THE ELECTRONIC STRUCTURES OF ORGANOMETALLIC ALKYNE AND VINYLIDENE COMPLEXES AS DETERMINED BY X-RAY AND ULTRAVIOLET PHOTOELECTRON SPECTROSCOPY (CYCLOPENTADIENYL, VALENCE, MANGANESE, CORE, VANADIUM).

Abstract

The chemistry and bonding of alkynes and vinylidenes in organometallic complexes have been investigated. A variety of these complexes have been synthesized and characterized by X-ray crystallography, temperature-dependent NMR, molecular orbital calculations, and most importantly, HeI, HeII and MgKα photoelectron spectroscopy (PES). The core and valence ionizations are found to be very informative with regard to the relative bond strengths and stabilities of these complexes. The first step involved preparation of the series of complexes R-CpM(CO)₂(alkyne) (R-Cp = Cp, MeCp and Me₅Cp). When M = Mn, Re (alkyne = 3-hexyne, 2-butyne and hexafluoro-2-butyne), the molecular mirror plane bisects the alkyne (horizontal conformation). PES shows the alkyne (π(⊥)) orbital forms a filled-filled interaction with the frontier metal orbital which is significantly destabilized. The ionizations derived from the two alkyne π orbitals are not split. When M = V, the alkyne (C₂H₂, 3-hexyne, etc.) coincides with the molecular mirror plane (vertical conformation). PES shows the alkyne π(⊥) orbital donates electrons to the electron deficient vanadium and the metal backbonds strongly to the alkyne. Electronic factors controlling the conformations in the d⁶ manganese case has been much discussed in the literature. Another factor not previously identified is necessary for understanding the conformation in the d⁴ vanadium case. The energy of the LUMO reveals that this factor is donation of cyclopentadienyl electrons into an empty d orbital of the electron deficient vanadium. Rearrangement of alkyne complexes to terminal vinylidene and bridging vinylidene complexes, similar to reactions of organic molecules on metal surfaces, were also investigated. The series of [R-CpMn(CO)₂]₂(μC=CHR') (R' = H, Me) (Chapter 6) and CpMn(CO)₂(C=CHBuᵗ) (Chapter 7) complexes were prepared. PES showed that the terminal vinylidene ligand has less filled-filled interaction with the metal and stabilizes the metal more than the alkyne does. The bridging vinylidene accepts more electron density from the metals and stabilizes the metals more than the terminal vinylidene. The removal of antibonding electrons from the HOMO of the metal fragment by the bridging vinylidene leaves net metal-metal bonding interaction and forms a stable dimetallocyclopropane structure.