Finally, reductive elimination from Ni(II)-H species V is considered thermodynamically feasible, yielding the dehalogenation by-product ArH, which was observed quantitatively by both F NMR and GCMS, and regenerating Ni(0), thereby closing the catalytic cycle. We assume that 2a already features high (Z):(E) ratios at this stage (vide infra). Nevertheless, we examined the subsequent possibility of an energy transfer process, where remaining (E)-2a is converted efficiently to (Z)-2a in the presence of photocatalyst Ir-3 (Figure 2B, Table). In the absence of light, no thermal background isomerization is observed (Entry 1). Likewise, without PC present poor selectivity is observed for UV-light or blue light irradiation, respectively (Entries 2-3). Only under standard reaction conditions high (Z)-selectivity is obtained (Entry 4). To further confirm the (E) to (Z) photo-isomerization, we followed the kinetics by photo-probe 1 H NMR spectroscopy. The results indicated full conversion within 5 min with a rate constant for the isomerization reaction (E → Z) of 1.5 s -1 (Figure ). We were surprised to observe a strong difference in reaction performance for various protected amino acids, which is in contrast to previous reports. Carbamate amine protecting groups like Boc performed exceptionally well (Figure , Table ). However, blocking the amine proton with a methyl group completely suppressed the reactivity and product 3 was not formed (Entry 2). Amides, like acetate and benzoate, gave slightly lower yield with maintained distinct (Z)-selectivity (Entries 3-4). Unfortunately, phthalimide or tosyl protection groups hampered the reaction and products 2e and 2f could be obtained (Entries 5-6). DFT calculations (B3LYP D3/def 2 level of theory, see SI) indicate steric demand of Ni(II) IV intermediates and distortion toward the (Z)-elimination product (Figure , gray box), similarly as reported by Newhouse for a cobalt catalytic system. Unfavorable geometries are obtained for 3 and 2e thus pointing toward a steric mismatch. Steric bulk at the final double bond was tolerated giving access to trifunctionalized olefins (Figure , bottom right). β-Methyl andphenyl phenylalanines were both tolerated, but showed a strong aryl halide dependence leading to 49% analytical yield for 4 with X4 and 85% yield for 5 with X3, respectively. α-Methyl phenylalanine 6 was well tolerated and a smaller dependence toward either of the conditions was observed. Further research is required to better understand the origin of steric and electronic influences on the reactivity.