The ability of tetrapyrrolic macrocycles to stabilize unpaired electrons and engage in pi-pi interactions is essential for many electron-transfer processes in biology and materials "engineering. Herein, we demonstrate that the formation of pi dimers is recapitulated in complexes of a linear tripyrrolic analogue of naturally occurring pigments derived from heme decomposition. Hexaethyltripyrrindione (H(3)TD1) coordinates divalent transition metals (i.e., Pd, Cu, Ni) as a stable dianionic radical and was recently described as a robust redox-active ligand. The resulting planar complexes, which feature a delocalized ligand-based electronic spin, are stable at room temperature in air and support ligand-based one-electron processes. We detail the dimerization of neutral tripyrrindione complexes in solution through electron paramagnetic resonance (EPR) and visible absorption spectroscopic. methods. Variable-temperature measurements using both EPR and absorption techniques allowed determination of the thermodynamic parameters of pi dimerization, which resemble those previously reported for porphyrin radical cations. The inferred electronic structure, featuring coupling of ligand-based electronic spins in the iv dimers, is supported by density functional theory (DFT) calculations.