Tetrahedral complexes absorption spectra

When a dark red solution of 2(4) + was oxidized by an excess of NO + BF4 , a green solution of 2(4)2+ was obtained. The CV is the same as for the starting complex, and the visible absorption spectrum shows a band at = 670 nm, 8 = 810 L/mol/cm, in CH3CN, typical of these tetrahedral Cu(II) complexes.16 A decrease of the intensity of this band was observed when monitoring it as a function of time. This fact is due... 

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...

In close analogy to 3-hydroxypyridine, the chromophores in pyridoxamine phosphate and other related species of types 2 and 3 (Fig. 42) possessing a tetrahedral centre at C-4, absorb at about 330 nm. The presence of an aldehyde function at this position, as in pyridoxal phosphate (Fig. 42, 1) shifts the absorption spectrum to 390 nm. The conversion of the aldehyde into a Schiff base is attended by a slight hypsochromic shift, but the protonation of the Schiff base produces a dramatic bathochromic shift and species of type 5 (Fig. 42) absorb at approximately 420-440 nm. Guided by the data from chemical model systems we examine the spectroscopic information obtained in the formation of binary and ternary complexes with several... 

The symmetry is lower in pentacoordinated complexes than in octahedral or tetrahedral geometries, and the spectrum is composed of several widely separated bands. High-spin pentacoordinated Co(II) complexes have absorption bands with intensities intermediate between those of tetrahedral and octahedral species. A characteristic spectrum of a complex with trigonal bipyramidal symmetry is given in Fig. 3. Further examples can be found in a recent review by Ciampolini (13) and in references therein. Ciampolini points out that the band around 16 kK is rather insensitive to field strength and that the band around 12 kK is split in compounds with lower symmetry (21). 

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