10). Utilising as imilar approach Scheme7.Reactivity of E 13 -imides towards CO 2 (Al only) and organic azides (In only).
Chem.E ur.J.2021, 27,[1941][1942][1943][1944][1945][1946][1947][1948][1949][1950][1951][1952][1953][1954] www.chemeurj.org 2020 The Authors. Published by Wiley-VCH GmbH to Betrand, where the isoelectronic phosphinonitrene can act as an itrogen transfer agent, [78] 1,3-di-p-tolylcarbodiimide was added to 44 to yield 45.A ddition of oxalyl chloride released the functionalised cyclic urea derivativew ith concomitantf ormation of the boron-chloride species (46). Compound 44 could then be regenerated in astep wise manner through conversion to the boronic acid species (47), followedb yd eprotonation in the presence of aK -sequestering cryptandl igand (Scheme 10). [77] As imilara pproacht ot he stabilisation of AlÀOb onds was undertaken (Figure 4, type E). Again startingf rom the corresponding anionic Al I nucleophiles( 26 and 33), reaction with 1equiv.o fN 2 Or esults in the formation of monoalumoxane anions (Scheme 11, 48 [79] and 49 [80] ). These are thought to contain some multiple bond characterh owever,i ti sd ominated by the anionic resonancef orm. [79] In the presence of additional N 2 O, five membered heterocyclesa re formed (50 and 51). The reactivity of 48 towards CO was examined to provide some insighti nto formation of an ethenetetrathiolate species, which was obtained from the reaction of 26 with CS 2 . [81] It was postulated that use of 48 would allow for isolation of intermediates due to the increased AlÀObond strengthinc omparison to AlÀS( bond dissociation energies (BDE): 501 kJ mol À1 vs. 332 kJ mol À1 ,r espectively). Addition of CO to 48 resulted in CÀ Cb ond formation in the form of the analogous ethenetetraolate ligand (54,S cheme 12). Mechanistic insights were provided by DFT calculations and highlightedt he importance of the potassiumc ounterion in this reaction, as severals tabilising interactions from K + were found in this transformation. [81] Reactivity studies with the heavier aluminium chalcogenides is limited. Coles reported the selenium analogue to 48,w hich was synthesised in the presence of ac ryptand ligand to sequestert he cation (55,S cheme 13 a), [82] whilst in the absence of the cryptandapolymeric AlÀSe species was obtained. Addition of as econd equivalent of selenium to 55 affords ap lanar three-membered AlSe 2 ring (56). Evidence for the high degree of polarity in E 13 -E 16 bonds is shown by the Lewis base stabilised Al = Te complex (57). [83] This monotopiccompound readily dimerises to 58,w ithl oss of one Lewis base, on mild heating  chalcogenide multiple bonds, [84][85][86][87] no onwards reactivity of these compounds has been reported. Heavier E 13 -E 16 organometallicc ontaining complexes,h owever,f ind themselves to be useful singles ource precursors for materials applications. [60] Group 14 Multiple Bonds E 14 -E 14

multiple bonds
Of the main group multiple bonds, E 14 elements are the most widely studied. For the lightest E 14 member,c arbon, its multiple bonds with itself and other elements account for the majority of known multiple bondsi ne xistence, both those occurring naturally ands ynthetically. [88] In contrast, heavier E 14 multiple bonds have only come to fruition in the last 40 years, starting with West's disilene. [1] Since then ap lethora of E 14 =E 14 double bonds have been isolated and been the subject of numerous reviews. [5,6,29,[89][90][91][92] With their 4v alence electrons, triple bond formation is much more facile in comparison to E 13 and disilynes( 59), [93] digermynes (60), [94] distannynes (61) [95] and diplumblynes (62) [96] have all been isolated ( Figure 5). Here, the lone pair effect on trans-bent geometries is clearly observed as alkynes are linear whilst diplumbynes bear RÀPbÀPb angles of nearly 908,a nd are therefore better described as diplumbylenes with af ormal singleb onda nd active lone pairs on each lead