Transition metal η 2-vinyl complexes

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Transition metal η 2-vinyl complexes, L n M(η 2-CRCR 2), adopt structures in which both alkenyl carbons are bound to the metal. The M(CRCR 2) unit is not planar in these 1-metallocyclopropene complexes. Instead, the CR 2 entity is twisted by ca. 90° away from planarity with the three-membered ring formed by M, C α, and C β. The ring carbons of η 2-vinyl ligands are bound asymmetrically to the metal with one short metal–carbon bond and one long metal–carbon bond. These geometrical features reflect double bond character between C α and the metal at the expense of the carbon–carbon double bond in the classic organic vinyl moiety. Coordination of both carbons of the four-electron donating η 2-vinyl ligand increases the electron count at the metal by two electrons relative to an η 1-bound vinyl ligand. This review summarizes synthetic methods which yield η 2-vinyl complexes, describes bonding and structural characteristics of η 2-vinyl ligands, tabulates NMR spectroscopic signatures of η 2-vinyl ligands, and outlines reactions of this growing class of compounds.