We report the syntheses, crystal structures, and characterization of the novel cis-dioxo molybdenum(VI) complexes [Tpm*Mo VI O 2 Cl](MoO 2 Cl 3) (1) and [Tpm*Mo VI O 2 Cl](ClO 4) (2), which are supported by the charge-neutral Tpm* ligand (Tpm* = tris(3,5-dimethyl-1pyrazolyl)methane). A comparison between isostructural [Tpm*Mo VI O 2 Cl] + and Tp*Mo VI O 2 Cl (Tp* = hydrotris(3,5-dimethyl-1-pyrazolyl)borate) reveals the effects of one unit of overall charge difference on their spectroscopic and electrochemical properties, geometric and electronic structures, and OAT reactivities, providing new insight into pyranopterin molybdoenzyme OAT reactivity. Computational studies of these molecules indicate that the delocalized positive charge lowers the LUMO energy of cationic [Tpm*MoO 2 Cl] + relative to Tp*MoO 2 Cl. Despite their virtually identical geometric structures revealed by crystal structures, the Mo VI /Mo V redox potential of 2 is increased by 350 mV relative to Tp*Mo VI O 2 Cl. This LUMO stabilization also contributes to an increased effective electrophilicity of [Tpm*MoO 2 Cl] + relative to Tp*MoO 2 Cl, resulting in a more favorable resonant interaction between the Mo complex LUMO and the HOMO of the PPh 3 substrate. This leads to a greater thermodynamic driving force, an earlier transition state, and a lowered activation barrier for the orbitally controlled first step of the OAT reaction in the Tpm* system relative to the Tp* system. An Eyring plot analysis shows that this initial step yields an O≡Mo IV-OPPh 3 intermediate via an associative TS, and the reaction is ~500-fold faster for 2 than for Tp*MoO 2 Cl. The second step of the OAT reaction entails the solvolysis of the O≡Mo IV-OPPh 3 intermediate to afford the solvent-substituted Mo IV product, and is 750-fold faster for the Tpm* system at-15 °C compared to the Tp* system. The observed rate enhancement for the second step is ascribed to a switch of reaction mechanism from a dissociative pathway for the Tp* system, to an alternative associative pathway for the Tpm* system. This is due to a more Lewis acidic Mo(IV) center in the Tpm* system.

Kβ valence-to-core (V2C) X-emission spectroscopy (XES) has gained prominence as a tool for molecular inorganic chemists to probe the occupied valence orbitals of coordination complexes, as illustrated by recent evaluation of Kβ V2C XES ranging from titanium to iron. However, cobalt Kβ V2C XES has not been studied in detail, limiting the application of this technique to probe cobalt coordination in molecular catalysts and bioinorganic systems. In addition, the community still lacks a complete understanding of all factors that dictate the V2C peak area. In this manuscript, we report experimental cobalt Kβ V2C XES spectra of low-spin octahedral Co(III) complexes with different ligand donors, in conjunction with DFT calculations. Cobalt Kβ V2C XES was demonstrated to be sensitive to cobalt-ligand coordination environments. Notably, we recognize here for the first time that there is a linear correlation between the V2C area and the spectrochemical series for low-spin octahedral cobalt(III) complexes, with strong field π acceptor ligands giving rise to the largest V2C area. This unprecedented correlation is explained by invoking different levels of π-interaction between cobalt p orbitals and ligand orbitals that modulate the percentage of cobalt p orbital character in donor MOs, in combination with changes in the average cobalt-ligand distance.