Which of the following can be reduced easily?

\mathrm{V}(\mathrm{CO})_6$can be easily reduced to \left[\mathrm{V}(\mathrm{CO})_6\right]^{-}$. $\mathrm{V}(\mathrm{CO})_6$ has a total of 17 bonding electrons, hence it is very reactive and unstable. \left[\mathrm{V}(\mathrm{CO})_6\right]$on the other hand has complete set of 18 bonding electrons as an electron is added into the bonding orbital when$\mathrm{V}(\mathrm{CO})_6$gets reduced to \left[\mathrm{V}(\mathrm{CO})_6\right]^{-}$. All others have 18 bonding electrons.