We successfully synthesized a novel fluorinated polyoxomolybdate, K5Mo4O14F, in which the unusual polyanion [Mo4O14F](5-) consists of face-sharing [Mo2O8F] bioctahedra linked with [MoO4] tetrahedra. This unique structural feature provides the very rare case that simultaneously violates Pauling's electrostatic valence (II) and atomic coordination (IV) rules, as well as the stable tendency governed by the polyhedral sharing (III) rule. The structural stability of K5Mo4O14F was confirmed from thermal experiments over a wide temperature range and further elucidated by first-principles calculations.