Spin-free complex

SPIN-PAIRED AND SPIN-FREE COMPLEXES LIGAND FIELD EFFECTS 357... 

Nucleophilic reactions of the spin-paired tris(o-phenanthroline) iron(II) ion are bimolecular 70-72). The tris complex is close to the spin-free complex in energy since dithiocyanatobis(o-phenanthroline) iron(II) exists in a spin-free = spin-paired equilibrium 53). The corresponding tris(o-phenanthroline)nickel(II) ion is unaffected by the same nucleophile, which probably rules out Sat2 attack on the organic ring as the predominant factor. 

Mn also occurs in both spin-free complexes such as [Mn(acac)3] , and in spin-paired complexes with two unpaired electrons such as Mn(CN) The Re complexes Uke Re(NH3) + ReCl4 are, however, diamagnetic. 

Comparison of Palmer s and Piper s estimate of 10 Dy (16.5—19 AK) for spin-paired Fe(bipy) , with the value of ca. 13.5 AK obtained from Table 1 for the hypothetical spin-free complex agrees reasonably with this expectation. 

Figure 8-10. A comparison of Coulombic and exchange contributions in the free-ion and high-spin octahedral complexes (see text).

The assessment of k is of some importance since it relates to the question as to how much if any of the free energy of activation barrier is due to the spin-forbidden character of the transition. From the experimental point of view, Eq. (49) shows that the transmission coefficient k and the activation entropy AS appear in the temperature-independent part of the rate constant and thus cannot be separated without additional assumptions. Possible approaches to the partition of — TAS have been discussed in Sect. 4 for spin transition complexes of iron(II) and iron(III). If the assumption is made that the entropy of activation is completely due to k, minimum values between 10 and 10 are obtained for iron(II) and values between 10 and 10 for iron(III). There is an increase of entropy for the transition LS -+ HS and thus the above assumption implies that the transition state resembles the HS state. On the other hand, volumes of activation indicate that the transition state should be about midway between the LS and HS state. This appears indeed more reasonable and has the... 

Table 6.2 Energies of the lowest spin-free states originating from the SH multiplet and the energies of the low-lying Kramers doublets of the DyZn3 complex.

The original objective in binding metal complexes to insoluble supports was undoubtedly to get over the solubility problems of homogeneous catalysts which made their separation after reaction such a problem. However, there have been other spin-off advantages. In particular the selectivity of the supported catalysts is often greater than their homogeneous counterparts because, in addition to the electronic and steric selectivity present in the free complex, further selectivity arises because the solvent channel leading up to the active site has both a size restriction and polar properties. This results in the diffusion rate of two... 

In this paper we will first review the manner in which spin-free permutation and point group symmetry arise. Some general concepts concerning time-dependent processes will be discussed. Spin-free processes in which spin is conserved will be studied, and spin-free spin conservation rules and examples will be given. Special attention will be given to processes in which spin apparently is not conserved, but is in actuality. In addition, we will treat processes in which spin is not conserved. The role of doublepoint group symmetry and of Franck-Condon factors will be developed. Special emphasis is given to spin-forbidden processes in methylene, benzene, and chromium(III) complexes. 

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