The reaction of two different carboranylcarboxylate ligands, 1‐CH3‐2‐CO2H‐1,2‐closo‐C2B10H10 or 1‐CO2H‐1,2‐closo‐C2B10H11, with MnCO3 in water leads to polymeric compounds 1 a and 1 b. Both compounds have been characterized by analytical and spectroscopic techniques. Additionally, electrochemical techniques have also been used for compound 1 a. X‐ray analysis revealed substantial differences between both compounds: whereas a six‐coordinated MnII compound with water molecules bridging two MnII centers has been observed for 1 a, a square pyramidal geometry around each MnII ion with terminal water molecules coordinated to each MnII center has been found for 1 b. The observed differences have been attributed to the existence of different substituents, CH3 or H, on one of the carbon atoms of the carboranylcarboxylate ligand. The reaction of 1 a and 1 b with coordinating solvents, such as ethers or Lewis bases, leads to the formation of new compounds with low (mononuclear 4 a, 4 b; dinuclear 3 a, 3 b; and trinuclear 2 a) or high nuclearity (hybrid polymer, 5 a), due to breakage of the corresponding polymer. X‐ray analysis shows that the structural core present in the polymeric materials is not maintained in the resulting compounds, with the exception of trinuclear compound 2 a. The magnetic properties of the compounds studied show weak antiferromagnetic coupling.