Frenking and coworkers attributed an increased reactivity from N-heterocyclic carbenes (NHCs) to P-heterocyclic carbenes (PHCs) also to a more pyramidalized phosphorus ligand. Pyramidalization in the PHC weakens the p-donation from phosphorus to the carbene carbon (hyperconjugation), resulting in a more electron-deficient carbene carbon. They associate this with a more stable LUMO energy, whereas we observed less steric Pauli repulsion. Bulky ligand PHCs should minimalize the pyramidalization, making the PHCs more stable and synthetically viable while increasing the activation barriers slightly. Most metallylene studies focus on modifying the ligand to increase the reaction barrier by decreasing the HOMO-LUMO gap. The HOMO and LUMO energies can be influenced directly by s-donating or p-donating ligands, or by the angle defined at the Group 14 tetrel element with its two ligands (CGeL angle in Figure ). The HOMO consists primarily of a lone pair orbital, while the LUMO consists primarily of a vacant porbital (Scheme 2b). s-donating ligands, containing more electropositive elements, inductively raise the HOMO in energy, while electronegative ligands inductively stabilize the HOMO (Figure ). The s-donating capability also changes the L-E-L angle as will be explained in a following paragraph. A p-electrondonating ligand, a mesomeric effect, increases the LUMO energy through lone pair donation from the ligand into the vacant p-orbital, whereas a p-electron-withdrawing group lowers the LUMO, as shown in Figure . An electronegative and p-electron donating ligand causes a larger HOMO-LUMO gap and thus a decreased reactivity. The decreased reactivity when having p-electron-donating nitrogen ligands has been experimentally observed for carbenes and silylenes. Frey et al. noticed that a carbene with one nitrogen ligand can perform H2 activation, but one with two nitrogen ligands cannot. Kosai et al. found that a silylene with one nitrogen ligand is thermally stable and able to activate C-H bonds. In contrast, a silylene with two nitrogen ligands is too stable to perform small molecule activation, while a silylene without nitrogen ligands (only bearing alkyl groups) is not thermally stable.