Why does cyanide pretend to be a weak field ligand in [Cr(CN)5]3-?

Inorganic Chemistry
Richard L Lord, Mu-Hyun Baik

Abstract

Chemical reasoning based on ligand-field theory suggests that homoleptic cyano complexes should exhibit low-spin configurations, particularly when the coordination sphere is nearly saturated. Recently, the well-known chromium hexacyano complex anion [Cr(CN)6](4-) was shown to lose cyanide to afford [Cr(CN)5](3-) in the absence of coordinating cations. Furthermore, (NEt 4)3[Cr(CN)5] was found to be in a high-spin (S=2) ground state, which challenges the common notion that cyanide is a strong field ligand and should always enforce low-spin configurations. Using density functional theory coupled to a continuum solvation model, we examined both the instability of the hexacyanochromate(II) anion and the relative energies of the different spin states of the pentacyanochromate(II) anion. By making direct comparisons to the analogous Fe (II) complex, we found that cyanide electronically behaves as a strong-field ligand for both metals because the orbital interaction is energetically more favorable in the low-spin configuration than in the corresponding high-spin configuration. The Coulombic repulsion between the anionic cyanide ligands, however, dominates the overall energetics and ultimately gives preference to the high-spin complex, ...Continue Reading

References

Apr 25, 2000·Acta Crystallographica. Section C, Crystal Structure Communications·I A RazakB S Kang
May 22, 2003·Journal of Computational Chemistry·E Van Lenthe, E J Baerends
Jul 12, 2003·Acta Crystallographica. Section C, Crystal Structure Communications·Miroslava MalarováWerner Massa

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Citations

Sep 11, 2009·The Journal of Chemical Physics·Víctor M RayónAntonio Largo
Apr 14, 2016·Dalton Transactions : an International Journal of Inorganic Chemistry·Maryam YousifStanislav Groysman
Feb 18, 2009·Angewandte Chemie·Mikio Nakamura
Nov 8, 2008·Angewandte Chemie·Jianfeng LiW Robert Scheidt
Mar 19, 2011·Dalton Transactions : an International Journal of Inorganic Chemistry·Bo LiYiying Zheng
Mar 21, 2013·Dalton Transactions : an International Journal of Inorganic Chemistry·Jordi Cirera, Santiago Alvarez
Feb 6, 2015·Chemistry : a European Journal·Daniel Charles Ashley, Mu-Hyun Baik

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