The spectroscopic properties (UV-vis absorption and luminescence) of Ru(II)- and Os(II)-based mononuclear and dinuclear complexes containing terdentate terpyridine-type (ttp) and related cyclometalating (dpb-, and tpbp2-) ligands are reported [ttp is 4’-p-tolyl-2,2’:6’,2"-terpyridine, dpbH is di-o-pyridyl-1,3-benzene, tpbpH-2 is 3,3’,5,5’-tetrapyridylbiphenyl]. For the dinuclear species, [(ttp)M(tpbp)M’(ttp)]2+, the biscyclometalating dianionic tpbp2-ligand bridges the two metal centers, which are held at a fixed distance of 11 angstrom [M, M’ = Ru, Os]. The five complexes are weakly luminescent at room temperature, lambda(max) approximately 790 nm, phi approximately 4 X 10(-5), and tau approximately 4 ns for the ruthenium complexes, and lambda(max) approximately 820 nm, and phi approximately 2-5 x 10(-6) for the osmium-containing complexes. The luminescence properties are compared to the electrochemical behavior and are discussed in terms of a high degree of covalency for the metal-ligand interaction. For the heterodinuclear [(ttp)Ru(tpbp)Os(ttp)]2+ complex, the Ru --> Os energy transfer step, which is exothermic by approximately 0.16 eV, is very efficient (k(en) greater-than-or-equal-to 2 X 10(9) s-1), and luminescence only occurs from the Os-based component. The energy transfer takes place according to an exchange-type mechanism.