Tetrahedral complexes thermodynamic stability

For reaction 20-1 mentioned above, carried out hypothetically in the vapor phase, the enthalpies have been estimated from thermodynamic data for the metals, M, in the series Mn2+, Fe2+,..., Cu2+, Zn2+. At the same time, from the spectra of the [M(H20)6]2 + and [MC14]2 " ions the values of A0 and A, have been evaluated and the differences between the two LFSE s calcu-J ated. Fig. 20-33 shows a comparison-between- these Two sets-of quantities — It is evident that the qualitative relationship is very close even though some quantitative discrepancies exist. The latter may well be due to inaccuracies in the AH values since these are obtained as net algebraic sums of the independently measured enthalpies of several processes. The qualitatively close agreement between the variation in the enthalpies and the LFSE difference justifies the conclusion that it is the variations in LFSE s that account for gross qualitative stability relations such as the fact that tetrahedral complexes of Co11 are relatively stable while those of Ni11 are not. 

Divalent nickel (3d ) forms tetrahedral and square-planar complexes of comparable energy, their relative thermodynamic stability depending on the identity of the ligands [6, p. 751]. The discussion of their interconversion that follows is abstracted from the symmetry analysis published by Knorr and the author [23]. 

The reduction mechanism changes for complexes containing sterically hindered danphen or catphen ligands. The 4-coordinated species [Ni(danphen)2] + and [Ni(catphen)2] + are reduced by one electron to the corresponding Ni complexes [108]. The distorted tetrahedral coordination required by these ligands stabilizes Ni complexes both thermodynamically (reduction occurs at ca -0.57 V) and kineti-cally towards ligand loss and, for [Ni(catphen)2] +, even towards reoxidation by O2 [108]. [Ni(catphen)2] +, on the other hand, cannot be oxidized to a Ni complex. 

ZnO surfaces are more complex than those of the rock-salt type oxides Uke MgO and NiO. ZnO crystalhzes in the wurtzite structure in which each Zn cation is tetrahedrally coordinated to four O anions and vice versa [105]. This crystal structure has no inversion center. The most important low-index surface planes are two polar planes, the Zn-terminated ZnO(OOOl) and 0-termi-nated ZnO(OOO-l) plane, and two neutral planes, ZnO(lO-lO) and ZnO(l 1-20). According to Nosker et al. [106] and Tasker [107], the two polar surfaces are thermodynamically unstable, however, they can be easily prepared and characterized experimentally, and do even show rather regular (1x1) LEED patterns [108]. This indicates that they are not stabilized by major reconstructions or other modifications. Therefore, it was believed for a long time that both polar surfaces exist in an unreconstructed bulk-Hke trimcation. Several contradicting proposals have been made to explain how the stability of the polar un-... 

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