The term "sigma complex" refers to a specific type of molecular structure in chemistry, particularly in coordination compounds. It describes a situation where a central metal atom is surrounded by ligands that can form multiple bonds, leading to a complex with unique properties. This can influence the reactivity and stability of the compound. In the context of transition metals, sigma complexes often exhibit interesting electronic characteristics due to the nature of the bonding interactions. These complexes are important in various fields, including catalysis and materials science, as they can affect how substances interact and transform in chemical reactions.