Zhang Lewis acid strengths predicted “Pyridine Compositions” 

SYNTHESIS AND CHARACTERIZATION OF MIXED OXIDES CONTAINING COBALT,

COPPER AND IRON AND STUDY OF THEIR CATALYTIC ACTIVITY

THOMAS MATHEW

CATALYSIS DIVISION NATIONAL CHEMICAL LABORATORY, PUNE, INDIA 

2.4.4.4. Correlation between theoretical and experimental results

    In the following an attempt has been made to correlate experimental results with theoretical (empirical) quantities. According to Lewis definition, acidity and basicity are electron accepting and electron donating properties that could contribute to the formation of a covalent bond. Larger the electronegativity, the stronger the electron accepting power can be considered as a first approximation [83]. According to Sanderson [84] the electronegativity of the metal ion would be expected to change linearly with its charge. Later Zhang [85] has proposed a numerical scale for the acid strengths Z of cations and he defined it as

Z = P - 7.7 Xz + 8 ……………(7)

where P and Xz are the polarizing power and electronegativity of the ion respectively. The advantage of such a scale is the predictive power for thermochemical and physical properties that obviously depend on the nature of the chemical bond established between the acid cation and the basic anion. In the case of inorganic compounds, this bond is called iono-covalent, meaning that this bond involves simultaneously electrostatic force (ionic part) and covalent forces resulting from the combination of atomic orbitals. To express this duality, for a given cation, Zhang took into account (a) the polarizing power and (b) the electronegativity for the covalent part. He calculated the polarizing power, electronegativity and acid strength of various cations [85] and it is note d that the acid strength of cations relevant to our compositions are in the order Fe3+>Fe2+»Co2+>Cu2+>Cu+.

    When an organic molecule like electron donating pyridine adsorbs on the surface, cations get reduced due to charge transfer. Thus the easily reducible metal ions like Cu2+ undergo fast reduction to Cu+ and hence the stability of metal ion-pyridine complex decreases. From the acid strength of various cations it is clear that reduced Cu+ has very poor acidity. In other words, for a given ligand, corresponding stability constants of metal ions are in the order Fe3+> Co2+ >Cu2+ and hence the acidity of these ions also follow the same order, as shown earlier.