Disilicon

Disilicon, or the silicon dimer Si2, is a diatomic molecule consisting of two silicon atoms. It is an example of a homo-nuclear diatomic of group 14 and is primarily observed in high-temperature gas-phase environments, such as plasmas, laser ablation plumes, combustion byproducts, and some astrophysical contexts, where transient Si2 units can form before reacting further.

Bonding and structure: The Si–Si bond in Si2 is relatively weak compared with C2; the molecule has significant multi-configurational character, and there is ongoing discussion about its exact bond order. The bond length is approximately 2.27 Å in the gas phase, with bond dissociation energies lower than many other covalent Si–Si bonds, reflecting the diffuse 3p-based bonding in silicon.

Spectroscopic and experimental notes: Si2 is detected by high-resolution spectroscopy and mass spectrometry; due to its homonuclear nature, pure rotational infrared transitions are weak, and many observations rely on electronic spectroscopy or matrix isolation techniques. Computational studies using ab initio and multireference methods provide insight into its electronic structure and confirm its status as a transient species rather than a stable molecule under ambient conditions.

Relevance: Si2 serves as a benchmark for bonding theories in heavier group-14 elements and provides insight into silicon's behavior in clusters and during silicon-oxide formation in materials processing.