Anions Derived from Squaric Acid Form Interionic π‐Stack and Layered, Hydrogen‐Bonded Superstructures with Organometallic Sandwich Cations: The Magnetic Behaviour of Crystalline [( η 6 ‐C 6 H 6 ) 2 Cr] + [HC 4 O 4 ] −

Depending on the stoichiometric ratio, squaric acid (3,4‐dihydroxy‐3‐cyclobutene‐1,2‐dione, H 2 SQA) reacts with [( η 6 ‐C 6 H 6 ) 2 Cr] in THF to form the crystalline material [( η 6 ‐C 6 H 6 ) 2 Cr][HSQA] ( 1 ) and in water to yield [{( η 6 ‐C 6 H 6 ) 2 Cr} 2 ][SQA]⋅6 H 2 O ( 3 ); it also reacts with [( η 5 ‐C 5 H 5 ) 2 Co][OH] in water to form [{( η 5 ‐C 5 H 5 ) 2 Co} 2 ][SQA]⋅6 H 2 O ( 4 ). Compound 1 is almost isostructural with the previously reported salt [( η 5 ‐C 5 H 5 ) 2 Co][HSQA] ( 2 ); its structure is based on π‐π stacks between the benzene ligands and the hydrogen squarate anionic chains (π‐π distance 3.375 Å). Compounds 3 and 4 are isomorphous and have a structure in which layers of organometallic cations intercalate with layers of water molecules hydrogen bonded to squarate dianions. All crystals contain charge‐assisted C−H δ + ⋅⋅⋅O δ − hydrogen bonds between the organometallic and the organic components, while negative O−H (−) ⋅⋅⋅O (−) and O−H⋅⋅⋅O (2−) interactions are present in the pairs 1 / 3 and 2 / 4 , respectively. In constrast to most organic salts of [( η 6 ‐C 6 H 6 ) 2 Cr] + and [( η 5 ‐C 5 H 5 ) 2 Co] + which are yellow, crystals of compounds 1 – 4 are orange. Reflectance spectra measured on the crystalline material 1 show the presence of an intense tail that can be assigned to a charge‐transfer transition through the [( η 6 ‐C 6 H 6 ) 2 Cr] + /[HSQA] − π‐stacking interactions, while the π stacking in 2 causes only a broadening of the band. The magnetic behaviour of 1 and 3 has been investigated by SQUID magnetometry. Both compounds are characterised by a weak antiferromagnetic interaction between the S =1/2 Cr centres of the [( η 6 ‐C 6 H 6 ) 2 Cr] + cations, which is significantly stronger in 1 due to the π‐stacking with the HSQA − anions.