The sequential addition of HCN and Al to a helium nanodroplet leads barrierlessly to a bent, planar, (2)A', HCNAl species, which is computed to be 20.3 kcal/mol below the separated reactants. A ro-vibrational band near 2690 cm(-1) is assigned to the v(1) (a') CH stretch. A Natural Bond Orbital analysis of this reaction product reveals that both ligand-to-metal sigma-donation and metal-to-ligand pi-donation are responsible for the strong bonding between HCN and Al. A significant donor-acceptor interaction between the C-N and C-H anti-bonding orbitals is responsible for the bent geometry and the 619 cm(-1) red shift of the CH stretch band upon complexation. (C) 2012 Elsevier B.V. All rights reserved.