Tetrahedral complexes electronic spectra

N-Do/Jor Ligands. The full account of the preparation and properties of V[N(SiMc3)2]3 has been published. (Et4N)3[V(NCSe)e] has been prepared and its electronic spectrum reported in several solvents. The electronic spectra of fVL lfNCSij complexes (L = py, 3-picoline, 3,4-lutidine, or 3,5-lutidine) are consistent with tetrahedral microsymmetry about the V " atom, and the magnetic properties of V complexes with the thiosemicarbazones of salicylaldehyde and pyruvic acid have been interpreted in terms of a tetragonal environment about the metal. ... 

In this case the planar complex is diamagnetic and possesses the usual narrow line, high-resolution diamagnetic spectrum. The tetrahedral complex in Td symmetry would possess a 3T ground state. In approximately tetrahedral nickel(II) complexes the orbital angular momentum is incompletely quenched the result is a very short electron spin relaxation time and an NMR spectrum with relatively narrow, paramagnetic shifted resonances. 

The products isolated from reactions of amides with transition metal halides usually contain coordinated halide (e.g. the formulations in Table 2). In some cases such as [Co(NMF)6][CoCLt], halide and amide are coordinated to different metal atoms, but when such compounds are dissolved in the neat ligand, halide can be replaced and at high dilution all the metal ions may be fully coordinated by the amide alone. The electronic spectrum resulting when this cobalt complex is dissolved in nitromethane has been interpreted as relating solely to the tetrahedral complex [CoC12(NMF)2]. 

S-Donor ligands. As an extension of previous studies on related complexes of iron(n), cobalt(n), nickel(n), and zinc(n), the structure of Mn[SPPh2-N PPh2S]2 has been determined by X-ray methods. The metal atom is co-ordinated in an approximately tetrahedral manner by the four sulphur atoms. The two MnS2P2N rings adopt the twisted boat conformation with S and P atoms at the apices. The single-crystal electronic spectrum has been measured and interpreted.92... 

BM, with an N2C12 donor set.120 The effect of pressure on the electronic spectrum of this complex has been examined.121 Thiomorpholine-3-thione, S=CNHCH2CH2S< H2, and thiazolidine-2-selenone, Se=CNHCH2CH2, form tetrahedral CoL2X2 (X = Cl, Br, or I) complexes spectral evidence suggests that co-ordination is via the N rather than the S or Se atoms.122... 

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...

The tetrahedral complex anion [Co(NCO)4] has been characterized by infrared and electronic spectra 292, 296, 432, 639) in the presence of a variety of cations. The infrared spectrum of the potassium salt is considerably more complex than that of the tetraethylammonium salt, indicating some loss of symmetry in the former compound. It is suggested that there is a strong interaction between K+ and O of the oyanate group which leads to the distortion (296, 743). 

U V- Vis. Spectroscopic measurements in the ultraviolet and visible range of the electronic spectrum (UV-Vis) can be used to probe electronic transitions in certain metal atoms and ion complexes. The energy of an electronic transition can depend upon the symmetry of the metal ion being different for transitions in a metal complex displaying tetrahedral (Td) symmetry from the same metal showing an octahedral (Oh) symmetry. Thus, it is possible to use UV-Vis spectroscopy to interrogate the symmetry of certain metal ions bound to oxide surfaces. We show here a few examples of the use of UV-Vis spectroscopy to characterized supported metal oxides. 

In the example of a tetrahedral complex, LMCT involves excitation of an electron from the ligand non-bonding o orbitals of a, and t symmetry, or the ligand non-bonding n orbitals of e, t, or tj symmetry (see Figure 6.21a), to a predominantly metal-based e or t/ orbital. In the case of [MnOJ, the four lower-energy transitions observed in the UV/visible spectrum have been assigned to the transitions Mn(e ) <—... 

NAD. The several possibilities for Zn -NAD interaction have been reviewed. However, the wide range of techniques that have been applied have failed to give conclusive evidence for direct metal-coenzyme interaction. Many studies have been inconclusive or contradictory. On the other hand there is some positive evidence for the maintenance of a four-coordinate geometry for the metal in the binary complex. Thus the shape and intensity of the electronic spectrum of Co(c)2Zn(n)2-LADH (discussed below) are consistent with the expected tetrahedral structure for the catalytic metal ion. However, there are no major changes in the spectrum of the binary complex, suggesting that the four-coordinate structure is maintained and that the coenzyme does not bind to the metal. 

---