Spin Crossover in Dinuclear Iron II Complexes

The interplay between magnetic coupling and ST in 2,2 -bipyrimidine (bpym)-bridged iron(ll) dinuclear compounds has been studied by magnetic susceptibility and Fe Mossbauer spectroscopy [24]. The molecular stmctures of [Fe(bpym) (NCS)2]2(bpym) and [[Fe(bt)(NCS)2]2(bpym) (bt = 2,2 -bithiazoline) are shown in Fig. 8.12. The magnetic behavior in the form of XmT versus Tplots, being the 

The effective hyperfine field at the iron nuclei of a paramagnetic nonconducting sample in an external field //ext may be estimated as //ext 

These results obtained in applied field clearly prove that the ST in the dinuclear compounds under study proceeds via [HS-HS] O [HS-LS] O [LS-LS]. Simultaneous ST in both iron centers of the [HS-HS] pairs, leading directly to [LS-LS] pairs, apparently can be excluded, at least in the systems discussed above. This is surprising in view of the fact that these dinuclear complexes are centrosymmetric, that is, the two metal centers have identical surroundings and therefore, experience the same ligand field strength and consequently, thermal ST is expected to set in simultaneously in both centers. In other dinuclear iron(II) complexes, however, thermally induced direct ST from [HS-HS] to [LS-LS] pairs does occur and, indeed, has been observed by Mossbauer measurements [30, 31]. 

An interesting Mossbauer study has been reported on the dinuclear SCO complex [Fe2 (PMAT)2](BF4)4-DMF (PMAT 4-amino-3,5-bis [(2-pyridylmethyl) amino]methyl -4H-1,2,4-triazole), where thermal ST occurs from [HS-HS] to the stable endproduct [HS-LS] [32]. The molecular structure and magnetic behavior of this complex was reported earlier by Brooker et al. [33, 34] (Fig. 8.15). At ca. 225 K, the complex undergoes a sharp half ST from the HS state, T2, to a state containing 50% HS and 50% LS, Af, isomers. The single-crystal structural analysis 

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