Layered alkali propano­ates M (+)(C(2)H(5)COO)(−); M (+) = Na(+), K(+), Rb(+), Cs(+)

The title alkali propano­ates poly[(μ(5)-propano­ato)alkali(I)], M (+)(C(2)H(5)COO)(−), with alkali/M (+) = Na(+), K(+), Rb(+) and Cs(+), show close structural similarity, which is manifested by the coordination of the cations by six oxygen atoms in a chessboard motif, forming a bilayer. This bilayer is situated between hydro­phobic layers composed of dangling ethyl chains from the carboxyl­ate groups. Stacking of these two-dimensional sandwiches, which are parallel to (001), forms the title structures. Each metal cation is coordinated by six O atoms in the form of a distorted trigonal prism. One pair of these oxygen atoms belongs to a bridging, bidentately coordinating carboxyl­ate anion, while each of the other four oxygen atoms belongs to different carboxyl­ate groups, which are in a bridging monodentate mode. Despite the close similarity, each of the studied alkali propano­ates crystallizes in a different space group. The atoms are in general positions, except for the cation in K(+)(C(2)H(5)COO)(−), which is situated on a mirror plane. Positional disorder of the methyl groups that are disordered over two positions is present in the Na(+) and K(+) propano­ates, in contrast to the Rb(+) and Cs(+) propano­ates. In the Na(+) compound, the occupational parameters of the disordered methyl groups are different compared to the K(+) compound where they are equal. This difference results in doubling of the a unit-cell parameter of the Na(+) compound with respect to that of the K(+) compound, otherwise the structures are homeotypic. In Cs(+) propano­ate, a disorder of the methyl H atoms is observed.