The new potentially tetradentate ligand precursor 2-[(bis(2-hydroxyethyl)amino)methyl]-4,6-bis-tert-butylphenol (H₃L) has been synthesized and characterized. The reaction of H₃L with cobalt(ii) acetate has afforded the novel mixed-valent tetra- and pentanuclear cobalt complexes [Co^II₂Co^III₂(OAc)₂(L)₂(HL)] (1) and [Co^IICo^III₄(OAc)₂(L)₄] (2). Single-crystal X-ray diffraction studies of these complexes indicate different coordination geometries for the divalent cobalt centres in each complex, with distorted trigonal bipyramidal and distorted tetrahedral coordination evident in 1 and 2, respectively. The variable temperature magnetic susceptibility data for complex 1 reveal behaviour dominated by antiferromagnetic coupling between the two cobalt(ii) centres. Their approximate trigonal bipyramidal coordination geometries give rise to a ⁴A′₂ ground term, allowing a spin-only treatment assuming local spin quantum numbers of S_i = 3/2. Fitting the data to the Heisenberg exchange Hamiltonian (Ĥ_ex = –2JS·₁S₂) results in J = –6.9(1) cm^–1 and g = 2.30(5).