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Bis(μ-carboxylato-O,O′)-diaquabis(1,10-phenanthroline)dicopper(II) dinitrate tetrahydrates, [Cu(RCOO)(phen)(H2O)]2(NO3)2·4H2O, where R=H, CH3, and (CH3)3C, were prepared and characterized by elemental analyses, electronic spectra, magnetic susceptibilities, and X-ray structure analysis. The magnetic susceptibility data conform to the usual dimer equation. The −2J values are 125 cm−1 for formate, 86 cm−1 for acetate, and 99 cm−1 for 2,2-dimethylpropanoate. The crystal structures of [Cu(HCOO)(phen)(H2O)]2(NO3)2·4H2O (1) and [Cu(CH3COO)(phen)(H2O)]2(NO3)2·4H2O (2) were determined by the single-crystal X-ray diffraction method. The crystallographic data are: Compound 1, monoclinic, I2⁄a, a=18.850(2), b=9.775(2), c=17.752(2) Å, β=99.03(1)°, V=3230.4(8) Å3, Z=4, R=0.055 for 2511 observed unique reflections; Compound 2, monoclinic, C2⁄c, a=18.563(4), b=14.272(5), c=14.126(3) Å, β=106.72(2)°, V=3584(2) Å3, Z=4, R=0.082 for 2092 reflections. The complexes consist of dimeric [Cu(RCOO)(phen)(H2O)]22+)] cations with five-coordinated copper ions linked by two carboxylato ions in a syn-syn bridging arrangement. The Cu···Cu distances are 3.103(2) Å for 1 and 3.063(3) Å for 2.

The structures of six dimeric copper(II) formates and acetates with picoline as an axial ligand, (I)-(VI), have been determined at room temperature and electron density distributions in lithium acetate dihydrate, (VII), and copper(II) formate diurea dihydrate, (VIII), have been studied at 120K. (VI) show no important geometrical differences in the Cu2(COO)4 cage between the copper acetates and formates. The observed deformation-density distributions in the CH3COO-(VII) and HCOO-(VIII) ions are in accord with theoretical expectations. Mulliken population analysis of eight carboxylate ions using STO-6G basis sets indicates that the electron population of the carboxylate group of the bridging ligand correlates with the -2J values of the binuclear copper(II) carboxylates. The key point that determines the strength of the spin-exchange interaction is the 2px orbital population of the carboxylate C atom (x is parallel to the C--R bond axis in the RCOO-ion). The electron population on a carboxylate O atom is shown to exhibit a roughly linear correlation with the pKa value of the parent carboxylic acid. The conundrum of why formate and fluoroacetate are exceptions to the correlation of -2J with pKa value has been solved in this paper.

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