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

Two electrons in two orbitals give rise to four states. When the orbitals are weakly coupled as in the case for the d orbitals of quadruple bond species, two of the states are diradical in character with electrons residing in separate orbitals and two of the states are zwitterionic with electrons paired in one orbital or the other. By measuring one-and two-photon spectra, the one-electron (ΔW) and two-electron (K) energies may be calculated, which are the determinants of the state energies of the four-state model for the two-electron bond. The K energy is thus especially sensitive to the size of the orbital as K is dependent on the distance between electrons. To this end, one- and two-photon spectra of Mo₂X₄(PMe₃)₄ are sensitive to secondary bonding interactions of the δ-orbital manifold with the halide orbitals, as reflected in decreasing K energies along the series Cl > Br > I. Additionally, the calculated one-electron energies have been verified with the spectroelectrochemical preparation of the Mo₂X₄(PMe₃)₄⁺ complexes, where the δ bond is a one-electron bond, and K is thus absent. The δ → δ* transition shifts over 10,000 cm^-1 upon oxidation of Mo₂X₄(PMe₃)₄ to Mo₂X₄(PMe₃)₄⁺, establishing that transitions within the two-electron δ bond are heavily governed by the two-electron exchange energy.