To determine which compound has the longest C–O bond length, we need to consider the electronic structure and bonding in each complex.$\newline$The C–O bond length is influenced by the back-donation of electrons from the metal to the $\pi^*$ orbitals of CO.$\newline$More back-donation results in a weaker C–O bond and thus a longer bond length.$\newline$Let's analyze each option:$\newline$• Option 1: $Ni(CO)_4 \newline$