Novel organometallic building blocks for crystal engineering. Synthesis and structural characterization of the dicarboxylic acid [Cr-0(eta(6)-C6H5COOH)(2)], of two polymorphs of its oxidation derivative [Cr-I(eta(6)-C6H5COOH)(2)](+)[PF6](-), and of the zwitterionic form [Cr-I(eta(6)-C6H5COOH)(eta(6)-C6H5COO)]

The synthesis and Structural characterization of the neutral diamagnetic or ganometallic dicarboxylic acid [Cr-0(eta (6)-C6H5COOH)(2)] (1) is reported. Oxidation of 1 by oxygen leads to formation of the paramagnetic dicarboxyl ic acid cation [Cr-I(eta (6)-C6H5COOH)(2)](+) (2), which has been isolated as the [PF6](-) salt in two crystalline polymorphic modifications, a monocl inic form alpha and a triclinic form beta. The neutral zwitterionic form [C rI(eta (6)-C6H5COOH)(eta (6)-C6H5-COOH] (3) has been obtained in a cocrysta l with [NH4][PF6] upon treatment of the acid salt [Cr-I(eta (6)-C6H5COOH)(2 )] + [PF6]-with ammonia. The hydrogen bonding interactions established by t he neutral and cationic acids are discussed and compared with those observe d in the zwitterionic form and in the related complexes Fe-II(eta5-C5H4COOH )(2)] and [Co-III(eta (5)-C5H4-COOH)(2)+[PF6](-).