All reagents for the syntheses were purchased from commercial sources and used as received, except for the starting potassium salt, K₃[Cr(C₂O₄)₃]・3H₂O, which was synthesized according to the literature [17] . Micro- analyses (carbon and hydrogen) were performed on a Vario EL (Heraeus) analyzer. The FT-IR spectrum was recorded from KBr pellets in the range of 4000 - 400 cm⁻¹ on a Perkin-Elmer spectrometer. The UV-Vis spectrum was obtained on a Perkin-Elmer Lambda 900 spectrophotometer in water at room temperature within the range 300 - 700 nm. The chemical analysis was obtained using Energy Dispersive X-ray Spectroscopy (EDX), with a JDX-7000F X-ray spectrometer. TGA experiments were performed with a Mettler Toledo TGA/ SDTA 851 thermal analyzer, heated from room temperature to 600˚C under nitrogen gas with a heating rate of 10˚C/min. The melting points were measured using an Electrothermal 9100 apparatus.

The title compound was obtained by metathesis from the potassium salt as follows: K₃[Cr(C₂O₄)₃]・3H₂O (0.98 g, 2 mmol) was dissolved in 50 mL of water. Simultaneously, an aqueous solution (30 mL) of SrCl₂・6H₂O (0.80 g, 3 mmol) was prepared and subsequently added in successive small portions into the K₃[Cr(C₂O₄)₃]・3H₂O one. The resulting violet mixture was stirred at 333 K for 1 hour and then filtered off. The filtrate was allowed to evaporate at room temperature. After a few weeks, violet needle crystals suitable for X-ray diffraction were isolated by filtration, and dried in air. Yield: 0.82 g (85.5%). M. p.: > 240˚C. Anal. Calcd. for C₆H₄CrKO₁₄Sr (478.81 g・mol⁻¹): C, 15.05; H, 0.84%. Found: C, 14.96; H, 0.79%. IR (KBr disk, cm⁻¹): ν = 3449 (w), 1708 (m), 1672 (s), 1419 (s), 1277 (s), 907 (w), 853 (s), 806 (s), 543 (s), 477 (s). UV-Vis (H₂O solution, nm): 417, 569.

A violet crystal with approximate dimensions 0.281 × 0.085 × 0.038 mm³ was selected under ambient conditions and attached to the tip of a nylon loop. It was mounted in a stream of cold nitrogen at 100 (1) K and cen- tered in the X-ray beam by using a video camera. The crystal evaluation and data collection were performed on a Bruker QuazarSMART APEXII diffractometer with MoK_α (λ = 0.71073 Å) radiation. The program SMART [18] was used for collecting frames of data, indexing reflections, and determining lattice parameters, SAINT [18] for integration of the intensity of reflections and scaling, SADABS [19] for absorption corrections, and SHELXTL [20] [21] for space group and structure determination and least-squares refinements on F². The structure was solved by direct methods using the program SHELXS-97 [22] and refined by full-matrix least-squares methods against F² with SHELXL-97 [22] . All non-hydrogen atoms were refined with anisotropic displacement coeffi- cients. All hydrogen atoms were included in the structure factor calculation at idealized positions and were al- lowed to ride on the neighboring atoms with relative isotropic displacement coefficients. The figures have been generated using Diamond 3.1e software [23] . Crystallographic data and refinement parameters are listed in Ta- ble 1. Selected bond distances and bond angles are given in Table 2.

Symmetry transformations used to generate equivalent atoms: ¹1/4 + X, 5/4 − Y, 1/4 + Z; ²1/4 + X, 5/4 − Y, −3/4 + Z; ³3/4 − X, 1/4 + Y, −1/4 + Z; ⁴1 − X, 1 − Y, +Z; ⁵1/2 + X, +Y, 1/2 + Z.

Our original attempt aimed at synthesizing the heterometallic neutral polynuclear complex, {Sr₃[Cr^III(C₂O₄)₃]₂}・nH₂O with the ionic ratio of 3Sr^II vs. 2Cr^III, analog of the well-documented {Ba₆(H₂O)₁₇[M^III(C₂O₄)₃]₄}・7H₂O (M = Cr; Fe) compounds [10] [11] . Unexpectedly, the tripotassium tris(oxalato)chromate(III) salt, K₃[Cr(C₂O₄)₃]・3H₂O, exchanged only two of its three potassium ions for one strontium(II) ion, yielding the air stable heterometallic neutral trinuclear complex, {[(H₂O)₂K(µ-H₂O)Sr]@[Cr(C₂O₄)₃]}_n (1). The competition between K⁺ and Sr²⁺ ca- tions appears as an important point to prevent formation of the expected Sr(II)/Cr(III)-oxalate coordination po- lymer, in line with the identical coordination number (8) of K⁺ and of Sr²⁺ ions in the title compound. Using Ba²⁺ in lieu of Sr²⁺ ions, a total exchange of three potassium cations was observed, demonstrating, therefore, a more pro- nounced affinity of Ba²⁺ ions to bind tris(oxalato)metalate(III) anions [10] [11] . Efforts were taken to better un- derstand the formation of 1 by carrying out two other syntheses with the K₃[Cr(C₂O₄)₃]・3H₂O:SrCl₂ stoichiome- tric ratios of 1:1 and 1:2. As expected, the same compound 1 was obtained, different only in purity and yield, the maximum yield being 89 % with the 1:1 ratio. From the above results, it is evident that, irrespective to the stoi- chiometric ratio of reagents, the title compound is readily formed if K₃[Cr(C₂O₄)₃]・3H₂O and SrCl₂ are combi- ned. The possible formation mechanisms producing compound 1 are summarized in Scheme 1.

