The magnetic properties of ore-bridged oxo-(carboxylato)-bridged and oxo-bis(carboxylato)-bridged iron(III) dimers are found to depend on the iron-(mu-O) distance as well as on the iron-(mu-O)-iron angle. With an angular and radial overlap model we account for both these dependencies. The use of an angular overlap model allows us to separate the sigma donor properties of oxide as a ligand into the s sigma and p sigma contributions.