The cobalt(II)–lanthanide(III) heteronuclear complexes [Ln(H 2 O) 4 Co 2 (TTHA)(SCN) 2 ]·H 3 O + (H 6 TTHA = triethylenetetraaminehexaacetic acid, Ln=La(2), Ce(3)) have been synthesized based on a binuclear building block of [Co 2 (H 2 TTHA)(H 2 O) 2 ] in [Co 2 (H 2 TTHA)(H 2 O) 2 ]·4H 2 O (1). Single-crystal structures show that complex 1 is a binuclear complex, containing the [Co 2 (H 2 TTHA)(H 2 O) 2 ] unit as a useful building block. Adding the La 3+ and Ce 3+ ions to this synthesis system, two new 3d-4f mixed complexes 2 and 3 were obtained. Complexes 2 and 3 show 3D framework, comprising an infinite 1D (one-dimensional) chain based on heterometallic Ln 2 Co 2 (Ln=La(2), Ce(3)). Further investigations such as IR spectra, UV–vis spectra, TGA, XRD and magnetic properties were studied.