Polymeric two- and three-dimensional, homo- and heterometallic oxalatebridged coordination compounds offer exciting opportunities, mainly in the fields of molecular magnetism and photophysics. Given that a large variety of magnetic phenomena have been reported so far from these molecular magnets, very limited experience is gained from elastic neutron scattering experiments. Therefore, with two examples, we will address the topic of the elucidation of magnetic structures by means of the neutron scattering technique. In addition, due to the possibility of the variation of different metal ions in varying oxidation states, interesting photophysical processes can be observed within the extended three-dimensional host/guest systems.

Based on a synthetic strategy, extended anionic, homo and bimetallic oxalato-bridged transition-metal compounds with two (2D) and three-dimensional (3D) connectivities can be synthesized and crystallized. Thereby, the choice of the templating counterions will determine the crystal chemistry. Since the oxalato bridge is a mediator for both antiferro and ferromagnetic interactions between similar and dissimilar metal ions, long-range magnetic ordering will occur. Examples of the determination of magnetic structures in 2D and 3D compounds by means of elastic neutron scattering methods will be discussed. In addition, due to the possibility of the variation of different metal ions in varying oxidation states, interesting photophysical processes can be observed within the extended three-dimensional host/guest systems.

In analogy to the [M^II(bpy)₃]²⁺ cations, where M^II is a divalent transition-metal and bpy is 2,2‘-bipyridine, the tris-chelated [M^III(bpy)₃]³⁺ cations, where M^III is Cr^III or Co^III, induce the crystallization of chiral, anionic three-dimensional (3D) coordination polymers of oxalate-bridged (μ-ox) metal complexes with stoichiometries [M^II₂(ox)₃]_n^2n- or [M^IM^III(ox)₃]_n^2n-. The tripositive charge is partially compensated by inclusion of additional complex anions like ClO₄^-, BF₄^-, or PF₆^- which are encapsulated in cubic shaped cavities formed by the bipyridine ligands of the cations. Thus, an elaborate structure of cationic and anionic species within a polymeric anionic network is realized. The compounds isolated and structurally characterized include [Cr^III(bpy)₃][ClO₄] [NaCr^III(ox)₃] (1), [Cr^III(bpy)₃][ClO₄][Mn^II₂(ox)₃] (2), [Cr^III(bpy)₃][BF₄] [Mn^II₂(ox)₃] (3), [Co^III(bpy)₃][PF₆][NaCr^III(ox)₃] (4). Crystal data:  1, cubic, P2₁3, a = 15.523(4) Ã…, Z = 4; 2, cubic, P4₁32, a = 15.564(3) Ã…, Z = 4; 3, cubic, P4₁32, a = 15.553(3) Ã…, Z = 4; 4, cubic, P2₁3, a= 15.515(3) Ã…, Z = 4. Furthermore, it seemed likely that 1,2-dithiooxalate (dto) could act as an alternative to the oxalate bridging ligand, and as a result the compound [Ni^II(phen)₃][NaCo^III(dto)₃]·C₃H₆O (5) has successfully been isolated and structurally characterized. Crystal data:  5, orthorhombic, P2₁2₁2₁, a = 16.238(4) Ã…, b = 16.225(4) Ã…, c = 18.371(5) Ã…, Z = 4. In addition, the photophysical properties of compound 1 have been investigated in detail. In single crystal absorption spectra of [Cr^III(bpy)₃][ClO₄][NaCr^III(ox)₃] (1), the spin−flip transitions of both the [Cr(bpy)₃]³⁺ and the [Cr(ox)₃]^3- chromophores are observed and can be clearly distinguished. Irradiating into the spin-allowed ⁴A₂ → ⁴T₂absorption band of [Cr(ox)₃]^3- results in intense luminescence from the ²E state of [Cr(bpy)₃]³⁺as a result of rapid energy transfer processes.

The mixed-metal ferromagnet {[P(Ph)4][MnCr(ox)3]}n, where Ph is phenyl and ox is oxalate, has been prepared and a two-dimensional network structure, extended by Mn(II)-ox-Cr(III) bridges, has been determined from single crystal X-ray data. Crystal data: space group R3c, a=b=18.783(3), c=57.283(24) Å, α=β=90, γ=120°, Z=24 (C30H20O12PCrMn). The magnetic susceptibility data obey the Curie-Weiss law in the temperature range 260–20 K with a positive Weiss constant of 10.5 K. The temperature dependence of the molar magnetization exhibits a magnetic phase transition at Tc=5.9 K. The structure is discussed in relation to the strategy for preparing molecular based ferromagnets and, in addition, it is a solution to the question of the dimensionality of the [MM'(ox)3]n network, which in principle can extend two- or three-dimensionally to the crystal lattice. The optical absorption spectra of the single crystals are assigned to the ‘CrO6' chromophores. Their polarization patterns reflect the electric dipole selection rules for D3 symmetry. A strong site selective luminescence from the chromium(III) 2E states is observed at low temperature and the system may be suitable for studying energy transfer mechanisms.