Silyl Substitution Effects on Metal−Pentadienyl Bonding: Synthesis, Structure, Photoelectron Spectroscopy, and Electronic Structure of a High-Valent Half-Open Zirconocene

The molecule Cp(3-Me3Si-6,6-dmch)ZrI2 (Cp = η5-cyclopentadienyl; 3-Me3Si-6,6-dmch = η5-3-trimethylsilyl-6,6-dimethylcyclohexadienyl) has been synthesized, and the molecular and electronic
structures have been investigated. Photoelectron spectroscopy shows that substitution of a trimethylsilyl
group in place of a hydrogen atom on the 6,6-dmch ligand destabilizes all of the ionizations of Cp(3-Me3Si-6,6-dmch)ZrI2 by ca. 0.1−0.2 eV compared to those of Cp(6,6-dmch)ZrI2. Density functional
calculations accurately reproduce the experimental structure of the molecule and agree with the observed
shift of all ionizations to lower energies. Interestingly, the ionizations are calculated to shift to lower
energies without the expected increase in electron density at the metal center. This apparent contradiction
is understood from analysis of one-center and two-center charge effects in the molecule. As a consequence
of these charge effects, the silyl substitution makes the 3-Me3Si-6,6-dmch ligand a slightly better donor
and, surprisingly, also a better acceptor ligand than the 6,6-dmch ligand.