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A metal-centered cycloaddition is a subtype of the more general class of cycloaddition reactions. In such reactions "two or more unsaturated molecules unite directly to form a ring", incorporating a metal bonded to one or more of the molecules. Cycloadditions involving metal centers are a staple of organic and organometallic chemistry, and are involved in many industrially-valuable synthetic processes.
There are two general types of metal-centered cycloaddition reactions: those in which the metal is incorporated into the cycle , and those in which the metal is external to the cycle. These can be further divided into "true" cycloadditions , and formal cycloadditions. Beyond that, they are classified by the number of atoms contributed to the cycle by each of the participants.
For example, olefin metathesis using a Grubbs catalyst typically involves a reversible cycloaddition. A Ruthenium alkylidene and an alkene react to form a metallocycle.