1984 Volume 57 Issue 6 Pages 1556-1561
X-Ray data of 16 structures having coordination geometry of the trans-MX2N4 type show that (i) an out-of-plane coordination bond length decreases as an in-plane distance increases, (ii) the degree of the correlation is metal ion dependent and varies in the order, Zn2+>Ni2+>Co3+. In the Ni(II) systems, complexes having stronger ligands (X=NCS−) at axial positions show the weaker correlation as compared with complexes with weaker axial ligands (Cl−). Electronic origin that causes such negative correlations has been investigated in terms of the potential energy surfaces obtained from ab initio MO calculations for model compounds, trans-[MCl2(NH3)4]. A section of the potential surface along the M–Cl and M–N bond axes is an ellipse and the ellipse has a tilt with respect to the metal-ligand bond axes. The dependence of the M–Cl distances on the M–N distances becomes stronger, as the tilt of the ellipse increases, and/or as the ratio of the length of long to short axes of the ellipse is increased. The degree of correlation expected from the potential energy surfaces are consistent, in all respect, with experimentally observed results. The interrelationships, found both experimentally and theoretically, are associated with softness of metal ions. Among the metal ions studied, Co(III) is the hardest, Zn(II) is the softest, and Ni(II) is intermediate.
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