A family of 4,4'- t Bu 2 -2,2'-bipyridine ( tBu bpy) ligands with substituents in either the 6-position, 4,4'- t Bu 2 -6-Me-bpy ( tBu bpy Me ), or 6 and 6'-positions, 4,4'- t Bu 2 -6,6'-R 2 -bpy ( tBu bpy R2 ; R = Me, i Pr, s Bu, Ph, or Mes), was synthesized. These ligands were used to prepare Ni complexes in the 0, I, and II oxidation states. We observed that the substituents in the 6 and 6'-positions of the tBu bpy ligand impact the properties of the Ni complexes. For example, bulkier substituents in the 6,6'-positions of tBu bpy better stabilized ( tBu bpy R2 )Ni I Cl species and resulted in cleaner reduction from ( tBu bpy R2 )Ni II Cl 2 . However, bulkier substituents hindered or prevented coordination of tBu bpy R2 ligands to Ni 0 (cod) 2 . In addition, by using complexes of the type ( tBu bpy Me )NiCl 2 and ( tBu bpy R2 )NiCl 2 as precatalysts for different XEC reactions, we demonstrated that the 6 or 6,6' substituents lead to major differences in catalytic performance. Specifically, while ( tBu bpy Me )Ni II Cl 2 is one of the most active catalysts reported to date for XEC and can facilitate XEC reactions at room temperature, lower turnover frequencies were observed for catalysts containing tBu bpy R2 ligands. A detailed study on the catalytic intermediates ( tBu bpy)Ni(Ar)I and ( tBu bpy Me2 )Ni(Ar)I revealed several factors that likely contributed to the differences in catalytic activity. For example, whereas complexes of the type ( tBu bpy)Ni(Ar)I are low spin and relatively stable, complexes of the type ( tBu bpy Me2 )Ni(Ar)I are high-spin and less stable. Further, ( tBu bpy Me2 )Ni(Ar)I captures primary and benzylic alkyl radicals more slowly than ( tBu bpy)Ni(Ar)I, consistent with the lower activity of the former in catalysis. Our findings will assist in the design of tailor-made ligands for Ni-catalyzed transformations.