Crystals suitable for X-ray crystallography obtained from a toluene solution allowed the solidstate structure of the product to be determined, confirming addition of the Al-Cu unit across the alkyne triple bond in syn fashion to give the aluminocuprated alkene, 2-Et (Figure ). As such, 2-Et represents the first example of an (aluminylalkenyl)copper complex. More broadly, while 1,2dimetallated species have previously been synthesized via Pt-catalysed approaches, the formation of 2-Et represents the first structurally authenticated report of direct 1,2-heterodimetallation by insertion of an alkyne into an M-M' bond. The 13 C NMR spectrum of 2-Et displays a pattern characteristic of the dimetallated alkene. The copper-bound olefinic carbon appears as a low-field resonance at 188.3 ppm (with doublet splitting arising from coupling to the phosphine ligand 2 J CP = 73 Hz), and the signal for the aluminium bound quaternary carbon is found at 154.1 ppm, i.e. slightly higher than previously reported alkenyl aluminium species. Although 2-Et can be cleanly isolated in crystalline form as the syn isomer, extended reaction times can be employed to enable onward reactivity, with complete consumption of 2-Et apparent after 24 h in toluene solution at room temperature. The 1 H NMR spectrum at this point shows the formation of a second species, characterized by quartet and doublet of quartets signals for the alkenyl ethyl CH 2 groups, similar to (but shifted from) those measured for 2-Et. The 31 P NMR spectrum shows a new signal at δ P = 58.5 ppm (cf. 55.5 ppm for 2-Et) and the corresponding C NMR spectrum displays a new downfield doublet at 183.0 ppm ( 2 J PC = 71 Hz), corresponding to a [ t Bu 3 PCu]-bound alkenyl carbon (cf. δ C = 188.3, 2 J CP = 73 Hz for 2-Et). The spectroscopic evidence implies the formation of a closely related alumino-cuprated alkene. We propose that this species is the corresponding anti dimetallated alkene 3-Et (Scheme 1). While this assignment could not be confirmed crystallographically for 3-Et, comparison with the related (structurally characterised) system 3-Pr,Ph (see below) confirms that stepwise syn to anti isomerization is viable. As such, this system is uniquely capable of allowing selective access to isolable syn or anti dimetallated alkenes, by appropriate control of the reaction conditions.