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 μ-L (μ-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 σ bond combinations (Pt IV ) to low-lying π* orbitals of the π acceptor ligands are responsible for long-wavelength absorption maxima λ max (CT) > 700 nm. UV/Vis and EPR spectroelectrochemical results for reversible reduction processes indicate the formation of [Pt IV ] 2 (μ-L • - ) and [Pt II ] 2 (μ-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(μ-bptz)PtMes 2 ] n forms an enormously stabilized radical anion ( n = 1−) with Δ E 1/2 = 1250 V and K c = 10 21.2 and a Pt III /Pt II mixed-valent state ( n = 1+) with Δ E 1/2 = 80 mV and K c = 23. This small K c value is attributed to the predominantly d σ orbital character of the redox orbitals on the Pt(II) centers.