Antiferromagnetic linking

The synthesis, structure, and magnetic properties of an octairon(III) citrate complex, [Feg(/r3-0)2(/i-OH)2(citrate)6(02CMe)2(imidazole)2], have been described. This complex consists of two equivalent tetranuclear units linked by two citrates the Fe ions are antiferromagnetically coupled in a four S = 5/2 spin system. [Feg0(citrate)8(H20)3] has a three-layer citrate-bridged structure. 

Lepidocrocite is paramagnetic at room temperature. The Neel temperature of 77 K is much lower than that of the other iron oxides and is the result of the layer-like structure of this mineral. The sheets of Fe(0,0H)6 octahedra are linked by weak hydrogen bonds, hence magnetic interactions are relatively weak. The saturation hyperfine field is also lower than for any other iron oxide (Tab. 6.2). In the antiferromagnetic state, the spins are ordered parallel to the c-axis with spins in alternate layers having opposite signs. A decrease of T by 5 K was observed for Al-lepidocrocites with an Al/(Fe-i-Al) ratio of 0.1 (De Grave et al., 1995). 

A novel tridentate coordination mode for the carbonato-nickel(II) system was reported by Escuer and co-workers (61). The trinuclear compound [Ni2(p-C03)(dmpd)4(H20)][Ni(dmpd)2(H20)2](C104)4 H20 was obtained from the reaction of basic solutions of nickel(II) perchlorate and 2,2-dimethylpropane-1,3-diamine (dmpd) with atmospheric C02. The three nickel atoms show octahedral coordination with three different environments, №( 0 -0,0 )(dmpd)2, №( 0 -0 ) (dmpd)2(H20) and Ni(dmpd)2(H20)2. The carbonate anion acts as a bridge between two nickel ions whereas the [Ni(dmpd)2(H20)2]2 + subunit is linked by hydrogen bonds to the dinuclear group (Fig. 7). The dinuclear [Ni2(p-C03)(dmpd)4(H20)]2 + subunit shows a moderately weak antiferromagnetic coupling with a J value of —7.8 cm-1. 

AgF252,53 and the HP form of PdF254 are also characterized by buckled layers of MF4 squares, but the bonding of the planes is different from that in the previous case. Instead of resulting from MF6 octahedra linked by comers and edges, the distorted octahedra are only shared by comers. In both compounds, the ferromagnetic layers are coupled antiferromagnetically. 

Jahn-Teller ordering at Cu2+ distorts antiferromagnetic CuF2 similarly to CrF2 (70a). The unpaired dxt-y electron spins are presumably coupled via the shorter 135° cation-anion-cation links. 

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