Tetragonally distorted nickel

This paper reviews the measurement of this parameter, both in cubic and in non-cubic complexes, and discusses its significance. A novel method for the rapid calculation of Dq and B from the spectra of cubic molecules of high-spin d, dP, d , and d ions is introduced, and the spectra of some tetragonally distorted nickel (II) complexes are interpreted in terms of unusually low, crystal field strengths for the axial ligands. 

The same theory may be applied to tetragonally distorted nickel(II) complexes which commonly show a splitting of the first spin-allowed transition, T2 M2. The energies and orbital representations of the two components generally observed are exactly the same as for chromium (III) above (Equation 15) (18). 

A crystal-structure determination on [Ni(PhCH2CS2)2] showed evidence of a Ni-Ni bond (Ni—Ni distance, 256 pm) in a bridging, acetate-cage, binuclear complex (363). Each nickel atom is 5-coordinate and is in a tetragonally distorted, square-pyramid spectroscopic evidence for a Ni-Ni bond has been obtained (364). The polarized crystal spectra showed more bands than predicted for a mononuclear, diamagnetic, square-planar nickel(Il), and the spectra are indicative of substantial overlap of the d-orbitals between the two nickel atoms. The bis(dithiobenzation)nickeKII) complex was found to exhibit unusual spectrochemical behavior (365). 

A quantitative consideration on the origin of the EFG should be based on reliable results from molecular orbital or DPT calculations, as pointed out in detail in Chap. 5. For a qualitative discussion, however, it will suffice to use the easy-to-handle one-electron approximation of the crystal field model. In this framework, it is easy to realize that in nickel(II) complexes of Oh and symmetry and in tetragonally distorted octahedral nickel(II) complexes, no valence electron contribution to the EFG should be expected (cf. Fig. 7.7 and Table 4.2). A temperature-dependent valence electron contribution is to be expected in distorted tetrahedral nickel(n) complexes for tetragonal distortion, e.g., Fzz = (4/7)e(r )3 for com-... 

Nickel(III) peptide complexes have a tetragonally-distorted octahedral geometry as shown by electron spin resonance studies (19) and by reaction entropies for the Ni(III,II) redox couple (17). Axial substitutions for Ni(III)-peptide complexes are very fast with formation rate constants for imidazole greater... 

In nickel(III) peptide complexes, there is a strong in-plane field provided by the deprotonated peptide linkages (117, 118). Two axially coordinated water molecules are present in the tetragonally distorted complexes which exchange much more rapidly than for the [14]aneN4 species with a substitution rate of constant >106 M l sec-1 for the formation of the imidazole complex (141). However, except for the terminal peptide group, equatorial substitution is very slow. Substitution and rearrangement (125) reactions of these species reveal acid-... 

The remarkable hexanuclear complex [ NiCp ] (81), prepared by the sodium naphthalenide reduction of nickelocene, undergoes an extensive series of reversible one-electron transfer reactions cyclic voltammetry shows waves relating the six species [ NiCp 6] (Z = -2 to 3). Chemical oxidation of 81, with Ag, gave the monocation whose structure shows only a small tetragonal distortion from the octahedral array of nickel atoms in the neutral precursor (198). 

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