Regioselective protonation of the eta(4)-anthracene ligand in [Mn(eta(4)-C14H10)(CO)(3)](-) to give an eta(5)-anthracenyl complex [Mn(eta(5)-C14H11)(CO)(3)] that undergoes a low-energy 1,4-hydride exchange reaction

Protonation of the terminal (eta(4)-anthracene) manganese tricarbonyl anion [Mn(eta(4)-C14H10)-(CO)(3)](-) gives the terminal (eta(5)-anthracenyl) man ganese tricarbonyl complex [Mn(eta(5)-C14H11)-(CO)(3)]. Single-crystal X-ra y diffraction analysis confirms that the proton adds to the hydrocarbon rin g, and labeling studies indicate that protonation is regiospecific and occu rs at the endo site. Magnetization transfer experiments establish rapid spi n population transfer from the exo site to the para ring position. This is proposed to involve an intermediate metal hydride, and dynamic NMR studies have been used to determine that the free energy barrier to this exchange i s ca. 14.5 kcal/mol at and below ambient temperatures. This is interpreted in terms of a modification of the mechanism of Lamanna and Brookhart for th e isomerization within 6-exo-H-1-[Mn(eta(5)-C6HD5)(CO)(3)].