The Energetics, Structures, and Dynamics of the δδ* Excited States of Quadrulpy Bonded Dimers

Abstract

The results of an experimental investigation of the electronic structures of the M2X4L4 (M = Mo, W; X = Cl, Br I; L = uncharged σ-donor ligand) class of quadruply bonded metal dimers are reported. These results are interpreted within a theoretical framework that is general for all types of weakly interacting orbitals; this is set out in Chapter I. In Chapter II we lay the structural foundation for this investigation by presenting the crystal structures of Mo2Br4(PMe3)4 and Mo2I4(PMe3)4 and comparing their bonding parameters with those previously reported for Mo2Cl4(PMe3)4. Resonance Raman and far-infrared spectral data of these complexes are also reported; comparisons to the available structural data indicate that the metal-metal distance and diatomic force constants, which are fingerprint parameters for the characterization of metal-metal interactions, display different ligand sensitivities. This chapter concludes with a discussion of the energies and intensities of the 1(δ → δ*) transitions of these complexes, cast both in terms of their molecular structures as well as within the framework of existing theories of the electronic structural significance of these latter two spectroscopic properties. The data for the M2X4L4 complexes lead to a reevaluation of these standard interpretations, and a new postulate, based on charge transfer mixing, is put forth that more adequately accounts for the energies and intensities of the 1(δ → δ*) transitions of nearly all quadruply bonded dimers. The 3(δδ*) state is discussed briefly in Chapter III, wherein it is shown that differing magnetic and spectroscopic properties of the Mo2X4(PMe3)4 and diphosphine-bridged β-Mo2X4(dmpe)2 dimers are readily accounted for by the simple theoretical framework set out in Chapter I. The analysis of these data leads to experimental estimates of the energies of the 3(δδ*) states of these compounds: L = PMe3, E = 5200 c-1; L = dmpe, E = 500 cm-1. In Chapter IV we consider the photophysical properties of the 1(δδ*) state in light of its energetics relative to the 3(δδ*) and δ2 states. Of particular interest is the finding that the photophysical properties of these complexes are perturbed by the nature of the coordination sphere to a different extent than are their 1(δ → δ*) energies and intensities. The extensive series of M2X4L4 complexes studied allows the estimation of the intrinsic radiative and nonradiative decay properties of this chromophore. Chapter V contains the main body of this thesis, namely the analysis of the high-resolution electronic spectra of the M2X4L4 complexes. The unprecedented vibronic detail revealed by these spectra is interpreted using the conceptual base developed over the preceding four chapters, and results in the proposal of a double-well potential energy surface for the 1(δδ*) state along nontotally symmetric coordinates; this arises from sudden polarization. In the final chapter, the photochemical manifestations of this new excited state description are outlined. Material that supplements the discussion in Chapters I-VI is presented in the appendices.