Crystal structures of potassium, ammonium, rubidium, and cesium tetrafluoborates

The crystal structures of potassium, ammonium, and rubidium tetrafluoborates have been redetermined. The structure of cesium tetrafluoborate has also been established. These 4 compounds are isomorphous and show orthorhombic symmetry, space group Pbnm. Atomic parameters have been obtained from a block diagonal least squares refinement using anisotropic temperature parameters. The final agreement residuals for observed reflections were KBF 4 , R = 0.072; NH 4 BF 4 , R = 0.075; RbBF 4 , R = 0.064; and CsBF 4 , R = 0.051.There are significant differences between some of the B—F bond distances in the [BF 4 ] − ions. The shortest N—(H)F distance in NH 4 BF 4 is 2.914 Å (estimated standard deviation 0.005 Å) which is long for a hydrogen bond. The positions of the hydrogen atoms in the NH 4 + ion could not be established.