The synthesis, structures and magnetic properties of a family of lanthanide complexes containing phosphonate ligands are reported. Reaction of hydrated lanthanide nitrate and ^tbutylphosphonic acid under reflux conditions in iso-butanol, in the presence of pivalic acid as a co-ligand produced five new lanthanide complexes; pyridine (py) was present as a base. The compounds formed are tetrametallic, with the general formula [pyH]₄[Ln₄(μ₃-OH)(O₃P^tBu)₃(HO₃P^tBu)(O₂C^tBu)₂(NO₃)₆] where Ln = Gd^III, 1; Tb^III, 2; Dy^III, 3; Ho^III, 4 and Er^III, 5. The metal sites within the complexes lie on the vertices of a triangle-based pyramid, with phosphonate ligands on the triangular faces linking the apical Ln site to the Ln sites in the base. Each lanthanide(III) site is eight-coordinate. Magnetic studies of the compounds show a decline in the product χ_MT with T; modelling the behaviour of 1 shows anti-ferromagnetic exchange between Gd^III centres within the triangle with a negligible interaction to the fourth Gd^III centre at the apex of the trigonal pyramid.