Ab initio calculations are employed to investigate attachment of Group II light metal atoms to hydrocarbon molecules with pairs of double carbon-carbon bonds. Comparison of the atom attachment to one and between two such bonds reveals cooperative binding for some cases, with some counter-intuitive features. Dependence of its manifestation on the system composition and geometry is investigated for a series of metal-hydrocarbon complexes. Attachment-induced catalytic isomerisation of the system is considered at molecular level as well. The complexes are characterized and compared in terms of geometry, stability and various electronic properties. (C) 2010 Elsevier B.V. All rights reserved.