Spectroelectrochemistry and Dimerization Equilibria of Chloro(terpyridine)platinum(II). Nature of the Reduced Complexes

[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) = −0.74 V and E° '_(0/-) = −1.30 V (vs AgCl (1.0 M KCl)/Ag). The first reduction couple is substantially positive of those observed for other M(II)−tpy complexes (for example, E° '_(0/-) = −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 π* orbital of tpy, stabilizing the (π*)^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) × 10 and 10(4) × 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(tpy)Cl^+][Pt(tpy)Cl]) = 18(4) × 10 M^(-1). Analysis of variable-temperature EPR spectra indicates that the first reduction is ligand-centered (^2B_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_(x^2 - y^2) is the likely acceptor orbital.