Mercury can form amalgams with metals, such as gold, however how exactly does the "reaction" if you can call it that take place?
My understanding of metal bonds is that the atoms of a metal are surrounded by a "sea" of electrons, however I don't understand how mercury can "penetrate" metals and weaken their structure or "dissolve" them.
How does it do this? Also does this apply to Gallium?
An amalgam is simply an alloy of some metal with mercury.
Alloys are simply solid solutions with metals. Since the mixture of multiple elements disrupts the metal bonding, the properties are often quite different than the components. (In some cases, alloys form so-called intermetallic compounds with defined stoichiometry, but more often, the ratios are very flexible.)
I wouldn't describe the mixture as a reaction, in much the same way that I wouldn't really call dissolving sodium chloride in water as a reaction. You're forming a mixture. Water disrupts the ionic bonding in NaCl, and mercury and other metals can also accept other components or impurities.
Gallium does alloy with a lot of metals, and indium-gallium eutectic alloy is a liquid at room temperature, as is "galinstan".
As to your question about mercury dissolving the metal atoms, consider that mercury is also a metal, so it also participates in similar metal bonding.
To my knowledge aluminum treated with gallium will happily reduce water to hydrogen the way aluminum amalgam does.
But if you want to reduce a nitroalkane you have to consider hydrogen overpotential of the gallium-aluminium compared to aluminium amalgam. That is what dictates the selectivity (substrate reduction vs. hydrogen evolution).
This is the same idea as in electrolytic reductions where you have to use electrodes made of suitable materials (mercury and lead are traditional choices) to be able to actually reduce your compound instead of making hydrogen gas.
