Complex low-spin

Spin-pairing in manganese(II) requires a good deal of energy and is achieved only by ligands such as CN and CNR which are high in the spectrochemical series. The low-spin complexes. [MnfCN) ] and [Mn(CNR)6] + are presumed... 

Table 26.S Spectra of ociahedral low-spin complexes of cobaU(lll)...

It is possible to observe spin-allowed, d d bands in the visible region of the. spectra of low-spin cobalt(lll) complexes because of the small value of 0Dq, (A), which is required to induce spin-pairing in the cobalt(lll) ion. This means that the low-spin configuration occurs in complexes with ligands which do not cause the low -energy charge transfer bands whieh so often dominate the spectra of low-spin complexes. 

Strategy First (1) find out how many 3d electrons there are in Fe3+. Now (2) consider how those electrons will be distributed if Hunds rule is followed that corresponds to the high-spin complex. Finally (3) sprinkle as many electrons as possible into the lower three orbitals this is the low-spin complex. 

This model of the electronic structure of complex ions explains why high-spin and low-spin complexes occur only with ions that have four to seven electrons (d4, d5, d6, d7). With three or fewer electrons, only one distribution is possible the same is true with eight or more electrons. 

Derive orbital diagrams for high-spin and low-spin complexes. 

Low-spin complex A complex that, for a particular metal ion, has the smallest possible number of unpaired electrons, 419... 

FIGURE 16.35 (a) A strong-field ligand is likely to lead to a low-spin complex (in this case, the configuration is that of Fe3+). (b) Substituting weak-field ligands is likely to result in a high spin complex. 

The majority of octahedral ferric complexes exhibit simple Curie or Curie-Weiss magnetic behavior (i.e., magnetic susceptibility 1/7. They can be classified as either "high spin or "low spin. In high-spin complexes, the lowest term (ground state) is Aig, which corresponds to the tag eg2 configuration. The low-spin complexes have the Tgg term as... 

Co(c-Hx2 tc)3BF4 is a 1 1 electrolyte in nitromethane and has a magnetic moment of 3.48 BM at room temperature, which is in between the value for a high-spin and a low-spin complex. 

The series of 3d elements from scandium to iron as well as nickel preferably form octahedral complexes in the oxidation states I, II, III, and IV. Octahedra and tetrahe-dra are known for cobalt, and tetrahedra for zinc and copper . Copper(II) (d9) forms Jahn-Teller distorted octahedra and tetrahedra. With higher oxidation states (= smaller ionic radii) and larger ligands the tendency to form tetrahedra increases. For vanadium(V), chromium(VI) and manganese(VII) almost only tetrahedral coordination is known (VF5 is an exception). Nickel(II) low-spin complexes (d8) can be either octahedral or square. 

A major consideration before the ligand exchange equilibria can be considered with reference to biological systems is the stability of a particular oxidation state in the biological medium. Low-spin complexes undergo rapid one-electron oxidation and reduction. As a biological system operates at a low redox potential, say —0.5 to 0.0 volts, reduced, i.e. low valence, states of the metals are to be expected. The metal complexes, Ru, Os, Rh, Ir, Pd, Pt and Au should be reduced to the metallic state in fact but for the slow speed of this reduction. The metals of Fig. 6 will tend to go to the following redox states ... 

Although the simple valence-bond approach to the bonding in coordination compounds has many deficiencies, it is still useful as a first attempt to explain the structure of many complexes. The reasons why certain ligands force electron pairing will be explored in Chapter 17, but it is clear that high- and low-spin complexes have different magnetic character, and the interpretation of the results of this technique will now be explored. 

---