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 spectr a, and solvatochromic behavior, the two 1,4-diazabutadiene title compl exes exhibit markedly different photoreactivities and underlying elect ronic structures, as evident from absorption and EPR spectra of the pe rsistent anion radical forms. The lowest excited state of the nonphoto reactive Pt-II system [(CyN=CH-CH=NCy)-PtMe(2)] has MLCT (metal-to-lig and charge-transfer, 5d --> pi) character, and the EPR spectrum of th e corresponding anion radical at (g) = 2.016 exhibits sizable metal/li gand 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 cha rge-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) bo nds (214.0(8) pm) relative to Pt-C(eq) in the ground state (204.5(5) p m). The anion radical of the Pt-IV complex has lost the long-wavelengt h absorption band in the visible; it shows a well-resolved EPR spectru m 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 fo r the similar frontier-orbital energy differences despite dissimilar u nderlying electronic structures.