Phenyl substituents in the 6-positions of terpyridine ligands are important for controlling the spin state of their iron(II) complexes. The introduction of phenyl substituents in both the 6- and 6″-positions leads exclusively to the orange high-spin iron(II) complexes, whilst the presence of a single 6-phenyl substituent results in spin-crossover systems (see scheme).
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Constable et al. (1999) studied this question.