Octahedral complexes, tetragonal distortions

Low-spin Cr(II) complexes are octahedral with tetragonal distortion (distorted from Oi, to Z>4/, symmetry). They show two absorption bands, one in the visible and one in the near-infrared region, caused by this distortion. In a pure octahedral field, there should be only one d-d transition (see Chapter 11 for more details). Cr(II) also forms dimeric complexes with Cr — Cr bonds in many complexes. The acetate, Cr2(OAc)4, is an example in which the acetate ions bridge between the two chromiums, with significant Cr—Cr bonding resulting in a nearly diamagnetic complex. 

In fact the vast majority of 6-coordinate complexes are indeed octahedral or distorted octahedral. In addition to the twist distortion just considered distortions can be of two other types trigonal and tetragonal distortions which mean compression or elongation along a threefold and a fourfold axis of the octahedron respectively (Fig. 19.8). 

Complexes of the divalent metals [M(ttcn)2]2+ undergo electrochemical oxidation to paramagnetic [M(ttcn)2]3+. Red [Pd(ttcn)2]3+ has a tetragonally distorted octahedral structure (d7, Jahn-Teller distortion) with Pd—S 2.356-2.369 A (equatorial) and 2.545 A (axial) in keeping with the ESR spectrum (gj = 2.049, gy = 2.009) which also displays 105Pd hfs. Similarly, electrochemical oxidation of the palladium(II) tacn complex (at a rather lower... 

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... 

Finally, as far as the [Fen(terpy)2]2+ complex is concerned, its molecular structure is known. Its octahedral geometry is tetragonally distorted, in that the bond distance between the iron(II) ion and the central nitrogen atom of terpyridine is considerably shorter (Fe-N= 1.89 A) than the distance between theiron(II) ion and the two more external nitrogen atoms (Fe-N = 1.99 A).110... 

It is noted that the (tetragonally distorted) octahedral Ni(II)-tetraazamacrocycle complexes of general formula trans[NiN4X2] are high spin (d8-dvz2dx-2dxi2d 1 dx2 y21). 

In these cases, there is the expected tetragonal distortion due to the Jahn-Teller effect in addition to the often mentioned octahedral distortion associated with polypyridine complexes. For the terpyridine complex there is the usual difference between the distances separating the metal and nitrogen atom of the central pyridine and that of the metal and the nitrogen atom of the peripheral pyridines. 

The particular values and power dependence for the d-s mixing term are also not too critical although a certain threshold must be achieved. Tetragonally elongated Jahn-Teller distortions of d9 CuNe species (36) and the trigonal geometry of the oxidized copper center in Type 1 metalloproteins (37) can be achieved with an inverse sixth order power dependence and an associated a6 parameter of at least 300,000 kcal mol-1 A6. However, since eds also depends on symmetry—e.g., it makes no contribution for octahedral complexes—there are many systems where d-s mixing has a minimal effect. 

The [Cr(en)3]2+ and [Cr(pn)3]2+ salts have reflectance spectra (Table 11) resembling those of the hexaammines, and the six N donor atoms are assumed to complete tetragonally distorted octahedra around the metal. Stability constant measurements (Table 39) have shown that the ions [Cr(en)(aq)]2+ (vmax= 18 300 cm-1, e = 25 dm3 mol-1 cm-1) and [Cr(en)2(aq)]2+ (vma = 17 500 cm-1, e = 17 dm3 mol-1 cm-1) exist in aqueous solution, but that, as in the copper(II) system, the third ethylenediamine molecule is only weakly bound, and care is needed to prevent loss of en from tris(amine) complexes in the preparations. Several bis(amine) complexes, e.g. [CrBr2(en)2], have been isolated, and these are assigned trans structures because of IR spectral resemblances to the corresponding oopper(II) complexes. Since the spectrum of [Cr(S04)(en)2] also shows the presence of bidentate sulfate, this is assigned a trans octahedral structure with bridging anions. 

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