Is Cyanide Really a Strong‐Field Ligand?

Abstract

Iron man or weakling? Ligand-field strengths are conveniently expressed by the empirical spectrochemical series. Although cyanide has been deeply entrenched as a strong-field ligand, a couple of recent examples cast doubt toward the position of this ligand, namely the high-spin (S = 2) states of [Cr(II)(CN)(5)](3-) and [Fe(II)(tpp)(CN)](-). tpp = meso-tetraphenylporphinate.

Axial Mn–CCN Bonds of Cyano Manganese(II) Porphyrin Complexes: Flexible and Weak?

“…Recently, the first (cyano)­iron­(II) porphyrin complex, five-coordinate [K(222)]­[Fe II (TPP)­(CN)], was isolated, which exhibited unusual temperature-dependent S = 0 ⇌ S = 2 spin crossover phenomena . [K(222)]­[Fe­(TPP)­(CN)] represents a case in which π back-bonding should be maximized and the CN – ligand should unequivocally lead to a LS species . The spin crossover behavior has been investigated using electronic structure methods .…”

Influence of the Introduction of Cyanido and Phosphane Ligands in Multifunctionalized (Mercaptomethyl)silane [FeFe] Hydrogenase Model Systems

“…Nevertheless, trimethylphosphane reveals a similar donor character and can serve as an abiological surrogate for CN -. [15] Inspired by the properties of [2Fe2S(Si)] complexes and the natural abundance of the strong donor ligand CN -, [16] [FeFe] hydrogenase model complexes 1a-c (Scheme 2) were treated with triphenylphosphane and tetraethylammonium cyanide to afford monosubstituted, asymmetric [2Fe2S(Si)] cluster compounds. The resulting products were investigated according to their chemical and structural properties by cyclic voltammetry and crystal structure analysis.…”

Comparison of Cyanide and Carbon Monoxide as Ligands in Iron(II) Porphyrinates

“…We recently reported the characterization of the first cyanoferroheme, five‐coordinate [K(222)][Fe(tpp)CN] 4. 9 The cyanide ligand field is insufficient to yield a completely low‐spin complex; rather, the species is a S =0⇌ S =2 spin‐crossover complex 10. Herein we report a new polymorphic form of [K(222)][Fe(tpp)CN], which is also a spin‐crossover complex 12.…”