centre. [96] Homodiatomic E 14 multiple bonds have been shown to react with an umber of substrates including small molecules. [97] It was the latter that first drew the comparisons to transition metals, [14][15] as digermyne showeda mbient temperature reactivity towards dihydrogen. [98] Even thoughd isilenes are arguably the most studied E 14 multiple bond, it was only recently that dihydrogen activation was achieved. [99] Ahighly trans-bent and twisted disilene (63)w as isolated,w hich is stabilised by sterically demanding N-heterocyclicimine (NHI) ligandsand hypersilyl( hypersilyl = Si(SiMe 3 ) 3 )g roups. Disilene 63 contains a long Si-Si doubleb ond (2.3134(7) ,a verage % 2.22 ), so can be best described as aw eakd ouble donor-acceptor bond. On reactionw ithd ihydrogen (1 bar) complete loss of the characteristic purple colour of 63 was observed within 10 minutes, this resulted in the formation of 1,2-disilane (64, Scheme14a). [99] Notably, 64 is the result of anti-addition,i n contrastt oa lkene hydrogenation where syn-additioni sf avoured. This experimental observation was explained through computational analysis as the staggered ligand arrangement reduces the stability,w hilst pre-organising the central Si=Si bond for concerted anti-addition. [99] Disilene 63 has also been shownt or eactw ith other small molecules such as NH 3 ,C O 2 and O 2 . [100] Further examinationo fd isilene dihydrogen activation was reported by Iwamoto. [101,102] This found that choice of stabilising ligand was key, as a p-accepting boryl group was required in order to achieve cleavage of dihydrogen (65 and 66). When the boryl group was replaced with an alkyl substituent (iPr) no reactivityw as observed. The rate of reaction could be further enhanced by use of ap ush-pulld isilene with a p-donating amino substituent (65,Scheme 14 b). [101,102] Other notable recent advances in disilene chemistry has focussed on the disilene-silylsilylene equilibrium (R 2 Si= SiR 2 $R 3 Si=SiR). This equilibrium has been previously inferred to explain unexpected reactionp roducts and thermally in-duced rearrangements.F or example, Inoue and co-workers reported the formation of at etrasilyldisilene (69)w hich was provent oe xist as the disilene in solution but largely reacted as as ilyl silylsilylene (69',S cheme 15). [103] On leaving as olution of 69 at room temperature, CÀHa ctivation of the tBu group of bis(silyl)silylene (69')i sobtainedt oy ield 70.[ 2 + +1]-cycloaddition of ethylene occurs from 69',r ather than [2+ +2]-cycloaddition of 69,t of orm 71.D ihydrogen activation was possible due to the ability to access 69',w hereas addition of NH 3 occurs at the disilene (69)t oy ield the hydroaminated species 73 (Scheme15). [103] Cowley and co-workers found direct evidencef or this disilene-silylsilylene equilibria using ab ase-coordination strategy. 4-pyrollidinopyridine( 4-PPy) allowed for isolation of disilene 74.4 -PPy was found to be labile and in the presence of excess NHCl igand, isomerisation to silylsilene 76 was achieved (Scheme 16). [104] This observation of 74 and 76 serves as direct evidencef or the transient nature of 75 and 75',w hich is also supported by computational studies. [104] The ability to control this equilibrium provides anew route to access to the more reactive silylsilylenes pecies. Two-coordinate acyclic silylenes are highly reactives pecies as they contain av acant coordination site and al one pair,a nd as such have shown facile bond activations towards small molecules and av ariety of substrates. [24,28,[105][106][107] It could be envisaged that new catalytic cycles, based on the ability to control this equilibrium, could be achieved in as omewhat similar fashion to that depicted in Scheme 1B.W herein the multiple bond (disilene) is off cycle and the "active" species is the low valent main group centre (silylsilylene).
The first example of am ain group multiple bond being employed in catalysis was reported by Sasamori and co-workers.
