Three isoelectronic hexacarbonyls $[\mathrm{V}(\mathrm{CO}){}_6]{}^{-}$, $\mathrm{Cr}(\mathrm{CO}){}_6$ and $[\mathrm{Mn}(\mathrm{CO}){}_6]{}^{+}$ show $\nu (\mathrm{CO})$ bands near $1860$, $2000$ and $2095\mathrm{c}{\mathrm{m}}^{-1}$ (in unknown order). Which assignment is correct and which species has the highest C-O bond order?

1. A$[\mathrm{V}(\mathrm{CO}){}_6]{}^{-}$ 1860, $\mathrm{Cr}(\mathrm{CO}){}_6$ 2000, $[\mathrm{Mn}(\mathrm{CO}){}_6]{}^{+}$ 2095; highest C-O order in $[\mathrm{Mn}(\mathrm{CO}){}_6]{}^{+}$
2. B$[\mathrm{Mn}(\mathrm{CO}){}_6]{}^{+}$ 1860, $\mathrm{Cr}(\mathrm{CO}){}_6$ 2000, $[\mathrm{V}(\mathrm{CO}){}_6]{}^{-}$ 2095; highest C-O order in $[\mathrm{V}(\mathrm{CO}){}_6]{}^{-}$
3. C$[\mathrm{V}(\mathrm{CO}){}_6]{}^{-}$ 1860, $\mathrm{Cr}(\mathrm{CO}){}_6$ 2000, $[\mathrm{Mn}(\mathrm{CO}){}_6]{}^{+}$ 2095; highest C-O order in $[\mathrm{V}(\mathrm{CO}){}_6]{}^{-}$
4. D$\mathrm{Cr}(\mathrm{CO}){}_6$ 1860, $[\mathrm{V}(\mathrm{CO}){}_6]{}^{-}$ 2000, $[\mathrm{Mn}(\mathrm{CO}){}_6]{}^{+}$ 2095; highest C-O order in $[\mathrm{Mn}(\mathrm{CO}){}_6]{}^{+}$