The first [ECE]Ni(II) pincer complexes with E = Si-II and E = Ge-II metallylene donor arms were synthesized via C-X (X = H, Br) oxidative addition, starting from the corresponding [EC(X)E] ligands. These novel complexes were fully characterized (NMR, MS, and XRD) and used as catalyst for Ni-catalyzed Sonogashira reactions. These catalysts allowed detailed information on the elementary steps of this catalytic reaction (transmetalation -> oxidative addition -> reductive elimination), resulting in the isolation and characterization of an unexpected intermediate in the transmetalation step. This complex, {[ECE]Ni acetylide -> CuBr} contains both nickel and copper, with the copper bound to the alkyne pi-system. Consistent with these unusual structural features, DFT calculations of the {[ECE]Ni acetylide -> CuBr} intermediates revealed an unusual E-Cu-Ni three-center two-electron bonding scheme. The results reveal a general reaction mechanism for the Ni-based Sonogashira coupling and broaden the application of metallylenes as strong a-donor ligands for catalytic transformations.