Synthesis, structural determination and photochromism characterization of two complexes with [MO2(O2CCOPh2)2]2− cores [M = Mo or W]

K2[MoO2(O2CCOPh2)2] · 2H2O (1) and K2[WO2(O2CCOPh2) · 1.5H2O (2) have been synthesized by the reactions of (NH4)2MS4 (M = Mo, W) and benzilate (hydroxydiphenylacetate) in water–alcohol medium at room temperature. The two complexes are characterized by elemental analysis, infrared spectra, 1H NMR and TGA/DTA. When irradiated to UV light, both complexes exhibit photochromism in the solid state and their UV-induced products have been characterized by X-ray photoelectronic spectra and electron paramagnetic resonance spectra.