Nitrogen ligands are, however, frequently utilized to stabilize metallylenes, as exemplified by the pioneering syntheses of 'Lappert's germylene and stannylene', which have two N(SiMe3)2 groups. Schultz and coworkers reported the oxidative addition of diethyldichalcogenanes Et2Ch2 (Ch = S, Se, Te) to 'Lappert's germylene and stannylene', for instance the activation of Et2S2 by 'Lappert's germylene' (Scheme 3a). Drieß et al. synthesized six-membered N-heterocyclic silylenes and germylenes (Scheme 3b), which Roesky and coworkers found to be able to activate NH3. DFT studies by Alberta et al. revealed that an excess of NH3 lowers the activation barrier as a second NH3 molecule assists the addition. Another type of N-heterocyclic germylenes are the germylene hydrides, Scheme 3b, which have shown to be capable of activating CO2, ketones, alkynes, diazo-and azo-compounds. 4-membered Nheterocyclic metallylenes called amidinato-metallylenes have been reviewed recently by Cabeza and García-Álvarez and have shown reactivity (Scheme 3b). A N,B,N-chelated 4membered N-heterocyclic germylene, Scheme 3b, was able to activate the P-Cl bond of Ph2PCl. Furthermore, N-heterocyclic imine-stabilized germylenes and stannylenes have been synthesized and are able to activate CO2 and N2O. Nheterocyclic metallylenes are often aromatic, leading to increased stability and easier synthesizability. Heavier aromatic benzene-1,4-diide analogues (C6R2) 2-have been synthesized as well, though their characteristics and reactivity are still being explored. When only considering the HOMO-LUMO energy gap (Figure and 2d), an electropositive ligand that raises the HOMO and a p-electron-withdrawing ligand that lowers the LUMO would lead to an increased reactivity. Besides ligands that change the HOMO and/or LUMO energy, the L-E-L angle also influences the HOMO energy. A larger angle corresponds to a smaller HOMO s-orbital character and therefore a higher HOMO energy, as will be clarified in this section. The L-E-L angle can be influenced in three manners: i) a smaller ring structure (3membered ring) has a more stable HOMO than a larger ring structure (5-membered ring) or acyclic ligands, ii) bulky ligands attached to the central atom have an increased steric hindrance which could lead to a larger L-E-L angle and thus a higher HOMO energy, and iii) strongly electropositive ligands can enlarge the L-E-L angle. Bertrand and coworkers attributed the correlation between an increased L-E-L angle and a higher HOMO energy of carbenes to the inductive lowering of the contributing orbitals from the ligand orbitals to the HOMO. A carbene (or metallylene) can adopt a linear geometry or varying degrees of bending. A linear geometry indicates an sphybridized center with two nonbonding degenerate p orbitals (the system is in triplet state). Bending will break the degeneracy and give an sp 2 -type hybridization (HOMO) plus a p orbital orthogonal to it (LUMO). A linear geometry is an extreme; therefore, the HOMO consists primarily of an sp 2 -type hybridized orbital (the lone pair orbital), while the LUMO consists primarily of the vacant p-orbital that is orthogonal to it. An increased bending of the metallylene (a smaller L-E-L angle) results in more s-character in the HOMO and therefore a lower HOMO energy and thus a worse reactivity. As dictated by Bent's Please do not adjust margins Please do not adjust margins rule, more electronegative substituents form bonds with a larger p-character. Therefore, an electronegative s-electron withdrawing ligand results in a higher p-orbital contribution from the central E element to the ligand in the E-L bond, while the lone pair HOMO gains a higher s-orbital contribution and hence a more stable HOMO. The higher p-character in the E-L bond leads to a smaller L-E-L angle and a lower HOMO. In contrast, electropositive ligands result in less p-orbital character in the E-L bond and therefore a higher p-character in the lone pair HOMO and thus a higher HOMO energy. Both the larger bond angle and the higher HOMO energy are caused by the different s-and p-character in the E-L bonding orbital and the lone pair. A larger L-E-L angle in more electropositive ligands is thus not the cause for a higher HOMO energy. However, an increased angle can be correlated with a higher HOMO energy and thus with an increased reactivity. Besides that, the L-E-L angle can be changed by having a ring structure or adding bulky side groups, which influence the s-and p-character and thereby the energy of the HOMO. Wilfling et al. studied the geometries of metallylenes while changing their ligands by X-ray crystallography and their properties by NMR, UV-vis spectra, and DFT computations. They showed that an increasing electronegativity of the ligands resulted in a higher degree of bending and a larger HOMO-LUMO gap, which is associated with larger hypsochromic UV-vis shifts and lower paramagnetic contributions to NMR shift values of the central tetrel atom. Time-dependent DFT calculations were performed to show that the lowest-energy transition is dominated by the HOMO-LUMO transition, which shows the strong correlation between a larger HOMO-LUMO gap with a larger singlet-triplet gap and a hypsochromic UV-vis shift. Song et al. studied the effect of inductive electronwithdrawing ligands, such as CF3, on the singlet-triplet gap and the reactivity of carbenes by DFT. The stabilization of the singlet state of a carbene with a CF₃ ligand was attributed to the orbital character of the lone pair orbital (HOMO). The lone pair adopted more s-character due to the higher electronegativity of the ligands. Boryl ligands are rather