The complexes [MCl2{N,N'-(BuNSe)-Bu-t(mu-(NBu)-Bu-t)(2)(SeNBu)-Bu-t}] [M = Cd (1), Hg (2)] were obtained in high yields by the reaction of tert-butylselenium di_imide Se-IV((NBu)-Bu-t)(2) with CdCl2 or HgCl2 in tetrahydrofuran. Recrystallization of 1 and 2 from acetonitrile (MeCN) afforded yellow crystals of 1 center dot MeCN and 2 center dot MeCN, respectively. Isomorphic 1 center dot MeCN and 2 center dot MeCN contain an unprecedented dimeric selenium diimide ligand, which is N,N'-chelated to the metal through exocydic imido groups. In addition to the complexes 1 and 2, the Se-77 NMR spectra of acetonitrile solutions of 1 center dot MeCN and 2 center dot MeCN indicated the presence of the dimeric (BuNSe)-Bu-t(mu-(NBu)-Bu-t)(2)(SeNBu)-Bu-t, monomeric Se-IV((NBu)-Bu-t)(2), and cyclic selenium imides. Density functional theory calculations at the PBEO/def2-TZVPP level of theory were used to assign the Se-77 resonances of the dimer. A comparison of Gibbs energies of formation of some metal dichloride complexes [MCl2{N,N'-Se-IV((NBu)-Bu-t)(2)}] and [MCl2{N,N'-(BuNSe)-Bu-t(mu-(NBu)-Bu-t)(2)(SeNBu)-Bu-t}] (M = Zn, Cd, He indicated that the formation of complexes containing a dimeric selenium diirnide ligand is favored over those containing a monomeric ligand for the group 12 metals. In the case of the group 10 metal halogenides (M = Ni, Pd, Pt), the Gibbs energies of the complexes with monomeric Se-IV((NBu)-Bu-t)(2) ligands are close to those containing dimeric (BuNSe)-Bu-t(mu-(NBu)-Bu-t)(2)(SeNBu)-Bu-t ligands. A plausible reaction pathway with a low activation energy involves the initial formation of [MCl2{N,N'-Se-IV((NBu)-Bu-t)(2)}] (M = Zn, Cd, Hg), which then reacts with another molecule of Se((NBu)-Bu-t)(2), leading to the final [MCl2{N,N'-(BuNSe)-Bu-t(mu-(NBu)-Bu-t)(2)(SeNBu)-Bu-t}] complex. Without the presence of group 12 metal halogenides, the [2 + 2] cyclodimerization of Se-IV((NBu)-Bu-t)(2) is virtually thermoneutral, but the activation energy is relatively high, which accounts for the kinetic stability of (BuNSe)-Bu-t(mu-(NBu)-Bu-t)(2)(SeNBu)-Bu-t in solution. A minor byproduct, eu [Cd7Cl14{N,N'-Se-II((NHBu)-Bu-t)(2)}(6)]center dot 4CH(2)Cl(2), was identified by X-ray crystallography as a heptanuclear cluster with selenium(II) diamide ligands N,N'-chelated to the cadmium centers.