New pathways in the reaction between the 1,2,4-triphosphole P3C2But2CH(SiMe3)2 and selenium: Crystal and molecular structures of three new cage compounds, P3Se4C2But2CH(SiMe3)2, P3Se3C2But2 H2CH(SiMe3)2 and P5Se2C4But4(CHSiMe3But)

Three new, fully structurally characterised cage compounds P3Se4C2But2CH(SiMe3)2, P3Se3C2But2H2CH(SiMe3)2 and P5Se2C4But4(CHSiMe3But), are formed (together with the previously reported 1,2,4-selenadiphosphole, P2SeC2But2) from the reaction of the 1,2,4-triphosphole P3C2But2CH(SiMe3)2, with elemental selenium. Possible mechanistic aspects concerning this and related reactions are discussed.