A binuclear complex refers an entity of atoms (i.e., a complex or more appropriately, a molecular entity or coordination entity) with two central ions (by extension, a mononuclear complex has one central ion, a trinuclear complex has three, and polynuclear complexes have more than three). The central ion is a metal ion and an electron acceptor, with ligands as electron donors. If surface sites are involved, binuclear refers to a ligand (L) binding to two metal (M) surface sites; if a ligand is bound to only one site, the adsorption complex is termed mononuclear. See figure below.

Figure 1. Illustration of mononuclear and binuclear (inner-sphere)
complexes of adsorbed metal ions on a surface.
From Maurice (2009), with permission.

See binuclear complex.