Crystal structure non-rigidity of central atoms for Mn(II), Fe(II), Fe(III), Co(II), Co(III), Ni(II), Cu(II) and Zn(II) complexes.

The ability of coordination polyhedra of different central atoms in the solid state to undergo distortions has often been discussed. These distortions are also correlated with the nature of the central atom. This paper deals with the conception of crystal structure non-rigidity for central atoms of some selected complexes by the using of statistical distributions of interatomic distances central atom-nearest ligand atom (M-L; M=Mn(II), Fe(II), Fe(III), Co(II), Co(III), Ni(II), Cu(II) and Zn(II)). Based on the vector equilibrium principle the dispersion of these empirical distributions has been introduced as a measure of non-rigidity of central atoms, decreasing in the order: Cu(II) > Fe(II)HS > Mn(II)HS > Ni(II)LS > Zn(II) > Fe(III)HS ≈ Co(II)HS > Ni(II)HS > Co(III)LS, where LS and HS denote low-spin and high-spin states, respectively. Starting from the distribution of M-L distances for certain types of central and ligand atoms, the contribution of atoms of the inner coordination sphere to the vector equilibrium of crystal structures is discussed. The structures of complexes with all M-L bond lengths of at least one symmetrically independent central atom in the interval of maximum frequency exhibit in the appropriate set the most stable inner coordination sphere. These compounds are classified both with respect to the geometry of their inner coordination sphere fulfilling the above condition and to their crystal structure aspect. The square-planar geometry is found to be the most stable for Cu(II) and Ni(II)LS complexes, tetragonal-pyramidal for Zn(II) complexes, while for Fe(II)HS, Mn(II)HS, Fe(III)HS, Co(II)HS, Ni(II)HS and Co(III)LS complexes it is the regular octahedral geometry. Except for one Mn(II)HS complex, in the discussed structures chromophores with the most stable geometry of the coordination polyhedron form part of the final relatively isolated structural unit (island) with expressively chemical bonds between atoms. These facts are illustrated by the stabilization effected by the crystal field and by the properties of the ligands. [ABSTRACT FROM AUTHOR]