Type 15 Lewis Acid/Base Complexation Chemistry
All covalent sigma-bonds can be regarded as Type 15 complexes. Reactions involving Type 15 complexes occur either occur via 1st order sequential mechanisms (decomplexation followed by complexation) or via 1st order concerted mechanisms.

For more information look in the Chemogenesis webbook sections on Lewis acids and Lewis bases and the Lewis acid/base interaction matrix.