Tetrahedral complex high-spin

The change from high- to low-spin configurations is necessarily discontinuous. A given complex is either on one side of the divide or the other. We conclude this section with a look at how the steric role of the d shell can affect angular geometries within a series of just high-spin, nominally tetrahedral nickel(ii) complexes. 

The role of coordinated ethylene is evidenced by the recent ab initio calculation performed by Espelid and Borve [121-123], who have shown that ethylene may coordinate in two different ways to the reduced Cr(II) species, either as a molecular complex or covalently bound to chromium. At longer Cr-C distances (2.36-2.38 A) an ethylene-chromium zr-complex forms, in which the four d electrons of chromium remain high-spin coupled and the coordination interaction is characterized by donation from ethylene to chromium. Cr(II) species in a pseudo-tetrahedral geometry may adsorb up to two equivalents of ethylene. In the case of a pseudo-octahedral Cr(II) site a third ethylene molecule can also be present. The monoethylene complex on the pseudo-tetrahedral Cr(II) site was also found to undergo a transformation to covalently bound complex, characterized by shorter Cr-C distances (about... 

In class I compounds (or complexes) the two sites are very different from each other and the valences are strongly localized. The properties of the complex are the sum of the properties of the constituting ions. The optical MMCT transitions are at high energy. The compounds are insulators. Here are some examples [60, 97]. In GaCl2, or Ga(I)[Ga(III)Cl4] there are dodecahedrally coordinated Ga(I) ions with Ga-Cl distances of 3.2-3.3 A and tetrahedrally coordinated Ga(III) ions with Ga-Cl distance 2.2 A. In [Co(III)(NH3)6]2- Co(II)Cl4)3 there are low-spin, octahedrally coordinated Co(III) ions and high-spin, tetrahedrally coordinated Co(II) ions. For our purpose this class is not the most interesting one. 

The bi- and tridentate phosphines dppp (l,3-bis(diphenylphosphino)propane) and dppep (bis (2-diphenylphosphinoethyl)phenylphosphine) have been complexed with Co and their divalent four- and five-coordinated thiophenolate complexes Co(dppp)(SPh)2 and Co(dppep)(SPh)2 have been isolated and structurally characterized.379 Somewhat related to dppp is the bidentate silane Ph2PCH2SiMe2CH2PPh2, which forms high-spin, pseudo-tetrahedral dihalocobalt(II) complexes.380... 

To illustrate the effect of geometry on the Cl shift. Table IV shows some data for several pairs of high spin tetrahedral and octahedral iron complex ions. In each case the shift of tetrahedral ion is more negative than that of the octahedral ion, and the difference in shift is rather substantial. We feel that in these cases the differences in shifts can be ascribed to differences in 4 covalency six electronegative ligands are more able to draw off 5 electron density than are four. Apparently it makes little difference whether the six ligands belong exclusively to a... 

A variety of geometries have been established with Co(II). The interconversion of tetrahedral and octahedral species has been studied in nonaqueous solution (Sec. 7.2.4). The low spin, high spin equilibrium observed in a small number of cobalt(Il) complexes is rapidly attained (relaxation times < ns) (Sec. 7.3). The six-coordinated solvated cobalt(ll) species has been established in a number of solvents and kinetic parameters for solvent(S) exchange with Co(S)6 indicate an mechanism (Tables 4.1-4.4). The volumes of activation for Co " complexing with a variety of neutral ligands in aqueous solution are in the range h-4 to + 1 cm mol, reemphasizing an mechanism. 

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