Mesityllithium was used to synthesize dimesitylcadmium and dimesitylmercury from CdCl2 and HgCl2, respectively. X-ray-crystallographic data show that the group 12 metal compounds M[Mes]2 (M = Zn, Cd, Hg) are isomorphous (monoclinic, P21/n). The asymmetric unit of M[Mes]2 (M = Zn, Cd, Hg) consists of one mesityl group bonded to the metal atom, which is related to the second substituent by an inversion center. In addition we have investigated the reaction of BBr3 with M[Mes]2 (M = Cd, Hg) for our understanding of the reactivity of donor-free group 12 mesityl compounds. The reaction of M[Mes]2 (M = Cd, Hg) with an excess of BBr3 produces MesBBr2. UV-induced conversion of Hg[Mes]2 in benzene yielded quantitatively mesitylene and mercury whereas irradiation of a chloroform solution of Hg[Mes]2 for 1290 min (λmax = 510 nm) gave mesitylene, Hg[Mes]Cl, and HgCl2 in a ratio of 6:4:1. Slow concentration of the reaction solution led to the deposition of X-ray quality crystals of the addition compound of two Hg[Mes]Cl molecules and HgCl2 (monoclinic space group P21/n).