Structures of DMF solvated potassium and sodium salts of [Fe(CO)4]2− and [M2(CO)8]2− (M = Fe, Ru)

► New type of interaction between alkali metal cation and [Fe(CO)4]2− anion. ► New extended two-dimensional structures of potassium salts of [Fe(CO)4]2− anion. ► New extended three-dimensional structures of sodium salts of [Fe(CO)4]2− anion. ► Solvent separated ionic pairs structure of [Na(DMF)6]2[Ru2(CO)8]. ► New type of structure of [Ru2(CO)8]2− anion.