DFT computations suggest that terminal allylphosphinidene pentacarbonyltungsten complex 1 can evolve either by intramolecular P + CC cycloaddition or by [1,2] H C to P shift to give 2 or 6. Upon thermolysis of the 7-allyl-7-phosphanorbornadiene precursor, the transient complex 1 gives two diastereomeric dimers 4a,b deriving from a head-to-tail P + CC intermolecular cycloaddition and another dimer 5 deriving from the [4 + 1] cycloaddition of 1 with 6, followed by an intramolecular hydrophosphination. © 2014 Taylor & Francis Group, LLC.