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The hardness,electronic,and elastic properties of 5d transition metal diborides with ReB2 structure are studied theoretically by using the first principles calculations.The calculated results are in good agreement with the previous experimental and theoretical results.Empirical formulas for estimating the hardness and partial number of effective free electrons for each bond in multibond compounds with metallicity are presented.Based on the formulas,IrB2 has the largest hardness of 21.8 GPa,followed by OsB2 (21.0 GPa) and ReB2 (19.7 GPa),indicating that they are good candidates as hard materials.