Spin Crossover in a Four-Coordinate Iron(II) Complex

The four-coordinate iron(II) phosphoraniminato complex PhB(MesIm)3Fe−NPPh3 undergoes an S = 0 to S = 2 spin transition with TC = 81 K, as determined by variable-temperature magnetic measurements and Mössbauer spectroscopy. Variable-temperature single-crystal X-ray diffraction revealed that the S = 0 to S = 2 transition is associated with an increase in the Fe−C and Fe−N bond distances and a decrease in the N−P bond distance. These structural changes have been interpreted in terms of electronic structure theory.