The key difference between metallylenes and carbenes is that the latter adopts in most cases a triplet electron configuration, like the simplest carbene, methylene, is more stable in its triplet state, whereas a metallylene predominantly exists as a singlet state because of a large singlet-triplet energy gap. Singlet metallylenes have a lone pair type orbital in the plane of the molecule and a vacant p-type orbital perpendicular to it, Scheme 2a. This electron configuration resembles the (un)occupied d and s orbitals in transition metal catalysts, enabling metallylenes to engage in similar types of chemistry as their transition metal analogs, such as small molecule activation. The vacant p-type orbital of a metallylene can mimic the interaction of a vacant d-type orbital of a transition metal (forward bonding), while the filled lone-pair orbital of the metallylene mimics the p-back donation from a transition metal towards a small molecule (Schemes 2b and 2c). For a metallylene to truly react as a transition metal, the vacant ptype orbital and filled lone-pair orbital must be sufficiently close in energy and have a good shape to effectively mix with the frontier orbitals of small molecules. A small HOMO-LUMO gap in a metallylene is a key ingredient for favorable orbital interactions with a small molecule. As found by Mendez et al. and Gronert et al., the HOMO-LUMO gap, and therefore the reactivity of the singlet state, is directly related to the singlettriplet gap and can be used as a measure of the electrophilicity and nucleophilicity of singlet carbenes and metallylenes; a smaller HOMO-LUMO gap corresponds to a smaller singlettriplet gap. .22 However, it is important to ensure that the triplet state does not become more stable than the singlet state. The importance of a change in the HOMO-LUMO gap or singlettriplet gap by adjusting the metallylene will be discussed in more detail (vide infra). Scheme 2. Metallylene a) in the singlet and triplet state, b) its orbital interactions with a small molecule, and c) transition metal orbital interactions.