The orbital interactions responsible for the large singlet-triplet gaps and low (or negative) electron affinities of stable silylenes are fairly well-understood. In the case of diaminosilylenes, for example, these are allyl-type p-conjugative interactions, with the LUMO identified as the fully antibonding p-orbital of the N-Si-N unit. In the case of pcomplexes such as decamethylsilicocene and silapyramidane, strong bonding interactions involving Si greatly increase the HOMO-LUMO gap of the carbocycle ligands. In the case of the silapyramidane, 20 for example, the fully bonding p-orbital of the cyclobutadiene moiety interacts head-on with an 3s/3pz hybrid orbital (Figure ). The resulting bonding and antibonding MOs constitute the HOMO-42 and HOMO-2 of the silapyramidane (both of which transform as 'a' under C4). The stability of the HOMO-2 reflects its largely Si 3s character. The two degenerate p-MOs of the cyclobutadiene are also suitably aligned to bond with the Si 3px and 3py orbitals to generate a pair of 'e' MOs, the HOMOs of the silapyramidane. In contrast, the fully antibonding p-orbital of the cyclobutadiene cannot (on account of its symmetry) engage in any energy-lowering interaction with Si orbitals and may be identified as the molecular LUMO (Figure ).