The phosphinoamide-linked Co/Hf complexes ICo((Ph2PNPr)-Pr-i)(3)HfCl (4), ICo((i)Pr(2)PNMes)(3)HfCl (5), and ICo((Pr2PNPr)-Pr-i-Pr-i)(3)HfCl (6) have been synthesized from the corresponding tris(phosphinoamide)HfCl complexes (1-3) for comparison with the recently reported tris(phosphinoamide) Co/Zr complexes. Very minor structural and electronic differences between the Zr and Hf complexes were found when the N-Pr-i-substituted phosphinoamide ligands [(Ph2PNPr)-Pr-i](-) and [(Pr2PNPr)-Pr-i-Pr-i](-) were utilized. The reduction products [(THF)(4)Na-{N-2-Co((Ph2PNPr)-Pr-i)(3)HfCl}(2)]Na(THF)(6) (7) and N-2-Co((Pr2PNPr)-Pr-i-Pr-i)(3)Hf (9) are also remarkably similar to the corresponding Zr/Co analogues. In the case of Hf/Co and Zr/Co complexes linked by the N-Mes ligand [(i)Pr(2)PNMes](-) (Mes = 2,4,6-trimethylphenyl), however, more pronounced differences in structure, bonding, and reactivity are observed. While differences associated with 5 are still modest, larger variations are observed when comparing the two-electron reduction product [N-2-Co((i)Pr(2)PNMes)(3)Hf-X][Na(THF)(5)] (8) with its Zr congener. In addition to structural and spectroscopic differences, vastly different reactivity is observed, with 8 undergoing one-electron oxidation to form ClHf(MesNP(i)Pr(2))(3)CoN2 (11) in the presence of MeI, while a two-electron oxidative addition process occurs in a similar reaction with the Zr derivative. The activity of 5 toward Kumada coupling was investigated, finding significantly diminished activity in comparison to Co/Zr complexes.