A series of new manganese(II) pivalate–phosphonate complexes with various nuclearities (Mn₄, Mn₁₀, and Mn₂₀) under aerobic conditions was synthesized by self-assembly of simple reagents. The constitution of polynuclear architectures is affected by the reaction temperature. The structures of the complexes were determined by single crystal X-ray analysis. Magnetic behavior of the compounds points to dominant antiferromagnetic exchange interactions. Antiferromagnetic coupling with J_Mn–Mn = −0.097(1) cm⁻¹, g_Mn = 1.993(1) for an equilateral tetrahedron model simulates the magnetic behavior of the tetranuclear complex Mn₄ with a cube-like core.