Studies of a number of the properties of pentacyanoferrate(II) complexes of pyridine, pyrazine, and related aromatic nitrogen heterocycles are presented. Each complex has a strong absorption band in the visible region which is assigned to a metal-to-ligand electron-transfer transition. The calculated oscillator strengths of a number of the bands and the measured oxidation potentials of the Fe(II)-Fe(III) couples in the complexes are also reported. The complexes can be reversibly protonated, presumably at a cyanide. The measured values of the pK& for the protonated complexes vary according to the identity of the ligand. The kinetics of ligand exchange were studied, yielding clear evidence for a limiting SN1 substitution mechanism. The spectra and the thermodynamic and kinetic data are interpreted in terms ofbackdonation from the pentacyanoferrate(II) moiety to the aromatic ligand.

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