On the question of mixed-valent states in ligand-bridged dinuclear organoplatinum compounds [RkPt(μ-L)PtRk]n, k = 2 or 4

Symmetrically dinuclear complexes between the bis-bidentate bridging ligands mu-L (mu-L = 3,6-bis(2-pyridyl)-1,2,4,5-tetrazine (bptz) or 2,5-bis( 1-phenyliminoethyl)pyrazine (bpip)) and the organoplatinum fragments PtMes(2) (Mes = mesityl), PtMe(2), or PtMe(4) were synthesized as deeply colored compounds. Low-energy charge-transfer transitions from metal d orbitals (Pt(II)) or metal-carbon sigma bond combinations (Pt(IV)) to low-lying pi* orbitals of the pi acceptor ligands are responsible for long-wavelength absorption maxima lambda(max) (CT) > 700 nm. UV/Vis and EPR spectroelectrochemical results for reversible reduction processes indicate the formation of [Pt(IV)](2)(mu-L(.-)) and [Pt(II)](2)(mu-L(.-)) species, however, the latter exhibit a significant metal contribution according to a Pt(II)/Pt(I) formulation Cyclic voltammetry reveals that the remarkable system [Mes(2)Pt(mu-bptz)PtMes(2)](n) forms an enormously stabilized radical anion (n = 1-) with Delta E(1/2) = 1250 V and K(c) = 10(21.2) and a Pt(III)/Pt(II) mixed-valent state (n = 1+) with Delta E(1/2) = 80 mV and K(c) = 23. This small K(c) value is attributed to the predominantly d(sigma) orbital character of the redox orbitals on the Pt(II) centers.