Weak spin complexes

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. 

Note that halides are weak field ligands giving rise to high spin complexes, while the cyanide ion is a strong field ligand creating low spin complexes. 

Spin selection rule The spin selection rule, AS = 0, specifies that there should be no change in the spin multiplicity. Weak spin-forbidden bands may occur when spin-orbit coupling is possible. Spin-forbidden transitions are more intense in complexes of heavy atoms as these lead to a larger spin-orbit coupling. 

Fig. 7. NMRD profiles calculated for slightly asymmetric, weakly deformable complexes with different electron spin quantum numbers (a) cylindrically-symmetric ZFS, E = 0 (b) maximum rhombicity E = DjS. Reprinted from J. Magn. Reson. vol. 146, Nilsson, T. Kowalewski, J., Slow-motion theory of nuclear spin relaxation in paramagnetic low-symmetry complexes A generalization to high electron spin , pp. 345-358, Copyright 2000, with permission from Elsevier.

The spin state of iron(II) in substituted 2,2 6, 2"-terpyridine, (72), complexes depends on the position of bulky substituents—phenyl substituents in both the 6 and 6" positions give a high-spin complex phenyl substituents in the 4 and 6 positions give a complex which exists both in high-spin and in low-spin forms. Crystal structure determinations gave Fe—N bond distances in both forms of the 4,6-diphenyl complex and of the 6,6"-diphenyl complex. Each ligand in the latter com )lex has one terminal pyridine very weakly bonded to the iron, with... 

The EPR powder spectra of the low-spin complexes [0s(NH3)5L][CF3S03]3 (L = H20 or (5)) have been analyzed the negative sign obtained for the axial splitting when L = (5) has been rationalized in terms of Os L backbonding. Crystal field parameters have also been derived. The spectral and electrochemical properties of [Ru(NH3)5L]" (Ru or Ru L =RC02 R = 4-py-A-Me+ or L = RCONH R = Ph, 4-py-A-Me", 4-py-A-H+) have been studied in detail as a function of pH the carboxamido Ru complexes are weak bases and are deprotonated only in strongly acidic solution. ... 

Pentadentate macrocyclic ligands L13-L15 accommodate Ni(II) ion with simultaneous dissociation of the two amide protons to form the square-pyramidal high-spin complex of [NiH 2L]°, 27. These complexes are air-sensitive and show a very low Ni(II)/Ni(III) oxidation potential of +0.24 V vs SCE (72, 73). They form 1 1 dioxygen adducts, 27-02, at room temperature in aqueous solution, which are formulated as Nilu-02 based on the EPR spectral data. The magnetic moment of the 1 1 02 adduct is 2.83 BM, which is interpreted in terms of weak interactions of Ni(III) with the superoxide where the spin coupling is weak. The oxygen uptake reaction is first-order with respect to both [02] and [NiH 2L]° in aqueous solutions and yields a second-order rate constant... 

As discussed in Chapter 11, there is a much greater tendency toward spin pairing in the heavier transition metals and consequently Ihe existence of high spin complexes is much less common than among the earlier metals. Thus, m contrast to Nidi), which forms tetrahedral, square planar, square pyramidal, trigonal bipyrarmdal, and octahedral complexes. Pd(ll) and Pull) form complexes that are almost universally low spin and square planar. A few weakly bonded five-coordinate adducts are known and... 

The steric effects of the substituents, such as in 3,5-dialkylpyrazoles, may induce the coordination of small ligands or counterions that usually do not coordinate to transition metal ions. This behaviour is believed to be responsible for the unusual decomposition of transition metal tetra-fluoroborates in the presence of such ligands.43 Otherwise, the coordination number changes, resulting in, for example, square-planar Ni11 (low-spin) complexes, even with rather weak ligands such as 1,2-dimethylimidazole.44... 

STRATEGY Decide from their positions in the spectrochemical series whether the ligands in the complex are weak-field or strong-field ligands. Then judge whether the complex is a high-spin or a low-spin complex. 

---