Probing the Halide Effect in the δ-Bond with One- and Two-Photon Spectroscopy

Area of Science:

• Inorganic Chemistry
• Quantum Chemistry
• Spectroscopy

Background:

• Two electrons in two orbitals can form four distinct electronic states.
• Weakly coupled orbitals, like the delta (δ) orbitals in quadruple bond species, exhibit diradical and zwitterionic states.
• The electronic structure of metal-metal quadruple bonds is crucial for understanding their reactivity and properties.

Purpose of the Study:

• To investigate the determinants of state energies in a two-electron bond model.
• To quantify the influence of one-electron (ΔW) and two-electron (K) energies on electronic states.
• To explore the relationship between secondary bonding interactions and the two-electron exchange energy (K).

Main Methods:

• Measurement of one-photon and two-photon electronic spectra.
• Calculation of one-electron (ΔW) and two-electron (K) energies from spectral data.
• Spectroelectrochemical preparation and analysis of oxidized molybdenum-dimer complexes (Mo₂X₄(PMe₃)₄⁺).

Main Results:

• The two-electron exchange energy (K) is sensitive to the distance between electrons and thus orbital size.
• Spectra of Mo₂X₄(PMe₃)₄ reveal secondary bonding interactions with halide orbitals, correlating with decreasing K energies (Cl > Br > I).
• Spectroelectrochemical oxidation confirmed calculated one-electron energies, showing a significant shift in the δ → δ* transition due to the absence of K in the one-electron bond.

Conclusions:

• The two-electron exchange energy (K) is a critical factor governing the electronic state energies of two-electron bonds.
• Secondary bonding interactions significantly influence the K energy in quadruple bond systems.
• The δ → δ* transition energy is strongly dependent on the two-electron exchange energy, highlighting its importance in describing the electronic structure of metal-metal multiple bonds.