Assertion: The C-O stretching frequency of $[\mathrm{Co}(\mathrm{CO}){}_4]{}^{-}$ is lower than that of $\mathrm{Ni}(\mathrm{CO}){}_4$. Reason: The negative charge on cobalt increases $\pi$ back-donation into the ${\pi }^{\ast }$ orbitals of $\mathrm{CO}$.

1. ABoth Assertion and Reason are true, and the Reason is the correct explanation of the Assertion
2. BBoth Assertion and Reason are true, but the Reason is not the correct explanation of the Assertion
3. CAssertion is true but Reason is false
4. DAssertion is false but Reason is true