Cover illustration: The unit-cell packing in ethylenediammonium catena-arsenite, projected on to (010) (normal to the catena-arsenite chain direction). Hydrogen bonds are indicated by dashed lines. See Lee & Harrison [Acta Cryst. (2004), C60, m215-m218].
The structures of the title compounds consist of layers of alkali cations and H2PO2− hypophosphite anions, with the latter bridging four cations within the same layer. The hypophosphite anion has a distorted tetrahedral arrangement, with the P atom at the centre and two O and two H atoms at the vertices.