Metal oxygen terminal frequencies

Figure 1.13 Raman spectra for a number of transition metal oxides supported on y-AI203 [75,102], Three distinct regions can be differentiated in these spectra, namely, the peaks around 1000 cm-1 assigned to the stretching frequency of terminal metal-oxygen double bonds, the features about 900 cm 1 corresponding to metal-oxygen stretches in tetrahedral coordination sites, and the low-frequency (<400 cm-1) range associated with oxygen-metal-oxygen deformation modes. Raman spectroscopy can clearly complement IR data for the characterization of solid catalysts. (Reproduced with permission from The American Chemical Society.)...

The 2,6-dichlorophenoxide ion can readily form chelate complexes with a number of metal ions, in which coordination is through the oxygen and the 2-chlorine atoms, while the 6-chlorine remains noncoordinated (and thus provides the reference NQR frequency vj for a terminal atom). There is some jr-bond character to the Cl-aromatic C bond (rj values of several percent are found). In principle these alter the usefiilness of equations (8) and (9)—in practice, insignificantly so—but the usefiilness of equation (6) is significantly compromised by this tt bonding. 

H-free surfaces. On hydrogen-free surfaces, the formation of terminal-CO species reveals the involvement of electron deficient (perturbed) Ni° metal sites. These sites can be generated consequently to (i) strong interactions of atomically dispersed (isolated) metal atoms with the support, and/or (ii) through-oxygen electron-exchange interactions with adjacent Ni impurity sites. Hence, the adsorption is weakened and the IR vCO frequency shifted upwards. As shown by Pearson [33], both incomplete reduction of metal ions and atomic dispersion of metal atoms are quite likely on surfaces of the electronically hard alumina. 

Consistent with p, -CO being more ketone-like, the IR y(CO) stretching frequency falls to 1720-1850 cm and p, -CO is more basic at O than terminal CO. For example, a Lewis acid binds more strongly to the p -CO oxygen and so displaces the equilibrium of Eq. 4.13 toward 4.5. Triply and even quadruply bridging CO groups with y(CO) in the range 1600-1730 cm are also known in metal cluster compounds, for example, (Cp Co)3(p -CO)2 (4.7). 

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