Unusual Electronic Configurations of Iron

In Chapters 6 and 7 the data for both high-spin and low-spin iron(II) and iron(III) complexes were discussed. Although these are the common electronic configurations for these oxidation states, there is also a minority group of compounds which show either ligand field crossover from high-spin to low-spin configuration or intermediate spin states, and in addition there are some uncommon oxidation states. 

It is also convenient to include here the dithiolene group of complexes, as well as the oxidation states iron(I), iron(IV), and iron(VI). The iron(IV) oxidation state also occurs in oxide systems such as BaFeOj, but these will be discussed separately in Chapter 10. Covalent diamagnetic complexes such as carbonyls are included in Chapter 9 the oxidation state of iron in these complexes spans both positive and negative values and includes iron(0) as in the binary carbonyls themselves. 

The Mossbauer parameters at 293 K and 77 K are clearly those of T2 and Ay configurations (Table 8.1). More extensive measurements [4] over this temperature range show that the spectrum of Fe(phen)2(NCS)2 exhibits lines due to both spin states in the vicinity of the transition region as shown in Fig. 8.1. This shows that the interconversion takes place on a time-scale of greater than 10 s, a factor also common to the other 7 2- i transitions, but contrasting with the A 1- X2 transition in iron(III) where the spectrum is a time-average. 

The tris-(2-aminomethylpyridine)iron(II) halides, [Fe(2-NH2pic)3]X2 (X=C1, Br, I), also show a spin crossover [5]. In this instance the halide anions have a very strong effect on the equilibrium, as shown by the magnetic moment versus temperature plot in Fig. 8.2. No interpretation can be formulated using a simple thermal excitation model. The chloride gives Mossbauer spectra indicating T2 at room temperature, Ai at 4-2 K, and both states in coexistence at 77 K, with no detectable interconversion within the observation time-scale. 

Attempts to prepare the selenocyanato derivative have been unsuccessful, but a compound nominally Fe(bipy)2.33(NCSe)2 can be obtained which contains both high-spin and low-spin Fe(II) in the ratio 1 2 [7], It shows no 

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The rr-allyliron tricarbonyl cations are unusual because the iron atom is two electrons short of the next rare gas configuration. 

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