Knowledge Bank

Early transition metal atoms, produced by laser ablation, are reacted with benzene vapor diluted in argon during co-deposited onto a low temperature CsI window. The resulting reaction products are trapped and the $M(C6H6)$ and $M(C6H6)2$ complexes are identified by benzene isotopic studies $(C6H6$, $\mathrm{<mrow\; data-mjx-texclass="ORD">13</mrow>}C6H6$, $C6D6)$. Density Functional Theory (DFT) frequency calculations are used to confirm the assignments. Calculated ground state energies predict these reaction energetically favorable. The $M(C6H6)$ and $M(C6H6)2$ products have $C6v$ and $D6h$ symmetries respectively. Based on the observed aromatic C-C breathing mode frequency shifts, the metal-carbon bond strengths are shown to increase down a group (from vanadium to tantalum) and decrease from left to right along a period (from scandium to chromium). These trends indicate that back donation of electrons from the metal d orbitals to the unoccupied $\pi \ast$ orbitals of benzene is the primary electronic interaction responsible for the bonding in these molecules. Analogous experiments in progress with $C60$ give similar product complexes.