Well metal-nonmetal covalent bonds are present in most of so called salts. Metal-metal bonds like in mercury (I) aren't so ubiquitous but still often found in multi-center complexes.

Mercury
- Mercury is the metal that can form a cation with a metal-metal bond.
- It is a unique metal because it is in a liquid state at room temperature and forms a diatomic molecule with a metal-metal bond.
- The metal-metal bond in mercury is due to the presence of unpaired electrons in the 6s orbital, allowing the formation of a covalent bond between two mercury atoms.

Copper
- Copper is another metal that can form a cation with a metal-metal bond.
- Copper can form metal-metal bonds in certain compounds due to its ability to form coordination complexes with ligands.
- The presence of d electrons in the outer shell of copper allows for the formation of metal-metal bonds.

Osmium
- Osmium is a transition metal that can form metal-metal bonds in certain compounds.
- Osmium is known for its ability to form stable metal-metal bonds due to its d electron configuration.
- The metal-metal bonds in osmium compounds are often seen in organometallic complexes.

Iron
- Iron is not known to form cations with metal-metal bonds.
- Iron typically forms ionic bonds with other elements and does not exhibit the ability to form metal-metal bonds like mercury, copper, or osmium.
- The electronic structure of iron does not favor the formation of metal-metal bonds.