Co[(CH
3PO
3)(H
2O)] (
1) and Co[(C
2H
5PO
3)(H
2O)] (
2) were prepared by the hydrothermal method and isolated as blue-violet platelet crystals. They were characterized by X-ray diffraction, FT-IR, TGA-DSC techniques and their magnetic properties studied by a dc-SQUID magnetometer. Compound (
1) shows an hybrid layered structure, made of alternating inorganic and organic layers along the
a-direction of the unit cell. The inorganic layers contain Co(II) ions six-coordinated by five phosphonate oxygen atoms and one from the water molecule. These layers are separated by bi-layers of methyl groups and van der Waals contacts are established between them. In compound (
2), the layered hybrid structure is rather similar to that described for compound (
1), but the alternation of the inorganic and organic layers is along the
b-direction of the unit cell. The magnetic behavior of (
1) and (
2) as function of temperature and magnetic field was studied. The compounds obey the Curie-Weiss law at temperatures above 100 K, the Curie
C, and Weiss
θ constants for the methyl derivative being and and for the ethyl derivative and , respectively. The observed magnetic moments for Co atom at room temperature (i.e.
μeff=5.18 and 5.38 BM, respectively) are higher than those expected for a spin-only value for high spin Co(II) (
S=3/2), revealing a substantial orbital contribution to the magnetic moment. The negative values of
θ are an indication of the presence of antiferromagnetic exchange couplings between the near-neighbors Co(II) ions, within the layers. [Co(C
nH
2n+1PO
3)(H
2O)] (
n=1,2) are 2D Ising antiferromagnets at low temperatures.
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