Bonding Scheme, Hydride Character, and Magnetic Paths of (HPO₃)^2– Versus (SeO₃)^2– Building Units in Solids

The abilities of the (HPO₃)^2– and (SeO₃)^2– anions as structure building units and as spin exchange paths between magnetic ions were investigated by preparing and analyzing the isostructural Fe₂(SeO₃)₃ and Fe₂(HPO₃)₃. In both compounds, the face-sharing Fe₂O₉ dimers are interconnected into chains by the (HPO₃)^2– and (SeO₃)^2– anions. The (HPO₃)^2– is the structural counterpart of the Se electron lone pair of (SeO₃)^2– due to the weak hydride character of the terminal hydrogen. However, they differ considerably as spin exchange paths between magnetic cations. Both compounds exhibit an effective magnetic dimer behavior, unexpectedly arising from the interdimer FeO···OFe exchange along the chain, but weaker in Fe₂(HPO₃)₃ by a factor of ∼3. It is consistent with the general tendencies of the phosphite anions to act as a weak magnetic mediator, which is caused by the through-bond effect of the P³⁺ ion in the FeO···P³⁺···OFe exchange path, much weaker than in the selenite phase in absence of P³⁺d contribution. Reasons for stronger exchanges through phosphates or sulfates are also discussed.