Density functional theory calculations predict a new lower energy route for the formation of the desired interligand addition product from the reaction between ethylene and nickel bis(dithiolene). The new route involves the initial binding of ethylene along the nickel–sulfur bond. The barrier heights for adding ethylene along this bond for the neutral and anionic nickel complexes are compared to each other as well as to a previously published previous mechanism. Selected structural parameters of the studied species have been analyzed to highlight the structural change on specific reactions. It was found that the ethylene/nickel bis-dithiolene reaction occurs preferably via the nickel–sulfur bond of the neutral species, forming a complex which then rearranges to a desired interligand adduct via a low barrier. © 2012 Wiley Periodicals, Inc.