Digermyne 77 was found to be active in the catalytic trimerisation of ar ange of phenylacetylenes to yield regioselective 1,2,4-triarylbenzenes( Scheme17). [19] This reactioni ss pecific to terminal arylacetylenes, as only stoichiometric reactions were observed with other unsaturated CÀCb onds. [108][109] The key to enablingturnover with arylacetylenes is the proposed equilibrium that exists between compounds 79 and 80.C alculated energyb arriers suggest that this is accessible at the higher temperatures (60 8C) at which the catalysis is performed. This equilibrium allows for accesst ot he germole-germylenes pecies (80)w hich contains the low valent Ge II centrew itha vacant coordination site for furthers ubstrate binding.T he regioselectivity of this reactioni sl ikely determined through the subsequent[ 1 + +2]-cycloaddition and intramolecular[ 4 + +2]-cycloaddition steps before release of the product andr egeneration of digermene 78.I ti sp roposed that digermyne 77 serves as ap re-catalyst to this transformation with 78 as the resting state. [19] Advances in reactivity for the heaviest E 14 -E 14 multiple bonds, that is, Sn and Pb, is somewhat hampered by their weak E 14 -E 14 bonds and therefore tendency to dissociate in solution. [92,110] Recent studies by Power and co-workers showed the reversibility of distannyne-stannylene in toluene solutions [111] as well as the reversibility of dihydrogen activation by distannynes. [112] Despite diplumbylenes being the first example of aheavier E 14 -E 14 triple bond, it was only recentlyt hat further examples emerged. [113] Ac ombined experimentala nd theoretical study found that London dispersion forces [31] were important in the stabilisation of diplumbylenes. Those which are less trans-bent contain increased multiple bond character compared with their more trans-bent counterparts (i.e. closer to 908). [113] E 14 ÀE 14' multiple bonds Since Brookf irst reported the isolation of as ilene, [3] several exampleso fm etallaalkenes (R 2 E 14 =CR 2 ,F igure 6) have been re-ported. [5,6,[114][115][116][117] The reactivity of these metallaalkenesh ave largely focussed on cycloadditionreactions of carbonyls and alkynes, where they have been found to follow the Woodward-Hoffman selectivityr ules. [118] Other E 14 =Cc ontaining species are metallavinylidenes( :E 14 =CR 2 ,F igure 6) where the terminal E 14 contains both an empty p-orbital and al one pair.I ng eneral, these compounds are reactive intermediates and require Lewis base stabilisation, however they have been shown to be ambiphillic in nature. [119] Extension towards E 14 -C triple bonds have also been achieved, both Ge [120] and Sn [121] derivatives were evidenced as transient species upon photolysis of their corresponding diazomethanes.U sing as imilars trategy,K ato and Baceiredo were able to isolate ab ase-stabilised silyne (82,S cheme 18). [122] This compound is stable up to À30 8C, however,a bove this temperature it undergoes a1 ,2-migration of the supporting ligand to form ap hosphaalkene (83,S cheme 18). Furthermore, the carbenic character of 82 was shown via trapping with tert-butyl isocyanide to form aketeneimine (84,Scheme 18). [122] The heavier mixed alkenes (i.e. E 14 =E 14' ,w here E=Si, Ge, Sn) are proposed to have similar p-bond strengths to their corresponding homo-diatomic multiple bonds (e.g. Ge=Ge and Ge= Si are similar). [123] Whilstt he synthesis and reactivity of metallaalkenes (E 14 =C) is rather well established, the heavier analogues are rare in comparison. [6,114] An example of as ilastannene (85) was found to react according to the polarity of the double bond with phenolsa nd thiophenols (Scheme 19 a). [124] Whilst the reactiono fd ioxygen with germastannene (88)r esults in coordination and formation of a4 -membered ring (89,Scheme19b). [125] Other examples of mixed E 14 multiple bonds are found within small inorganic ring systems [126] and even as heteroallenes (R 2 E 14 =E 14 =E 14 R 2 ). [127][128] Although the latter complexes may be better described as containing