Tetramesityldiiron is an organoiron tetramesityl compound with the formula Fe<sub>2</sub>(C<sub>6</sub>H<sub>2</sub>(CH<sub>3</sub>)<sub>3</sub>)<sub>4</sub>. It is a red, air-sensitive solid that is used as a precursor to other iron complexes. It adopts a centrosymmetric structure. The complex is a Lewis acid, forming monomeric adducts, e.g. Fe(C<sub>6</sub>H<sub>2</sub>(CH<sub>3</sub>)<sub>3</sub>)<sub>2</sub>pyridine<sub>2</sub>. The complex is prepared by treating ferrous halides with the Grignard reagent formed from mesityl bromide: