Sequential reaction of a N5O3 octadentate tripodal ligand with Ni2+ and subsequently with Cu2+ and azide ligand afforded the first example of a heterobridged (phenoxo/mu(1,1)-azido) pentanuclear heterometallic (Ni4Cu) compound, which exhibits a centrosymmetric vertex-sharing defective double-cubane structure. The study of the magnetic properties reveals that the compound shows ferromagnetic interaction interactions, leading to an S = 9/2 spin ground state. Density functional theory calculations on the X-ray structure and model compounds predict ferromagnetic interactions through the magnetic exchange pathways involving each couple of metal ions.