Results from the Energy Dispersive X-ray Spectroscopy (EDX) analyses of 1 are shown in Figure 1. It is observed in this EDX spectrum the presence of chemical elements O, K, Sr and Cr, suggesting clearly the success in the preparation of compound 1.

The IR spectrum exhibits weak absorption bands centered at 3449 cm⁻¹ and attributable to the well-known ν_O-H vibrations of the H₂O molecules that are coordinated to the K⁺ and Sr²⁺ sites. The band at 1700 cm⁻¹ is assigned to ν_C=O whereas those at 1419 cm⁻¹ are attributed to ν_O-C=O [24] . The sharp peaks at 1277 cm⁻¹ and 907 cm⁻¹ may be tentatively assigned to the ν_C-O and ν_C-C vibrations [24] , respectively. Medium to weak bands appearing in the region below 600 cm⁻¹ may be attributed to vibrations within the coordination spheres around the metallic centers. These results are, in fact, consistent with the presence of [Cr(C₂O₄)₃]³⁻ and H₂O in the material concerned.

The ultraviolet-visible spectrum in aqueous solution showed peaks at 417 and 569 nm attributed to the spin-al- lowed ⁴A_2g(F) → ⁴T_1g(F) and ⁴A_2g(F) → ⁴T_2g(F) transitions, respectively, within the octahedral complex anion [Cr(C₂O₄)₃]³⁻ contained in 1. It is obvious that the present electronic absorption spectrum is virtually superimposable with that reported [10] [12] [13] [25] since the spectral information thus obtained solely relate to the [Cr(C₂O₄)₃]³⁻ species.

The thermal behavior of 1 has been studied by thermogravimetric analysis (TGA) and differential thermal analysis (DTA) in the temperature range of 25˚C to 600˚C at a heating rate of 10˚C per minute under nitrogen atmosphere. TGA and DTA curves are depicted in Figure 2. The overall TGA curve consists of two-step weight loss process. The first step between 80˚C and 150˚C corresponds to a weight loss of 11.4% (calc. 11.3%) which is due to release of the three coordinated water molecules. During the second step, beyond 150˚C, a gradual decomposition of the crystal network takes place with formation of a final residue which is proved to be a mixture of SrCrO₄ and K₂CO₃ compounds.

The molecular structure of 1 has been determined by X-ray study, which shows that it crystallizes in orthorhom- bic chiral space group Fdd2 with three-dimensinal arrangement. As depicted in Figure 3, the asymmetric unit of 1 consists of one cationic motif-formally [(H₂O)₂K(µ-H₂O)Sr]³⁺, and one anionic [Cr(C₂O₄)₃]³⁻ entity. The anionic building stone, [Cr(C₂O₄)₃]³⁻, functions as a metalloligand or else, as an internetting bridge. Hence, this internetting bridge interconnects, across its O atoms, the independent sites K, Sr1 and Cr1 into a three-dimensional polymeric lattice network.

Two independent K centers (K1 and K2) are eight-coordinate both by H₂O and oxalate ligands and are present 50% in the asymmetric unit. Atom K1 is coordinated to two water molecules (OW13 and OW14). Atom K2 is coordinated to two water molecules (OW14 and OW15). The water molecules OW13 and O14W bridge the K1 and Sr1 centers and the K1 and K2 centers, respectively. The water molecule OW15 belongs solely to the K2 center. A molecular drawing of compound 1 with complete coordination spheres of the metal atoms is shown in Figure 4 and selected geometrical parameters are presented in Table 2. As can be seen from Figure 4, the eighth coordination of the K2 center is completed by an unusual short K2 - K2⁴ metallic bond of 2.214 (4) Å. All the K-O bond lengths fit well within the range of previous results [26] .

The Sr1 site experiences also eight-coordinate coordination sphere, involving one chelation via the “external” O atoms (O6,O8) and to six monodentate linkages, i.e. one across an O atom of water molecule (OW13) and five across “external” O atoms (O2, O4, O4¹, O10, O12). Taking into account the coordination of aqua ligands to K and Sr centers, the cationic motif in the asymmetric unit of 1 can be formulated [(OW15)K2_1/2(µ-OW14)K1_1/2(µ-OW13)Sr1]³⁺ or simply [(H₂O)₂K(µ-H₂O)Sr]³⁺.

The Cr1 center is exclusively coordinated by oxalates to produce the well-established helical coordination geometry of [Cr(C₂O₄)₃]³⁻ ions. This coordination to chromium occurs across the six “internal” O atoms (O1, O3, O5, O7, O9, O11), giving rise to Cr1-O bond lengths ranging from 1.971 (3) to 1.983 (6) Å and O-Cr1-O bond angles ranging from 82.18 (14) to 172.34 (14). These bond lengths and bond angles are in good agreement with those found in other tris(oxalato)metalate(III) complexes [10] -[15] [26] .

The packing diagram of compound 1 viewed down the crystallographic c-axis is shown in Figure 5. The bulk structure of 1 is consolidated by coulombic interactions between the ionic motifs and a three-dimensional network of hydrogen bonding of the type O-H・・・O, with O・・・O distances ranging from 2.626 (14) to 2.898 (8) Å (Table 3). It should be noted that, to the best of our knowledge, compound 1 constitutes the first chiral, non-hydrated representative of oxalate-based coordination polymers with general formula {M(I)/M(II)/M^III(C₂O₄)₃}・nH₂O.