Octahedral molecules species

The procedure for setting up the Cotton-Kraihanzel secular equations is now described. As an example, the octahedral molecule M(CO)5X is considered (see Fig. 7). The species of the CO-stretching vibrations... 

The geometry of the XeFj+ ion is of special interest. The dimensions are compared with those of related pseudo-octahedral molecules in Table 2. It is a characteristic of each of these species that the M-Y distance is shorter than the M-X. 

G) Some of these alkyls can add two molecules of chelating phosphines to give red, low spin (5 = j), octahedral [MnC2P4] species. The first described was the product of the chelating dialkyl -xylyl—[Mn o-QH4(CH2)2 (dmpe)2]—iit which the M—C bonds (2.11 A) are shorter than in the tetrahedral species (2.15 A) the other is the dimethyF species [Mn(CH3)2(dmpe)2]. 

Solution. The shapes of the FMOs for both compounds are shown below. Both species have a two-electron vacancy in one of their hybrid orbitals. Because most square planar compounds are 16-electron species and most octahedral compounds obey the 18-electron rule, a square planar compound will be isolobal with an octahedral molecule that has two greater valence electrons. 

In Fig. 8-13 are plotted lattice energies for MCI2 species. The metal ions are high-spin and lie in octahedral sites in the lattice. The double-hump form of the curve is obviously similar to that for the hydration energies we have just discussed. The reasons for the observed trend in lattice energy are virtually identical to those described for hydration energies. In one system, a metal(ii) ion is octahedrally coordinated by six water molecules within a liquid medium in the other, a metal(ii) ion is octahedrally coordinated by six chlorine atoms within a solid lattice. 

These two-step features, which will be further proved by the FTIR spectra of adsorbed CO, can be summarized as follows. The adsorption of CO, being accompanied by the increase of the coordination munber due to the formation of mono- and dicarbonyl species, causes a shift of the d - d transitions toward the values more typical of the octahedral coordination. Furthermore, in the presence of CO (electron donor molecule) more energy is required to transfer electrons from O to Cr as a consequence, the O Cr(II) CT transition shifts at higher frequencies (from 28000-30000 to 33 700cm ). At increasing CO pressure the CO Cr(II) CT transition also becomes visible (band at 33400 cm ). Analogous features have been reported in the past for NO adsorption on the reduced Cr/Si02 system [48,82]. 

The role of coordinated ethylene is evidenced by the recent ab initio calculation performed by Espelid and Borve [121-123], who have shown that ethylene may coordinate in two different ways to the reduced Cr(II) species, either as a molecular complex or covalently bound to chromium. At longer Cr-C distances (2.36-2.38 A) an ethylene-chromium zr-complex forms, in which the four d electrons of chromium remain high-spin coupled and the coordination interaction is characterized by donation from ethylene to chromium. Cr(II) species in a pseudo-tetrahedral geometry may adsorb up to two equivalents of ethylene. In the case of a pseudo-octahedral Cr(II) site a third ethylene molecule can also be present. The monoethylene complex on the pseudo-tetrahedral Cr(II) site was also found to undergo a transformation to covalently bound complex, characterized by shorter Cr-C distances (about... 

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