spectrum (at 60.3 ppm). X-ray quality crystals grown from hexane allowed the structure of the product 4-Et to be determined (Figure and Scheme 4). This structure confirms that insertion of CO has occurred selectively into the Cu-C (rather than the Al-C) bond, forming a five-membered AlCCCO metallocycle featuring a pendant CuP t Bu 3 unit. The C=O bond length (1.279(2) Å) is typical of an organic carbonyl function and the heterocycle is essentially planar. Spectroscopically, the C NMR spectrum of 4-Et shows a very low-field shifted doublet at 288.9 ppm, ( 2 J CP = 66 Hz) arising from the copper-bound CO unit. By means of comparison, iridium acyl species, key intermediates in the Cativa process, show chemical shifts in the range 200-230 ppm. The unusually high shift of the acyl group in 4-Et reflects O-coordination to the Lewis acidic Al centre, leading to deshielding of the carbonyl functionality. Consistent with this, the FT-IR spectrum of 4-Et displays a carbonyl stretching vibration at 1200 cm -1 , i.e. significantly lower than typical C=O stretches. To our knowledge, 4-Et is the first example of a crystallographically characterised copper acyl complex. The proximity of the Lewis acidic aluminium site in 4-Et is implicit in this, offering additional stabilization of the acyl unit in the crystal via an O→Al interaction (d(O-Al) = 1.8843(12) Å) which completes a trigonal bipyramidal coordination geometry at aluminium (τ = 0.61). The presence of the proximal aluminium centre is also important mechanistically -with the CO insertion process being found to be exclusively accessible to systems featuring a syn configuration of the two metal centres. Presumably this reflects the fact that a short-lived copper carbonyl intermediate would be activated towards migration of the alkenyl fragment by Lewis acid activation at O. As such, the anti (aluminylalkenyl)copper system 3-Pr,Ph shows no reactivity towards CO under the conditions examined.