Non-reversible (2+2) alkyne binding. Addition of 1 equiv. of a terminal alkyne RC≡CH (R = Ph, SiMe3 or n-Bu, 2-Py) to a solution of 1 in C6D6 at room temperature resulted in an immediate colour change from dark to bright orange, in each case leading to the quantitative formation of the cycloaddition products 2a-d (Figure ). These reactions appear to be non-reversible. 2a-d were all isolated in yields of >95 % and are stable in both solution and in the solid state. 2b was characterised by single crystal X-ray diffraction (Figure ). 2a-d all give rise to very similar and characteristic NMR signals for the coordination of the alkyne. For example, the 31 P{ 1 H} NMR spectrum of 2a exhibits a mutually coupled spin system comprising a doublet at δP = 29.0 ppm (2P) and a triplet resonance at 19.9 ppm (1P) being consistent with the chemical non-equivalence of the axial and equatorial phosphine ligands. In the 1 H NMR spectrum, the bridging hydrides appear as broadened virtual triplet at δH = -14.71 ppm and a doublet of triplets at δH = 10.02 ppm ( 3 JHP = 13.4 and 5.6 Hz) can be found for the ArC≡CH proton of the coordinated alkyne. No evidence for C-H activation of the sp C-H bond was found, despite the acidity of terminal alkynes.