Mean metal-oxygen bond

Strength Dq of the sixfold coordinated Cr " ion (i.e., the energy of its band) corresponds to the inverse fifth power of the mean metal-oxygen bond distances ... 

Quantitative phase analysis, agreement factors and refinement details, unit cell parameters, mean metal-oxygen bond distances and interatomic angles, and the effective coordination number (ECoN) at cubic site of orthogallate perovskites are listed in Tables 12.2 and 12.3. 

Mean metal -oxygen bond distances (A) and interatomic angles (°) ... 

Polar oxide surfaces present a wide variety of electronic and atomic characteristics, which are dependent upon the crystal structure, the ionicity of the metal-oxygen bonding, the surface orientation and its stoichiometry. The nature of the microscopic processes responsible for the cancellation of polarity provides a means to introduce a classification among these surfaces. 

Terminal-CO vibrating at 2123 cm, which is stabilized only at <160 K, detects the presence of Nf sites [3]. CO adsorption on metal ions is weak, due to the low ti/ct bonding contribution. Thus, application of low temperature regimes is essential for irreversible adsorption of CO on metal cations [32]. The presence of Ni sites indicates that the catalyst examined is incompletely reduced under the conditions applied. Earlier studies [3] have consistently found, that the complete reduction of Ni ions on alumina is difficult to accomplish. Incomplete reduction of a number of transition metal ions, including Ni, seems to be the trend on alumina and alike electronically "hard" support materials [33]. According to Pearson [33], the electronic hardness of a metal oxide means the occurrence of weakly polarizable metal-oxygen bonds. 

Estimations of the mean amplitudes of vibration for metal—oxygen bonds in MO (M = Mo or W) have been reported together with a force constant of 3.7 mdyne forWO . Raman andi.r. vibrational data for Mo 0 "and Mo 04 ... 

Due to the subtle difference between Ga " and Cr " ionic radii at octahedral site, both mean metal-oxygen (Tables 12.2 and 12.3) and local chromium-oxygen (listed, along with other optical parameters, in Table 12.4) bond distances are constant within the standard deviation. On the basis of this... 

Simultaneous presence ofbarium and titanium in the solution of citric acid causes the effect that can not be described as the superposition of the solutions of barium citrate and titanium citrate. In addition to the familiar Ti-0 bonds and shift of carboxylic groups NMR peaks, the central carbon peak in the C-NMR spectra is displaced to higher frequency (Fang, 2001 Kakihana, 1999b), which means a deprotonation of-OH group and most likely formation of metal-oxygen bonds. This observation is particularly interesting if one takes into account p/Q = 10.96 and low pH of citric acid solution. Barium and titanium participate in the formation of the metal complex with Ba Ti CA ratio of... 

In true aquo complexes, the water molecule is firmly bound to the metal atom by means of a partial covalent bond and is known as coordinated water. However, in some other cases, the metal-oxygen bond may be almost ionic... 

A calculation of the partial charge on the water in a few hydroxides shows that, as long as i5(H20) < 0, the hydroxide is indeed obtained. However, if 5(H20) > 0, the hydroxide is unstable and forms an oxyhydroxide or a hydrated oxide in aqueous solution (Table 2.1). This means that in stable hydroxides the metal-oxygen bond is strongly ionic. An increase in tlie covalent nature of the bond favors the formation of oxyhydroxides or oxides. 

At the outset it is important to clarify the scope of this discussion by the elimination of areas which will not be considered. When one notes that the term "activation of carbon monoxide" may mean a process as little perturbative of the C-0 bond as its end-on attachment to a metal atom in carbonyls, or as strongly perturbative as its dissociation to atoms on a metal surface, the need for limits becomes obvious. In this discussion we will consider only the activation of carbon monoxide in the sense that isolable products are formed by the addition of hydrogen to the molecule without complete rupture of all carbon-oxygen bonds, oxygenates are formed. 

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