Electronic absorption , square-planar

Fig. 8. Electronic absorption spectra of some planar square Pt(II) complexes of chloride and ammonia in aqueous solution (from Chatt, J., Gamlen, G. A., Orgel, L. E. J. Chem. Soc. 486 (1958))...

One of the earliest series of metal complexes which showed strong, redox-dependent near-IR absorptions is the well-known set of square-planar bis-dithiolene complexes of Ni, Pd, and Pt (Scheme 4). Extensive delocalization between metal and ligand orbitals in these non-innocent systems means that assignment of oxidation states is problematic, but does result in intense electronic transitions. These complexes have two reversible redox processes connecting the neutral, monoanionic, and dianionic species. 

Type II copper enzymes generally have more positive reduction potentials, weaker electronic absorption signals, and larger EPR hyperfine coupling constants. They adopt trigonal, square-planar, five-coordinate, or tetragonally distorted octahedral geometries. Usually, type II copper enzymes are involved in catalytic oxidations of substrate molecules and may be found in combination with both Type I and Type III copper centers. Laccase and ascorbate oxidase are typical examples. Information on these enzymes is found in Tables 5.1, 5.2, and 5.3. Superoxide dismutase, discussed in more detail below, contains a lone Type II copper center in each of two subunits of its quaternary structure. 

L. I. Elding, and L. F. Olsson, Electronic absorption spectra of square-planar chloro-aqua and bromo-aqua complexes of palladium (11) and platinum (11), J. Phys. Cherru 82(1), 69—74 (1978)... 

Macrocyclic complexes (continued) nickel(II), 44 93-94 eatalysis, 44 119-125 configurational isomerization, 44 126 electrochemical properties, 44 112-113 electronic absorption spectra, 44 108-112 reactions, 44 118-119 square-planar and octahedral species, 44 116-118... 

The following sections are purposely separated into specific structural classes of square planar Pt" complexes of the general formulae Pt(NAN)(C=CR)2, [Pt(NANAN) (OCR)]+, Pt(NANAC)(C=CR), rra s-Pt(PR3)2(OCR)2, and d.v-Pt(PAP)(( =CR)2, where NAN is a bidentate 2,2 -bipyridine, NANAN and NANAC are tridentate polypyridines, PR3 is a monodentate phosphine, and PAP is a bidentate phosphine ligand. The final section of this work is dedicated to recent electronic structure calculations on these molecules with an emphasis on the successful application of DFT (density functional theory) and TD-DFT (time-dependent density functional theory) methods towards understanding the absorption and emission processes of these chromophores. 

Square-planar metallo(diimine)(dithiolene) complexes generally display intense, solvatochromatic absorptions in the visible region of the spectrum that are not found in the corresponding metallo-bis(dithiolene) or metallo-bis (diimine) complexes. Futhermore, the LLCT transition energy does not vary appreciably as a function of the metal ion. Extended Hiickel calculations on Ni, Pt, and Zn metallo(diimine)(dithiolene) complexes indicate that the HOMO is comprised almost entirely of dithiolene orbital character (Figure 2), while the LUMO was found to possess essentially all diimine n orbital character (112, 252, 268). In stark contrast to the spectra of square-planar Ni and Pt metallo (diimine)(dithiolene) complexes, the psuedo-tetrahedral complexes of Zn possess extremely weak LLCT transitions. Now, it is of interest to discuss the differences in LLCT intensity as a function of geometry from a MO point of view. This discussion should help to explain important orientation-dependent differences in photoinduced electron delocalization and charge separation. 

Figure 24 displays the high energy (E > 25,000 cm-1) region of the room temperature electronic absorption spectrum for Zn(bpy)(tdt), where bpy = 2,2 -bipyridine. The LLCT transition occurs at 22,470 cm-1 (445 nm) with very weak absorption intensity (e = 72 M 1cm 1). The origin of the weak LLCT is a function of the symmetry of this psuedo-tetrahedral complex. A MO diagram for Zn(bpy)(tdt), derived from extended Hiickel calculations, is presented in Fig. 25. Irrespective of whether the metallo(diimine)(dithiolene) complex is square-planar or psuedo-tetrahedral, the point symmetry is C2V, and all intermediate geometries possess C2 symmetry. When the dithiolene and diimine planes are orthogonal (psuedo-tetrahedral geometry) the HOMO — LUMO transition represents a b2 —> b one-electron promotion and is electric dipole forbidden. However, the HOMO —> LUMO transition in a square-planar...

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