These species usually do not exist in nature, but they can be synthesized.
- Silver has been reduced in liquid ammonia to give $\ce{Ag-}$.
- A lot of anionic metal carbonyl complexes $\ce{M(CO)_{n}^{m-}}$ have been synthesized:
###-1
- $\ce{[V(CO)6]-}$, $\ce{[Nb(CO)6]-}$, $\ce{[Ta(CO)6]-}$, $\ce{[Mn(CO)5]-}$, $\ce{[Ir(CO)4]-}$, $\ce{[Co(CO)4]-}$, $\ce{[Rh(CO)4]-}$
-2
- $\ce{[Ti(CO6)]^{2-}}$$\ce{[Ti(CO)6]^{2-}}$, $\ce{[Zr(CO6)]^{2-}}$$\ce{[Zr(CO)6]^{2-}}$, $\ce{[Hf(CO6)]^{2-}}$$\ce{[Hf(CO)6]^{2-}}$, $\ce{[Fe(CO4)]^{2-}}$$\ce{[Fe(CO)4]^{2-}}$, $\ce{[Ru(CO4)]^{2-}}$$\ce{[Ru(CO)4]^{2-}}$, $\ce{[Os(CO4)]^{2-}}$$\ce{[Os(CO)4]^{2-}}$
###-3
- $\ce{[V(CO5)]^{3-}}$$\ce{[V(CO)5]^{3-}}$, $\ce{[Nb(CO5)]^{3-}}$$\ce{[Nb(CO)5]^{3-}}$, $\ce{[Ta(CO5)]^{3-}}$$\ce{[Ta(CO)5]^{3-}}$, $\ce{[Mn(CO4)]^{3-}}$$\ce{[Mn(CO)4]^{3-}}$, $\ce{[Re(CO4)]^{3-}}$$\ce{[Re(CO)4]^{3-}}$
###-4
- $\ce{[Cr(CO4)]^{4-}}$$\ce{[Cr(CO)4]^{4-}}$, $\ce{[Mo(CO4)]^{4-}}$$\ce{[Mo(CO)4]^{4-}}$
There's a review by John E. Ellis: Highly Reduced Metal Carbonyl Anions: Synthesis, Characteri's zation, and Chemical Properties on these compounds.
**Edit**
While Christian Aichinger's comment concerning $\pi$-backbonding in metal carbonyls is correct, it is important to realize that $\ce{CO}$ is a neutral ligand!
With other words: The Mond process: $\ce{Ni(s) + 4CO -> Ni(CO)4}$, using nickel powder and carbon monoxide, is not a redox reaction and the oxidation state of the metal still is $0$ in the metal carbonyl.
