The synthesis and characterisation of bis(diisopropylphosphoryl)ferrocene Fe[(η5-C5H4)P(O)iPr2]2, bis(diisopropylthiophosphoryl)ferrocene Fe[(η5-C5H4)P(S)iPr2]2, and bis(diisopropylselenophosphoryl)ferrocene Fe[(η5-C5H4)P(Se)iPr2]2, the oxidised derivatives of bis(diisopropylphosphino)ferrocene