The magnetic susceptibilities of hexanuclear gadolinium clusters in the compounds Gd(Gd(6)Zl(12)) (Z = Co, Fe, or Mn) and CsGd(Gd(6)Col(12))(2) are reported and subjected to theoretical analysis with the help of density functional theory (DFT) computations. The single-crystal structure of Gd(Gd6CoI12) is reported here as well. We find that the compound with a closed shell of cluster bonding electrons, Gd(Gd6CoI12), exhibits the effects of antiferromagnetic coupling over the entire range of temperatures measured (4-300 K). Clusters with unpaired, delocalized cluster bonding electrons (CBEs) exhibit enhanced susceptibilities consistent with strong ferromagnetic coupling, except at lower temperatures (less than 30 K) where intercluster antiferromagnetic coupling suppresses the susceptibilities. The presence of two unpaired CBEs, as in [Gd6MnI12](3-), yields stronger coupling than when just one unpaired CBE is present, as in [Gd6FeI12](3-) or [Gd6CoI12](2-). DFT calculations on model molecular systems, [Gd6CoI12](OPH3)(6) and [Gd6CoI12](2)(OPH3)(10), indicate that the delocalized cluster bonding electrons are highly effective at mediating intracluster ferromagnetic exchange coupling between the Gd atom 4f(7) moments and that intercluster coupling is expected to be antiferromagnetic. The DFT calculations were used to calculate the relative energies of various 4f(7) spin patterns and form the basis for construction of a simple spin Hamiltonian describing the coupling within the [Gd6CoI12] cluster.