= Solution
Neutral CO ligands require formal Cr($-\mathrm{II}$), hence $d^8$. With five two-electron <carbonyl ligands>,
$$
\boxed{N_e=8+5(2)=18.}
$$
A \b[<trigonal-bipyramidal molecular geometry>] is expected, analogous in <electron> count to $\mathrm{Fe(CO)_5}$. 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.
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