Triple metal-metal bonds of M2(CH2CMe3)6 (M = Mo, W) investigated by photoelectron spectroscopy and density functional theory
The variable PES of a pair of group 6 triply metal-metal bonded alkyl compounds (M2(CH2CMe3)6 (M=Mo, W) were obtained for the first time and benchmarked againest state-of-art DFT calculations. M. de Simone et al. Organom. 2022
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Photoelectron spectroscopy (PES) is a powerful technique for measuring the quantized energies of electrons in molecules, and the ability to vary the incident photon energy enables determination of their role in chemical bonding. Such data has proved vital in the development, validation, and benchmarking of theoretical methods for the study of electronic structure. Investigations of the nature of chemical bonding between transition metal atoms (M) in molecular compounds began in the 1960s and have increased dramatically over the past half century, with a significant emphasis placed on the synthesis, structure, spectroscopy, and chemical reactivity of compounds containing metal-metal multiple bonds. A remarkable subclass of the latter are dinuclear molybdenum(III) and tungsten(III) compounds of the type M2X6 (M = Mo, W; X = bulky anionic ligand), which exhibit staggered, ethane-like geometries and short metal-metal bonds that are the embodiment of 24 metal-metal triple bonds (Fig 1a-c). In this study, the electronic structure of a pair of M2X6 compounds where the bulky anionic ligand is a sigma-bonded neopentyl group (X = CH2CMe3) were compared and contrasted.
PE spectra were measured for both compounds over a photon energy range from 20 – 70 eV. Significant variations in the band intensities were observed with the ionizations associated with metal-based orbitals increasing in relative intensity with increasing photon energy (Fig. 1d-e). Ionizations from the M-M π- and s-bonding orbitals showed the largest intensity increase and occurred at the lowest ionization energy. M-C ionizations lie between 8 and 10 eV and they also increase in intensity relative to the broad bands between 10 and 16 eV that are associated with C-C and C-H ionizations from the neopentyl ligands. |
More detailed assignments were enabled by density functional theory (DFT) calculations using the Amsterdam Density Functional (ADF) modelling suite. Whereas in the case of Mo2(CH2CMe3)6 the first ionization band is associated with both metal-metal π- and s-bonding orbitals, for W2(CH2CMe3)6 the first band is assigned to a spin-orbit split π-ionization (Fig. 1f-g). Agreement between calculated and experimentally measured ionization energies was excellent. A marked difference between the bonding patterns of orbitals of Mo2(CH2CMe3)6 and those of W2(CH2CMe3)6 is the stabilization of those orbitals containing 6s character for W and is attributable to relativistic effects. The calculations also gave very good agreement with the structural parameters determined by x-ray diffraction and the electronic absorption spectra of the two compounds measured in alkane solution.
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Triple metal-metal bonds of M2(CH2CMe3)6, (M=Mo, W) investigated by photoelectron spectroscopy and density functional theory. |
