If Fe₁₂O₁₂ is neutral, binding Cd²⁺ gives a +2 complex. Cd²⁺ is closed-shell (d¹⁰, S=0), so the multiplicity comes from Fe centres. In practice, Fe–O cages are usually antiferromagnetic, so the lowest state is modelled as a singlet (multiplicity = 1).

As usual in life, things are a little more complicated. 1st of all, what do you consider a Fe12O12 nanocage? What is its multiplicity and structure? And, how do you know? For the structures of gas-phase metal oxide clusters and how they change upon metal doping, addition or replacement you may look up the work of K.R. Asmis over the last ~20 years. This will also show you that there is no simple answer to be expected if your input is just "gaussian calculation". Prediction can strongly depend on the theoretical method. In many cases you will need the experiment to decide what theoretical method actually gives a realistic picture.