Octahedral ligand fields, intermediate

Intermediate Octahedral Ligand Fields and Substitution Reaction Stereomobility... 

Methyl-coenzyme M reductase participates in the conversion of CO2 to CH4 and contains 6-coordinate nickel(II) in a highly hydrogenated and highly flexible porphyrin system. This flexibility is believed to allow sufficient distortion of the octahedral ligand field to produce low-spin Ni" (Fig. 27.7) which facilitates the formation of a Ni -CHs intermediate. 

No zirconium(III) complexes with oxygen donor ligands have been isolated. However, the electronic absorption spectra of aqueous solutions of Zrl3 have been interpreted in terms of the formation of aqua complexes (equation 4).29 The spectrum of a freshly prepared solution of Zrl3 exhibits a band at 24 400 cm-1, which decays over a period of 40 minutes, and a shoulder at 22000 cm-1, which decays more rapidly. The 24400 cm-1 band has been assigned to [Zr(H20)6]3+, and the 22000 cm-1 shoulder has been attributed to an unstable intermediate iodo-aqua complex. If it is assumed that the absorption band of [Zr(H20)6]3+ is due to the 2T 2Ee ligand-field transition, the value of A is 24 400 cm. This corresponds to a A value of 20 300 cm-1 for [Ti(H20)6]3+ 30 and 17 400 cm-1 for the octahedral ZrCl6 chromophore in zirconium(III) chloride.25... 

Rate equations of this type are normally associated with the formation of associative intermediates or the involvement of deprotonated ligand forms in reactions with two (or more) competitive pathways. However, in the case of octahedral metal complexes of ligands such as 2,2 -bipyridine or 1,10-phenanthroline, such mechanisms do not appear to be likely. Associative mechanisms would involve seven-co-ordinate intermediates, which are likely to be sterically strained and electronically disfavoured on ligand field grounds. Furthermore, this type of ligand does not appear to contain any strongly acidic protons which are likely to be involved in reactions with aqueous hydroxide ion (but see later). 

In the same samples, a second absorption feature was detected that is associated with the dopant ions themselves. These ligand-field transitions allow distinction among various octahedral and tetrahedral Co2+ species and are discussed in more detail in Section III.C. The three distinct spectra observed in Fig. 4(b) correspond to octahedral precursor (initial spectrum), tetrahedral surface-bound Co2+ (broad intermediate spectrum), and tetrahedral substitutional Co2+ in ZnO (intense structured spectrum). Plotting the tetrahedral substitutional Co2+ absorption intensity as a function of added base yields the data shown as triangles in Fig. 4(b). Again, no change in Co2+ absorption is observed until sufficient base is added to reach critical supersaturation of the precursors, after which base addition causes the conversion of solvated octahedral Co2+ into tetrahedral Co2+ substitutionally doped into ZnO. Importantly, a plot of the substitutional Co2+ absorption intensity versus added base shows the same nucleation point but does not show any jump in intensity that would correspond with the jump in ZnO intensity. Instead, extrapolation of the tetrahedral Co2+ intensities to zero shows intersection at the base concentration where ZnO first nucleates, demonstrating the need for crystalline ZnO to be... 

---