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 center dot center dot center dot O-Fe exchange along the chain, but weaker in Fe2(HPO3)(3) by a factor of similar to 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 center dot center dot center dot P3+center do