Electronic Structure Factors of Carbon-Hydrogen Bond Activation. The Photoelectron Spectroscopy of (Cyclohexenyl)manganese Tricarbonyl

The He I and He II ionizations for (cyclohexenyl)manganese tricarbonyl, a molecule which exhibits an activated C-H bond, are reported. Comparisons are made to the electronic structures of (methylcyclopentadienyl)manganese tricarbonyl, (cyclohexadienyi)manganese tricarbonyl, and (cyclohexadiene)iron tricarbonyl. Electronic structure factors contributing to the initial activation of the C-H bond are discussed in terms of two limiting descriptions that have been presented in the literature. These descriptions are labeled a activation, involving the donation of C-H σ bonding orbital electron density into the empty metal orbitals, and σ* activation, involving electron density flow in the opposite direction, i.e., from filled metal levels into the empty C-H σ* antibonding level. Both processes are shown to be possible based only on geomertry, symmetry, and overlap considerations. The experimental data demonstrate that the principal electronic structure mechanism for the early stages of C-H bond lengthening and interaction with the metal exhibited in (cyclohexenyl)manganese tricarbonyl is a activation. The data are also related to variable-temperature NMR studies of this complex which show that fluxionality proceeding through a 16 e“ intermediate is more favorable than that through an 18 e” intermediate with a full metal-hydrogen bond. Both bond strength and ionization energy data show that the C-H activation of (cyclohexenyl)manganese tricarbonyl stops at the agostic stage because there is no net gain in carbon-carbon or metal-carbon bonding to compensate the loss of the C-H bond.