A series of seven molybdenum(III) complexes with the general formula of [Mo(diimine)Cl 4 ] − were synthesized and characterized by X-ray diffraction, IR, cyclic voltammetry (CV), and UV−vis. The complexes were discovered to be highly solvatochromic, showing shifts in λ max between ∼120 and 170 nm in solvents ranging from water to acetone. Varying the substituents on the diimine ligand influenced the absorption energy such that electron-withdrawing groups induced a red shift while electron-donating groups exhibited the opposite effect. The complexes were surprisingly stable in both acidic and basic solutions, and in the case where carboxylic acid substituents were present, additional shifts in the absorption maxima were observed, corresponding to the state of protonation of these groups. Both the Mo IV/III and Mo III/II redox couples were observed in CV experiments and were complemented with density functional theory (DFT) calculations.

Four compounds with tetrachalcogenidomolybdate dianions were examined under identical conditions by single crystal Xray diffraction and density functional theory (DFT) calculations. Structural comparisons of bond lengths and angles were made looking at effects of counter-cations and type of chalcogenide (S or O). Ultimately, bond lengths and angles were found to be fairly regular for each anion and in good agreement with computational results, with exceptions likely stemming from close contacts in the crystal packing. A trend of increasing bond lengths was observed with increasing cation size. The most notable structural deviation occurred in [MoO 2 S 2 ] 2À , where the theoretical structure has a smaller OÀMo-O angle and a larger SÀMo-S angle, which was opposite in the crystal structure. This is likely attributed to NÀH-S and NÀH-O interactions. Both experimental and computational results suggest that uniform charge delocalization in all four structures. The HOMO-LUMO gap increases significantly with increasing number of oxygen atoms and electronic transitions were determined to be all ligand to metal charge transfer (LMCT).[a] A.