Reaction of [Fe3(μ3-O)(O2C(t)Bu)6(HO2C(t)Bu)3](O2C(t)Bu) and [Ln2(O2C(t)Bu)6(HO2C(t)Bu)6] (Ln = lanthanide) with three different phosphonic acids produce a family of highly symmetrical {Fe6Ln6P6} clusters with general formula [Fe6Ln6(μ3-O)2(CO3)(O3PR)6(O2C(t)Bu)18], where R = methyl 1, phenyl 2, or n-hexyl 3. All the clusters present an analogous metal frame to the previously reported {Ni6Ln6P6} both being related to the well-known Wells-Dawson ion from polyoxometallate chemistry. These highly symmetrical clusters have, or approximate very closely to, D3d point symmetry. Both Fe(III) and Gd(III) ions are magnetically isotropic and could thus exhibit promising magnetocaloric properties; hence we investigated the {Fe6Gd6P6} compounds accordingly. Modeling the magnetic data of [Fe6Gd6(μ3-O)2(CO3)(O3PPh)6(O2C(t)Bu)18] by the finite-temperature Lanczos method gave a strong antiferromagnetic Fe···Fe interaction (J(Fe-Fe) = -30 cm(-1)) and very weak Gd···Gd and Gd···Fe exchange interactions (|J| < 0.1 cm(-1)). The strong antiferromagnetic Fe···Fe interaction could account for the relatively smaller -ΔSm value observed, compared against the {Ni6Gd6P6} analogues.