Metal, ligand, and symmetry influences on metal-metal bonds: Photoelectron spectroscopy and theory

Abstract

Three sets of metal-metal bonded systems of the form M₂(L ͡ L)₄ have been studied by gas-phase ultraviolet photoelectron spectroscopy and electronic structure calculations to understand the electronic structures of and bonding in these molecules. The ligand sets range from the relatively poor electron donor trifluoroacetate ligand, to hydroxymethylpyridinate (mhp), and finally to the relatively strong electron donor N,N'-diphenylformamidinate (form) ligand. Not only does this study elucidate the methods by which metal and ligand interact throughout a series of differing electron donor ligand sets, but it also presents a cohesive understanding of the electronic structures of these systems in terms of overall molecular symmetry. In particular, the relative stabilities and orbital characters of the metal-metal bonding and antibonding orbitals are probed to understand the ability of a particular ligand set to affect the ability of two metal atoms to bind together. First, compounds of the form M₂(form)₄ (M = Cr, Mo, W, Ru, Rh, Pd) are examined. The spectra of the metal-metal quadruple bond-containing systems (i.e., M₂(form)₄ where M = Cr, Mo, W) are used to identify several metal- and ligand-based ionization features, which can then be used to identify the additional metal-based features in the spectra of the remaining systems. Given the ease with which functional groups can be added to the formamidinate ligand, a series of substituted Mo₂(form)₄ systems have been prepared and their ionization data have been compared with solution-phase electrochemical results. Next, the electronic structures of M₂(O₂CCF₃)₄ (M = Mo, Rh) are studied. Variable energy photon experiments reveal a predominance of ligand character in the M-M σ and π orbitals, despite the relatively poor overall electron donor ability of the ligand. The means by which such a ligand can interact by symmetry with these metal orbitals are studied by computational methods. Finally, the bonding in M₂(mhp)₄ (M = Cr, Mo, W, Ru, Rh) systems is probed. The lower symmetry of these molecules and the intermediate donor properties of this ligand set allow for correlation with the electronic structures of M₂(form)₄ and M₂(O₂CCF₃)₄. Unlike for the higher symmetry systems, ligand involvement in the M-M δ bond is observed and can be understood in terms of molecular symmetry arguments.