A TALE OF 2 COMPLEXES, [PTME(N)(RN=CH-CH=NR)] (N=2 AND N=4, R=CYCLOHEXYL) - WHY DO PT-II AND PT-IV COMPLEXES EXHIBIT VIRTUALLY IDENTICAL REDOX BEHAVIOR AND COLORS

In spite of their very similar cyclic voltammograms, absorption spectra, and solvatochromic behavior, the two 1,4-diazabutadiene title complexes exhibit markedly different photoreactivities and underlying electronic structures, as evident from absorption and EPR spectra of the persistent anion radical forms. The lowest excited state of the nonphotoreactive Pt-II system [(CyN=CH-CH=NCy)-PtMe(2)] has MLCT (metal-to-ligand charge-transfer, 5d --> pi*) character, and the EPR spectrum of the corresponding anion radical at (g) = 2.016 exhibits sizable metal/ligand orbital mixing. On the other hand, the structurally characterized Pt-IV complex [(CyN=CH-CH=NCy)PtMe(4)] (C2/c; a = 2021.6(2), b = 805.3(1), c = 1254.2(1) pm; beta = 111.05(1)degrees; V = 1905.7(4) x 10(6) pm(3) Z = 4) has a low-lying photoreactive LLCT (ligand-to-ligand charge-transfer, sigma(Pt-C) --> pi*) excited state in which the axial Pt-C bonds are activated, as already suggested by the longer Pt-C(ax) bonds (214.0(8) pm) relative to Pt-C(eq) in the ground state (204.5(5) pm). The anion radical of the Pt-IV complex has lost the long-wavelength absorption band in the visible; it shows a well-resolved EPR spectrum at (g) = 1.9945 with pi-ligand and Pt-194 hyperfine structure and a small g anisotropy. A qualitative MO scheme is presented to account for the similar frontier-orbital energy differences despite dissimilar underlying electronic structures.