Among the following pairs of complexes, in which case the △_o value is higher for the first one?

a) [CoF₆]³⁻ and [CoNH₃]⁶⁺
b) [Rh(H₂O)₆]³⁺ and [Co(H₂O)₆]³⁺
c) [Co(H₂O)₆]²⁺ and [Co(H₂O)₆]²⁺
d) [Co(NH₃)₆]³⁺ and [Co(CN₆)]³⁻