DFT calculations have been performed with the B3LYP functional using the Gaussian09 suite of programs. The B3LYP functional is well known to provide excellent numerical accuracy in the estimation of magnetic exchange interaction values. Here, we have estimated the pairwise exchange interactions using nine truncated models (Figure ) as the DFT calculation is computationally expensive with complete structure of 1. These models were constructed without altering the first coordination sphere of each pair of Fe III centres, substituting neighbouring Fe III sites with diamagnetic Ga III ions. OH/H2O moieties were employed for the truncated O atoms. Ahlrichs' triple-ξ plus polarisation basis set (TZVP) was used for the Fe, O, N, Br atoms and the split valence plus polarisation (SVP) basis set was used for the Ga, C and H atoms. One high spin (HS, the electrons in the two Fe III centres are "spin up") and one broken symmetry (BS, the electrons in one Fe(III) centre are "spin up", while those in the other are "spin down") configuration were considered for the exchange interaction, estimated using Noodleman's approach (spin-Hamiltonian ( ) and ( )). We have used the formula