Spectroscopic identification of Y(C4H6) isomers formed by yttrium-mediated C-H bond activation of butenes

Y(C$_{4}$H$_{6}$) was observed from the reactions of laser-vaporized Y atom with 1-butene (CH$_{2}$=CHCH$_{2}$CH$_{3}$) and iso-butene (CH$_{2}$=C(CH$_{3}$)$_{2}$) in a pulsed molecular beam source, and its structural isomers were investigated with mass-analyzed threshold ionization spectroscopy combined with electronic structure calculations and spectral simulations. Y(C$_{4}$H$_{6}$) was identified as a five-membered metallacycle [Y(CH$_{2}$-CH=CH-CH$_{2}$)] from the Y + 1-butene reaction and a tetrahedral structure [YC(CH$_{2}$)$_{3}$] from the Y + iso-butene reaction. The metallacycle has a C$_{s}$ structure with Y binding to the two terminal carbon atoms, whereas the tetrahedron has C$_{3v}$ symmetry with Y binding to the tertiary carbon atom of trimethylenemethane. Both isomers have a doublet ground state with the highest molecular orbital being largely a Y 5s character. Ionization removes the metal based electron, and the resultant singlet ion has a similar structure to the neutral complex. However, the adiabatic ionization energy [46309(5) cm$^{-1}$] of the tetrahedron is considerably higher than that [43473(5) cm$^{-1}$] of the cyclic structure