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How do rare-earth metals react with acids hydrogen and hydrofluoric acid?

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A line graph showing electron probability distributions for gadolinium, with separate curves for 4f, 5s, 5p, 5d, and 6s orbitals. The x-axis shows distance from the nucleus in atomic units and the y-axis shows probability P2(r). — rare-earth element Electron probabilities, P²*(r)*, for the 4f, 5s, 5p, 5d, and 6s electrons of gadolinium.

Rare-earth metals exhibit distinct chemical behaviors when exposed to acids and hydrogen:

Reaction with Acids: Rare-earth metals react vigorously with most acids, releasing hydrogen gas and forming the corresponding rare-earth–anion compound. However, they do not react in this manner with hydrofluoric acid (HF). Instead, when placed in hydrofluoric acid, the metals form an insoluble $R F_{3}$ coating that effectively prevents any further reaction.1
Reaction with Hydrogen: These metals readily react with hydrogen gas to form $R H_{2}$ compounds. Under conditions of strong hydrogen pressure, the trivalent rare-earth metals (with the exception of scandium) can further react to form the $R H_{3}$ phase.12

It is important to note that rare-earth hydrides are sensitive to air and must be handled in controlled environments, such as glove boxes, to maintain their integrity.2