Coordination Compounds

Among $\mathrm{V}(\mathrm{CO})_6, \mathrm{Cr}(\mathrm{CO})_5, \mathrm{Cu}(\mathrm{CO})_3, \mathrm{Mn}(\mathrm{CO})_5, \mathrm{Fe}(\mathrm{CO})_5,\left[\mathrm{Co}(\mathrm{CO})_3\right]^{3-},\left[\mathrm{Cr}(\mathrm{CO})_4\right]^{4-}$, and $\operatorname{Ir}(\mathrm{CO})_3$, the total number of species isoelectronic with $\mathrm{Ni}(\mathrm{CO})_4$ is _________.

[Given, atomic number: $\mathrm{V}=23, \mathrm{Cr}=24, \mathrm{Mn}=25, \mathrm{Fe}=26, \mathrm{Co}=27, \mathrm{Ni}=28, \mathrm{Cu}=29, \mathrm{Ir}=77$ ]