Transition-Metal Capping to Suppress Back-Donation to Enhance Donor Ability

The design of strong donor ligands has been one of the crucial tasks in the development of ligands for transition-metal complexes. One effective strategy is suppression of back-donation from a metal to a ligand by electron donation from adjacent main-group atoms bearing lone-pair electrons. We now report another method for suppression of back-donation: capping an accepting orbital by coordination of a transition metal. A lone pair on the triple-decker ruthenocene-type stannole complex can coordinate an Mo­(CO)5 moiety. In the resulting complex, back-donation from a Mo to an Sn moiety is effectively suppressed because of the occupation of a potentially electron-accepting 5p orbital on the Sn atom by coordination of the Ru moieties. The triple-decker ruthenocene-type stannole complex can also coordinate an Ag­(I) atom to form a 1:2 complex, where two bulky ligands are oriented in a perpendicular fashion.