Four heterotrinuclear (Re2MII)-M-IV compounds of general formula (NBu4)(2)[{(ReBr4)-Br-IV(mu-ox)}(2)M-II(Him)(2)] [NBu4+ = tetra-n-butylammonium cation, ox = oxalate, Him = imidazole; M = Mn (1), Co (2), Ni (3), and Cu (4)] have been synthesized by using the novel mononuclear complex [(ReBr4)-Br-IV(ox)](2-) as a ligand toward divalent first-row transition metal ions in the presence of imidazole. Compounds 1-4 are isostructural complexes whose structure contains discrete trinuclear [{(ReBr4)-Br-IV(mu-ox)}(2)M-II(Him)(2)](2-) anions and bulky NBu4+ cations. The Re and M atoms are six-coordinated: four peripheral bromo and two oxalate-oxygens (at Re), and two cis-coordinated imidazole molecules and four oxygen atoms from two oxalate ligands (at M), build distorted octahedral surroundings. Two peripheral [ReBr4(ox)](2-) units act as bidentate ligands through the oxalate group toward the central [M-II(Him)(2)] fragment affording the trinuclear entities. The values of the intramolecular Re center dot center dot center dot M separation are 5.62(1) (1), 5.51(1) (2), 5.46(1) (3), and 5.55(1) angstrom (4). Magnetic susceptibility measurements on polycrystalline samples of 1-4 in the temperature range of 1.9-300 K show the occurrence of intramolecular antiferro- [J = -1.1 cm(-1) (1)] and ferromagnetic interactions [J = +3.9 (2), +19.7 (3), and +14.4 cm(-1) (4)], the Hamiltonian being defined as (H) over cap = -J [(S) over cap (M)((S) over cap (Re1) + (S) over cap (Re2))] The larger spin delocalization on the oxalato bridge in 1-4 when compared to the trinuclear (Re2MII)-M-IV complexes with chloro instead of bromo as peripheral ligands (1'-4') accounts for the strengthening of the magnetic interactions in 1-4 [J = -0.35 (1'), +14.2 (3'), and +7.7 cm(-1) (4')]. An incipient frequency dependence of the out-of-phase ac signals of 3 at very low temperatures is reminiscent of a system with slow relaxation of the magnetization, a phenomenon characteristic of single-molecule magnet behavior.