Symmetry square-planar

Upon NO adsorption some broadening and simultaneously, some increase in the intensity of the signals took place, which indicate a complex redox transformation on the suifrice me lic sites of the catalysts. In the interaction between NO and Cu-ZSM-5 mainly Cu ions of lower symmetry (square planar or square pyramidal) are involved. 

The low spin alternative is a doublet state ion whose EPR spectrum is readily seen at 77 K. g-values typically lie in the region 1.7-3.5 although there are examples in which the g-values lie beyond these limits. The ground state of low-spin Co is notoriously varied and depends heavily upon the stereochemistry, the nature of the ligands and especially to any axial perturbation. In C2y(x) symmetry, "square planar" complexes, e.g. Co(acacen), have a dy2 ( A2) ground state which has two other doublet states close by (d 25 1 xz l) which interact by spin-orbit coupling since... 

Penta-atomic polyhalide anions [XY4] favour the square-planar geometry (D4h) as expected for species with 12 valence-shell electrons on the central atom. Examples are the Rb+ and Cs+ salts of [C1F4], and KBrp4 (in which Br-F is 189 pm and adjacent angles F-Br-F are 90 ( 2°). The symmetry of the anion is slightly... 

The example of COj discussed previously, which has no vibrations which are active in both the Raman and infrared spectra, is an illustration of the Principle of Mutual Exclusion For a centrosymmetric molecule every Raman active vibration is inactive in the infrared and any infrared active vibration is inactive in the Raman spectrum. A centrosymmetric molecule is one which possesses a center of symmetry. A center of symmetry is a point in a molecule about which the atoms are arranged in conjugate pairs. That is, taking the center of inversion as the origin (0, 0, 0), for every atom positioned at (au, yi, z ) there will be an identical atom at (-a ,-, —y%, —z,). A square planar molecule XY4 has a center of symmetry at atom X, whereas a trigonal planar molecule XYS does not possess a center of symmetry. 

Now look at octahedral complexes, or those with any other environment possessing a centre of symmetry e.g. square-planar). These present a further problem. The process of violating the parity rule is no longer available, for orbitals of different parity do not mix under a Hamiltonian for a centrosymmetric molecule. Here the nuclear arrangement requires the labelling of d functions as g and of p functions as m in centrosymmetric complexes, d orbitals do not mix with p orbitals. And yet d-d transitions are observed in octahedral chromophores. We must turn to another mechanism. Actually this mechanism is operative for all chromophores, whether centrosymmetric or not. As we shall see, however, it is less effective than that described above and so wasn t mentioned there. For centrosymmetric systems it s the only game in town. 

Entry 3 involves a catalyst derived from (/ , Trans-cyclohexane- 1,2-diamine. The square planar Cu2+ complex exposes the re face of the dienophile. As with the BOX catalysts, this catalyst has c2 symmetry. 

A few thioether-ligated copper(II) complexes have been reported, however (cf. Section 6.6.3.1.2) (417) (essentially square planar), (418) (two crystalline forms one TBP and other SP),361 (419) (SP),362 (420) (SP),362 (421) (TBP),362 (422) (SP),363 (423) (SP),363 (424) (two independent complexes SP and octahedral),364 (425) (TBP).364 In the complexes (420) and (421), EPR spectra revealed that the interaction between the unpaired electron and the nuclear spin of the halogen atom is dependent on the character of the ligand present. For (424) and (425), spectral and redox properties were also investigated. Rorabacher et al.365 nicely demonstrated the influence of coordination geometry upon CV/Cu1 redox potentials, and reported structures of complexes (426) and (427). Both the Cu1 (Section 6.6.4.5.1) and Cu11 complexes have virtual C3v symmetry. 

Analogous considerations apply for tetracoordinate fragments M(CO)4. Fig. 30 shows some of the possible conformations of these fragments. As before, the directional orbitals that develop for particular values of the angle 6 (refer to Fig. 30) allow prediction of possible interaction with donors or of dimerization. Also, the level shifts for variation of 6 in both cases can be calculated, as well as for the squashing mode rearrangement of a tetrahedral into a square-planar coordination. The qualitative confomiational preferences implied by these patterns have been checked, as for the pentacoordinate case, by comprehensive EHT calculations for all dn systems of all conceivable symmetries. 

EPR. The EPR of the Cu complex 62c has been reported as a 1% powder sample at 77 K and is given in Table XI (Section IV.A), which compares the EPR data for 62c to Cu[pz(SMe)8] (48) (Scheme 9), Cu(TPP) and Cu(pc). The spectrum is typical of a monomeric square-planar copper with axial symmetry. The EPR spectrum for 62c closely matches that of 48 implying that although the peripheral tridentate coordination geometry has an effect on the ir-clcctronic structure of the pz ring it does not effect the electronic properties of the central Cu2+ ion, whose unpaired electron density is in a a orbital. 

ESEM results on the interaction of silica-exchanged Cu(II) with a range of adsorbates showed that one or two adsorbate molecules were able to coordinate to the Cu depending on the chemical interaction, polarity and size (7[2). Differences in A, were observed for N- and O-coordinated ligands, but these seem to reflect a change in coordination symmetry and not a difference in adsorbate ligand number. N-coordinated ligands form approximately square planar... 

A. square planar B. octahedral with symmetry axes 45° to the silicate layers C. octahedral or square pyramidal with symmetry axis at 90° to the silicate layers. 

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