ON THE QUESTION OF MIXED-VALENT STATES IN LIGAND-BRIDGED DINUCLEAR ORGANOPLATINUM COMPOUNDS [RKPT(MU-L)PTRK](N), K = 2 OR 4

Symmetrically dinuclear complexes between the bis-bidentate bridging l igands 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 fragm ents PtMes(2) (Mes = mesityl), PtMe2, or PtMe4 were synthesized as dee ply colored compounds. Low-energy charge-transfer transitions from met al d orbitals (Pt-II) or metal-carbon sigma bond combinations (Pt-IV) to low-lying pi orbitals of the pi acceptor ligands are responsible f or long-wavelength absorption maxima lambda(max) (CT) > 700 nm. UV/Vis and EPR spectroelectrochemical results for reversible reduction proce sses indicate the formation of [Pt-IV](2)(mu-L.-) and [Pt-II](2)(mu-L. -) species, however, the latter exhibit a significant metal contributi on according to a Pt-II/Pt-I formulation Cyclic voltammetry reveals th at the remarkable system [Mes(2)Pt(mu-bptz)PtMes(2)](n) forms an enorm ously 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 Del ta E-1/2 = 80 mV and K-c = 23. This small K-c value is attributed to t he predominantly d(sigma) orbital character of the redox orbitals on t he Pt(II) centers.