Two new isostructural phosphine coordination materials, Ln-PCM-21 (Ln = Pr, Nd), have been obtained using a tris(p-carboxylated) methyltriphenylphosphonium ligand that is formally dianionic when triply deprotonated, allowing access to materials based on uncommon metal-to-ligand ratios. The polymers of the formula [Ln(3)(mptbc)(4)]X center dot solv (X = Cl, NO3) are cationic and contain unusual, linear oxo-bridged [Ln(3)](9+) clusters. Magnetic susceptibility data for both the Pr and Nd analogues has been compared to models based on three contrasting approaches.