I am playing Darius. I play Cleave. Opponent uses Hard Bargain (non-repeated). Can I pay 1 energy and use Darius Legend Ability to pay Hard Bargain's effect?

Yes. You can activate Darius's Legend ability during Hard Bargain's resolution to generate the energy you need to pay the [2].

Here's how it works:

• Hard Bargain is on the Chain above Cleave. When it resolves, it gives you the option to pay [2] or have Cleave countered. This payment happens during the resolution of Hard Bargain.
• Darius's ability (exhaust → [Reaction], [Legion] — [Add] 1 energy) is an activated ability that Adds resources and has Reaction. Per Rule 429.3, such abilities can be activated "during the playing or resolution of other spells and abilities, any time that those spells or abilities require resources be paid." And per Rule 429.3.a, it "immediately finalizes and resolves, even during the resolution of spells and abilities."
• Since you've already played Cleave this turn, Darius's condition ("Get the effect if you've played a card this turn") is satisfied.

Sequence: When Hard Bargain asks you to pay [2], you activate Darius (exhaust him, add 1 energy to your Rune Pool), then pay that 1 energy plus 1 more energy from your runes/channeled sources to meet the [2] cost. Cleave is saved from being countered.

Note: Darius only generates 1 energy, so you still need to supply the other 1 energy from your Rune Pool (runes or other sources).