LAUSR

Nonclassical iron(II) polyhydride complexes supported by PNP-pincer ligands were found to be very active catalysts for the Z-selective hydroboration and head-to-head dimerization of terminal alkynes. Iron(II) bis(acetylide) complexes are identified as catalytically active species in both transformations, which proceed via different reaction pathways, however. These species as well as intermediates relevant for the elucidation of selectivity could be trapped and fully characterized spectroscopically. X-ray structures of key intermediates are presented. Detailed DFT calculations provide a deeper insight into the activation of the precatalysts, and comprehensive catalytic cycles are elucidated. The proposed mechanisms provide a consistent picture of the different reaction pathways and fully explain the stereoselective steps of these transformations.