An exceedingly stable diiron(II,III) complex ion (tz)Fe(CN)525- with comproportionation constants between 108 (in H2O) and 1019 (in CH3CN). Angewandte Chemie, International Edition, (38)20:3072--3074, 1999. [PUMA: bridged comproportionation decacyano diiron iron pentacyano prepn stability;iron stability;tetrazine tetrazine valence valence;mixed valent]