az erovalent central atom which is supported by coordinated tetrylenes (i.e. R 2 E 14 ! E 14 ! E 14 R 2 ). [32,129] E 14 -E 15 multiple bonds It was not until 1981 that Beckerr eported the first example of ah eavier E 14 ÀE 15 bond (CP, phosphaalkyne). [4] Since this report, many examples of phosphaalkenes have emerged and, as such, are beyond the scope of this article. [130,131] Additionally, arange of silaimines have been isolated, which showincreased reactivity compared to their imine counterparts. Their reactivity is also well documented and therefore will not be discussed herein. [132][133][134][135][136] Heavier imine analogues (Ge, Sn) [6,137,138] have also been isolated although are much rarer in contrastt os ilaimines.Arecent study by Fulton and co-workerss howedt hat reactivity of ag ermanimine (90)e xhibits" metalloid" type behaviour.T he reactivity of 90 resembles that of transition metal imido complexes on reaction with heterocumulenes ([2+ +2]-cycloaddition to form compounds 91 and 92,S cheme 20 a), and that of imines as Diels-Alder reactivity was also observed ([4+ +2]-cycloaddition, compounds 93-96,Scheme 20 b). [139] Heavier combinations of E 14 ÀE 15 are also known,o fw hich silaphosphines are the most common and have recently been reviewed. [140] Examples of Ge and Sn-phosphorous complexes are rare, but ar ecent example by Inoue and co-workers showed that heavier nitrile analogues have interesting properties. [141] Use of N-heterocyclic phosphinidene( NHCPs) ligands allowedf or the isolation of compounds 97 and 98 (Scheme 21). These compounds contain short E 14 ÀPb onds and were additionally shown through DFT calculations to contain some multiple bond character,d ue to the resulting resonance structures. Surprisingly,c ompounds 97 and 98 showedn or eactivity towards small molecules (H 2 ,C Oa nd CO 2 ), however interesting reversible reactivity towards diphenylketene was observed (Scheme 21). [2+ +2]-cycloaddition occurs at room tem-perature, however,o nh eating 99 (80 8C) and 100 (100 8C) retro-cycloaddition of the ketene was achieved. [141] This reversibilityp romptedt he examination of 97 and 98 in catalysis. Both compounds weref ound to be catalytically active in the hydroboration of aldehydesa nd ketones,w ith low loadings andf ast reaction times at room temperature observed. [141] This preliminary study highlights the potentialf or these heavierm ultiple bonds in catalysis. Particularly those of Ge and Sn whereint he stability of the lower oxidation state increasesc ompared to the lighter congeners. Other recent achievements within E 14 ÀE 15 multiple bonds have resulted in the first examples of as tibasilene [142] (Si-Sb double bond) and an arsagermene [143] (Ge-As double bond). Albeit, no reactivity has been reported. E 14 -E 16 multiple bonds E 14 -E 16 multiple bonds are probably the most widely studied,i f you consider carbonyl containing compounds and efforts in the use of CO 2 as aC 1f eedstock for commodity chemicals. [144] In contrast heavierc arbonyls are rare due to the high polarity of the resultingE 14 ÀE 16 bond. This, however,d oes have its advantagesa sp oly(siloxanes) (R 2 SiO) n have found widespread materialu se. It was not until the last decades that Kipping's dream [145] was realised and now severale xamples of silanones (i.e. compounds containing aS i =Od oubleb ond) have been isolated. [146] The highr eactivity and instability of Si=Ob onds was shown by Inoue and co-workers, in which the first acyclic three-coordinate silanones (101 and 102)w ere found to react with small molecules such as CO 2 and methanol. [147] The acyclic silanones (101 and 102)a re indefinitely stable in the solid state but in solutiont hey decompose readily (t 1/2 for 101 is 7h in C 6 D 6 ). Interestingly,m onitoring solutions of 101 and 102 revealed different migration pathways depending on the different substitution at the supporting silyl ligand (Scheme 22). In the case of SiMe 3 substituents (101)a1,3-silyl migration was observedt of orm an intermediate disilene complex (103), analogous to keto-enol isomerisation. However,u se of as uper silyl supporting ligand (tBu 3 Si, Compound 102)r esultsi nformation of at wo-coordinate N,O-silylene (104). The differing reactivity of the two complexes wass hown on reaction with ethylene (Scheme22). The intermediate disilene (103)u ndergoes [2+ +2]cycloaddition to form af our-membered ring (105)w hereas the silylene (104)f orms at hree-membered ring (106). [147] Further reactivity of 102 has shown the potential of multiple bonds in synthesis. One of the versatile reactions in the organic chemist's toolkit is the Wittig reaction, wherein carbonyl compounds are used to prepare alkenes. As such, it was shown by Inoue andc o-workers that analogous reactivity is in fact possible with heavierc arbonyls. [148] This sila-Wittig reactivity is shownt ob ep ossible with ar ange of non-stabilised ylides (Scheme 23) resulting in highs electivity towards the Z-silenes. This reactivity shows the similarities that can be found between carbon and silicon. Additionally,t his provides new synthetic routes to silaalkenes.
Whilst silanones are somewhat established, examples of silaldehydes remainr ared ue to their reduced kinetic stabilisation. Lewis acid and base stabilisation methods enabled isolation of silaaldehydes,b ut single bond characterw as observed due to the push-pull stabilisation. [149][150][151][152][153] Reports of the reactivity of such compounds are also scarce.H owever,t he Lewis acid-base stabilised silaaldehyde (107)r evealed its carbonyl like reactivity (Scheme 24), on its reaction with phosphine which afforded the thermally stable silaphosphene ( 108). [153] Heavier E 14 -O multiple bonds, that is, Ge, Sn, and Pb, are knownb ut are much rarer in comparison to the silicon analogues. [146] The first example of am onomeric germanone was reported by Ta mao and co-workers. It was found to undergo a Scheme21. Heavier nitrile reversible reactivity with diphenylketene.
Scheme22. Ligand dependant transformations of silanones and subsequent reactivity with ethylene.

Conclusions and Outlook
Main group multiple bonds have proven themselves to be a powerful tool in the modernm ain group chemist's toolkit. Whilst al arge variety and combinations of E 13 /E 16 multiple bonds now exist, the reactivity of these complexesh as only really begun to emergei nt he last decades. Thish as particularly been exemplified by the discoveryt hat main group multiple bonds contain transition metal like properties and are therefore capable of facile activation of strongb onds such as dihydrogen.O ne of the key factors for the development of this chemistry has been the correct choice of supporting ligands. Not only has this enabled the isolation of the multiple bond, but also has ad irect influenceo nt he reactivity.F or example, comparison of the recentlyi solated dialumenes( Al=Al double bond) movingf rom as ilyl-based ligand to ana ryl ligand now enablesd ihydrogen activation. This is also observed with Braunschweig's diborynes (BBt riple bond) with the difference in the ligands controlling the reactivity towards CO and H 2 .
Main group multiple bonds have also shown more than just small molecule reactivity.T heir use in synthesis has also been highlighted enabling new routes to functionalised compounds. For example, the use of ab oron-oxygen double bond as an Otransfer reagent, as well as silicon-oxygen bonds in sila-Wittig reactivity,t he latter of which has enabled isolation of new silaethenes which also displayn ovel reactivity.
Finally, examples of multiple bondsi nc atalysis have begun to emerge. Digermynes and dialumenes have shown the stability of these dinuclears ystems is key to enabling turnover.I ti s anticipated that many more examples of catalytic application of main group multiple bonds will emerge over the next decade. With parallels being drawn to transition metals,c atalytic cycles such as those highlighted in Scheme 1w ill become more obtainable as furtheru nderstanding of the intrinsic nature of metal-metalb onds is realised.