Universal `bond valence versus bond length' correlation curve for manganese-oxygen bonds

The oxidation-state independent `bond valence (s) versus bond length (r)' correlation curve for manganese-oxygen bonds has been closely approximated using the modified two-parameter Trömel s = f(r) function [Trömel (1983). Acta Cryst. B39, 664-669], s = [(r₀ - l)/(r - l)]², where r₀ = 1.763 (2) Å and l = 1.148 (9) Å. The r₀ and l refinable parameters of the above function can be regarded as the alternative bond-valence parameters intended for use in the modern bond-valence model [Brown (2009). Chem. Rev. 109, 6858-6919] in cases where the traditional bond-valence parameters (r₀; n) and (r₀; b) fail.