Discrete Mixed-Valence Metal Chains: Iridium Pyridonate Blues The generous financial support from DGES and MCyT-PNI (Projects PB98-641 and BQU2000-1170) is gratefully acknowledged.

Binuclear complexes with head-to-tail (HT) configurations are appropriate for building tetrametallic chains, contrary to previous speculations. Moreover, the isolated HT,HH tetrametallic species are converted into the thermodynamically more stable HH,HH compounds.