Two lanthanide-based complexes, LM⁴–UBO–1–Yb and LM⁴–UBO–2–Dy, adopt square antiprismatic geometries. LM⁴–UBO–1–Yb is slightly distorted, while LM⁴–UBO–2–Dy is more rigid. A multivariate analysis of 185 complexes shows that [BW₁₁O₃₉]⁹⁻ and [PW₁₁O₃₉]⁷⁻ stabilize SAPR, while [GeW₁₁O₃₉]⁸⁻ and [SiW₁₁O₃₉]⁸⁻ favor bicapped trigonal prism geometries.

Two novel lanthanide-based complexes, [NBu₄]₃[YbH(PW₁₁O₃₉)(phen)₂]·H₂O (LM ⁴ –UBO–1–Yb) and K₁₃[Dy(SiW₁₁O₃₉)₂]·21H₂O (LM ⁴ –UBO–2–Dy), are synthesized and characterized to explore how Keggin monolacunary polyoxometalates (KMLPOMs) affect Ln^III geometry. SHAPE analysis shows both complexes adopt square antiprismatic (SAPR) geometries with pseudo-D4d symmetry. LM⁴–UBO–1–Yb has a slightly distorted SAPR geometry, with a displacement parameter (D) of 1.068 Å, influenced by organic ligands, while LM ⁴ –UBO–2–Dy is more rigid (D = 1.298 Å). A structural multivariate analysis of 185 KMLPOM-based Ln^III complexes identifies four main types: mononuclear inorganic, mononuclear hybrid, dinuclear hybrid, and dinuclear inorganic. Structural trends reveal that [BW₁₁O₃₉]⁹⁻ and [PW₁₁O₃₉]⁷⁻ prefer SAPR geometries, while [GeW₁₁O₃₉]⁸⁻ and [SiW₁₁O₃₉]⁸⁻ favor bicapped trigonal prism (BTPR) geometries. D values below ≈1.10 Å are linked to BTPR geometries, while higher values align with SAPR geometries. This study highlights the heteroatom's ionic radius as a key factor in determining geometry and offers predictive guidelines for designing Ln^III-KMLPOM systems.