A hydrogen-mediated Ru-C to Ru-B bond conversion was observed experimentally and supported by the theoretical calculations. Treatment of [eta(5):sigma(c)-Me2C(C5H4)(C2B10H10)]Ru(COD) (1) bearing a Ru-C(cage) a bond with PR3 in the presence of H-2 gave Ru-B(cage) bonded complexes [mu(5):sigma(B)-Me2C(C5H4)(C2B10H10)]RuH2(PR3) (R = Cy (2), Ph (3)) (sigma(C): Ru-C(cage) sigma bond; sigma(B): Ru-B(cage) sigma bond). Complex 3 was converted to [eta(5):sigma(B)-Me2C(C5H4)(C2B10H10)]Ru(L-2) in the presence of L-2 (L-2 = dppe (4), PPh3/P(OEt)(3) (5), PPh3/pyridine (6)) via liberation of H-2 upon heating. These complexes were fully characterized by various spectroscopic techniques, elemental analyses, and single-crystal X-ray diffraction studies. DFT calculations show that this conversion process is both kinetically and thermodynamically favorable and requires involvement of a hydride ligand.