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The absorption spectra, luminescence spectra, and luminescence lifetimes of the isomeric [M(bph)(bpy)] and [M(phpy)2] complexes M = Pt(II) or Pd(II), bph2− = 1,1′‐biphenyl‐2,2′‐diyl dianion, phpy− = 2‐phenylpyridine‐2′‐yl anion, and bpy = 2,2′‐bipyridine have been investigated and compared with those of [M(bpy)2]2+ complexes and of the free protonated ligands H2bph, Hbpy+, and Hphpy. In the absorption spectra, the region below 320 mm is dominated by ligand‐centered (LC) transitions, whereas metal‐to‐ligand charge transfer (MLCT) transitions are responsible for the bands present in the near UV/VIS region. The MLCT bands move to higher energies on replacing Pt with Pd and in going from [M(bph)(bpy)] to the [M(phpy)2] isomer. For the mixed‐ligand complexes, evidence for both M → bph2− (at higher energies) and M → bpy bands is found. The structured luminescence observed at 77 K shows lifetimes of 4.0 and 1.1 μs for [Pt(phpy)2] and [Pt(bph)(bpy)], respectively, and 480 and 250 μs for the analogous Pd complexes. On the basis of the energy and lifetime data, the luminescence of the Pt(II) complexes is assigned to the lowest triplet MLCT excited state, whereas for the Pd complexes the luminescent state is thought to result from a mixture of MLCT and LC triplet levels.

The X-ray structure of cis-bis(2,6-diphenylpyridinato-N,O')platinum(11) shows the first example of helical chirality for a 'square planar' co-ordination compound with nonchiral ligands; 1H NMR investigations indicate no racemization of the compound in solution on the NMR time scale.

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A synthetic route leading to bis‐heteroleptic cyclometalated complexes is described. The complexes [2‐(2′‐thienyl)pyridinato‐N, C‐3′]{2‐[3′‐(trimethylsilyl)2′‐thienyl]pyridinato‐N, C3′}platinum(II) ([Pt(thpy) (TMS‐thpy)]; I) and (l‐phenylpyrazolato‐N2, C2′)[2‐(2′‐thienyl)pyridinato‐N, C3′]platinum ([Pt(Phpz)(thpy)]; II) are characterized by UV/VIS, NMR, and mass spectroscopy. Thermal and photochemical oxidative addition reactions yield two out of the 10 possible pairs of enantiomers of octahedral Pt(IV) compounds.

ChemInform Abstract The title investigations are carried out for the two complexes (I) and (II) and the results are compared with those concerning the free protonated ligands and the previously studied complex (III). Luminescence originates from the lowest MLCT excited state. Correlations between spectroscopic and electrochemical data are discussed.

Depending on excitation wavelength, the spectra can be very simple or quite complex. However, in all cases, a relatively small number of sites, some probably with distributions of their own, can be used to fit all observed spectra. This indicates that remarkably few sites are present or that the spectra of the different sites are narrow enough to permit selective excitation of a relatively small number of sites at any one time.The wide differences in emission behavior of our complexes are explained on the basis of several criteria. We suggest that criteria can be used to fabricate systems that can show enhanced microscopic site selectivity.The relatively small number of sites coupled with the narrowness of the 77 K spectra suggests that these systems are candidates for Shpol'skii spectra or fluorescence line narrowing. Particularly promising are spectra measured from hydrocarbon glasses at lower temperatures than we can currently reach.Acknowledgment. J.N.D. and B.A.D. gratefully acknowledge support by the National Science Foundation (Grants CHE 86-00012 and 88-17809). We also thank Hewlett-Packard for the gift of the Model 8452A spectrophotometer and Henry Wilson for his kind assistance. C.M.L. thanks the donors of the Petroleum Research Fund, administered by the American Chemical Society, for support of this research.

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