AbstractComplex transition‐metal oxides exhibit a wide variety of chemical and physical properties which are a strong function the local electronic states of the transition‐metal centres, as determined by a combination of metal oxidation state and local coordination environment. Topochemical reduction of the double perovskite oxide, LaSrCoRuO₆, using Zr, yields LaSrCoRuO₅. This reduced phase contains an ordered array of apex‐linked square‐based pyramidal Ru³⁺O₅, square‐planar Co¹⁺O₄ and octahedral Co³⁺O₆ units, consistent with the coordination‐geometry driven disproportionation of Co²⁺. Coordination‐geometry driven disproportionation of d⁷ transition‐metal cations (e.g. Rh²⁺, Pd³⁺, Pt³⁺) is common in complex oxides containing 4d and 5d metals. However, the weak ligand field experienced by a 3d transition‐metal such as cobalt leads to the expectation that d⁷⁺ Co²⁺ should be stable to disproportionation in oxide environments, so the presence of Co¹⁺O₄ and Co³⁺O₆ units in LaSrCoRuO₅ is surprising. Low‐temperature measurements indicate LaSrCoRuO₅ adopts a ferromagnetically ordered state below 120 K due to couplings between S=¹/₂ Ru³⁺ and S=1 Co¹⁺.