[Pt(tpy)Cl](+) (tpy is 2,2’:6’,2(")-terpyridine) undergoes reversible one-electron reductions in 0.1 M TBAH/DMF (TBAH is tetrabutylammonium hexafluorophosphate) at E(+/0)(o’) = -0.74 V and E(0/-)(o’) = -1.30 V (vs AgCl (1.0 M KCI)/Ag). The first reduction couple is substantially positive of those observed for other M(II)-tpy complexes (for example, E(0/-)(o’) = -1.36 V for [Zn(tpy)Cl-2]), a finding that suggests there is coupling between the empty 6p(z) orbital of square planar Pt(II) and the pi* orbital of tpy, stabilizing the (pi*)(1) radical state. The dimerization constants of both [Pt(tpy)Cl](+) and [Pt(tpy)Cl] in 0.1 M TBAH/DMF were determined spectroelectrochemically and found to be 8(1) x 10 and 10(4) x 10 M(-1), respectively. On the basis of variable-concentration studies, a species observed at an intermediate level of reduction is formulated as the mixed-valence dimer [(Pt(tpy)Cl)(2)](+) with K-mix = [(Pt(tpy)Cl)(2)(+)]/([Pt(tpyCl+][Pt(tpy)Cl]) = 18(4) x 10 M(-1). Analysis of variable-temperature EPR spectra indicates that the first reduction is ligand-centered (B-2(2)) With substantial contributions from Pt(II) 5d(yz) (4-6%) and 6p(z). (3-4%). The second reduction is tentatively assigned as metal-centered; 5d(x2-y2) is the Likely acceptor orbital.