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Neutral CO ligands require formal Cr($-\mathrm{II}$), hence $d^8$. With five two-electron carbonyl ligands,A trigonal-bipyramidal molecular geometry is expected, analogous in electron count to $\mathrm{Fe}(\mathrm{CO}{)}_5$. Strong pi-acceptor ligands stabilize a low oxidation state through backbonding, and this five-coordinate structure accommodates the closed-shell 18-electron configuration. The negative oxidation state is a formal assignment; it does not mean that a free chromium dianion is trapped inside the molecule.