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.