The reduction chemistry of the newly emerging 2-phosphaethynolate (OCP)(-) is not well explored, and many unanswered questions remain about this ligand in this context. We report that reduction of [Th(Tren(TIPS))(OCP)] (2, Tren(TIPS)=[N(CH2CH2NSiPr3i)](3-)), with RbC8 via [2+2+1] cycloaddition, produces an unprecedented hexathorium complex [{Th(Tren(TIPS))}(6)(mu-OC2P3)(2)(mu-OC2P3H)(2)Rb-4] (5) featuring four five-membered [C2P3] phosphorus heterocycles, which can be converted to a rare oxo complex [{Th(Tren(TIPS))(mu-ORb)}(2)] (6) and the known cyclometallated complex [Th{N(CH2CH2NSiPr3i)(2)((CH2CH2SiPr2CHMeCH2)-C-i)}] (4) by thermolysis; thereby, providing an unprecedented example of reductive cycloaddition reactivity in the chemistry of 2-phosphaethynolate. This has permitted us to isolate intermediates that might normally remain unseen. We have debunked an erroneous assumption of a concerted fragmentation process for (OCP)(-), rather than cycloaddition products that then decompose with [Th(Tren(TIPS))O](-) essentially acting as a protecting then leaving group. In contrast, when KC8 or CsC8 were used the phosphinidiide C-H bond activation product [{Th(Tren(TIPS))}Th{N(CH2CH2NSiPr3i)(2)[(CH2CH2SiPr2CH)-C-i(Me)CH2C(O)mu-P]}] (3) and the oxo complex [{Th(Tren(TIPS))(mu-OCs)}(2)] (7) were isolated.