electropositive and p-electronwithdrawing, making them a good candidate to increase the reactivity of metallylenes. Aldridge and coworkers were able to activate H2 by silylenes with one boryl ligand and for stability one nitrogen ligand. These silylenes have a mesomeric pushpull and inductive pull-push pattern to stabilize the silylene and increase its reactivity. The push-pull stabilization of singlet carbenes can be schematically drawn as in Scheme 3c. Germylenes that contain boryl ligands have also been synthesized. Various bulky ligand combinations, including aryl groups, amine, boryl, and silyl ligands, were explored. Boryl and silyl ligands seemed to be most promising. For instance, an aryl(silyl)-germylene was able to undergo H2 activation. The aryl(boryl)-germylene exhibited excessive reactivity, undergoing intramolecular reactions and hence being unable to activate small molecules, Scheme 3d. Competing side reactions are thus a serious challenge for the generation of stable metallylenes and the use of activated metallylenes. Borylstannylenes have, however, been successful in the activation of H2, H2O, NH3, and Si-H bonds, Scheme 3e. An Nheterocyclic silylene borane is also capable of activating H2, H2O, CO2, O2, and N2O, with the boron atom serving as an electron acceptor for the activated small molecule. More information can be found in the review from Duan and Cui entitled 'Boryl-substituted low-valent heavy group 14 compounds'. Silyl ligands have shown to be useful as well; for instance, a silylene bearing two silyl ligands was able to activate hydrogen. The silylene was stable because it was in equilibrium with the product of a 1,2 shift of one of the trimethylsilyl (TMS) groups from the silyl ligand to the central silicon atom, Scheme 3f. The minireview by Fujmori and Inoue reports various other silylenes, especially acyclic silylenes which bear wide L-Si-L angles and small HOMO-LUMO gaps, that are capable of the activation of H2, NH3, ethylene, CO2, CO, P4. Activating N2 by silylenes remains, however, a challenge. These experimental findings highlight how ligand choice influences both the electronic structure and reactivity of silylenes. To further understand how such structural features impact electronic properties, computational studies have examined how ligand effects shape the L-E-L angle and, in turn, the bonding characteristics of metallylenes Nechaev studied various metallylenes by DFT, categorizing them based on their ligands that stabilize the metallylene by steric shielding, inductively electron-withdrawing substituents, p-electrondonating groups, or aromatic stabilization. It was noted that upon bonding of the metallylene with another species, the L-E-L angle increases (as can be seen in Figure for the reactivity of another set of metallylenes). The increased angle was attributed to a change in hybridization: electron density transfers from the metallylene to the newly forming bond, therefore changing the hybridization of the central atom E participating in the E-L bond from predominantly p (sp 2 hybridization) to sp 3 , resulting in a wider L-E-L angle. As noted previously, the L-E-L angle enlarges with larger ring sizes (or acyclic ligands) or might increases with bulky ligands. The effect of ring size has been explored using ring-expanded N-Please do not adjust margins Please do not adjust margins heterocyclic carbenes (NHCs). The increase in ring size of NHCs from 5-to 6-to 7-to 8-membered rings, corresponding to L-C-L angles from 106° to 122°, significantly decreases the singlettriplet gap from 73.1 kcal mol -1 to 49.3 kcal mol -1 . An (increased) reactivity was observed with the larger ringexpanded NHCs for NH3 activation. Wang and Ma studied computationally the effect of ring-size on silylenes and germylenes, reporting lower H2 activation barriers for the larger rings. Bulky acyclic ligands could in principal increase the L-E-L angle thereby raising its HOMO, but steric hindrance can also circumvent reactions with small molecules as shown by Vermersch et al. Power and coworkers observed that sterically hindered metallylenes undergo side-reactions compared to less bulky analogues. While sterically encumbered germylenes and stannylenes can activate hydrogen, the activated metallylene subsequently eliminates one of its bulky ligands, and in the case of stannylene, dimerizes, thereby maintaining the oxidation state II. The sterically encumbered ligands and the greater stability of the oxidation state II for the heavier p-block elements (due to the inert-pair effect ) offers an explanation for these findings. Besides that, as noted above, bulky groups attached to a p-electron donating nitrogen or phosphorus ligand minimalize pyramidalization of the ligand. This results in more hyperconjugation towards the less electron-deficient tetrel atom and therefore a lower reactivity. Bulky ligands could thus increase the activity of a metallylene via increasing the L-E-L angle but could lower the activity due to steric hindrance with incoming small molecules and when to a pelectron donating ligand. Metallylenes with more unconventional ligands have been utilized in small molecule activation as well. Inomata et al. utilized a cationic NHC and a tungsten complex as ligands in a germylene for hydrogen activation. This study highlights not only the innovative approaches available for hydrogen activation, but also demonstrates the similar reactivity of metallylenes with transition metals as cationic NHCs are common ligands in transition metal chemistry. Stabilization of metallylenes by neutral NHCs 80 and N-heterocyclic silylenes is seen more frequently.