trans-Dihydride complexes are important in many homogeneous catalytic processes. Here vibrational spectroscopy and density functional theory (DFT) methods are used for the first time to reveal that 4d and 5d metals transmit more effectively than the 3d metals influence of the ligand trans to the hydride and also couple the motions of the trans-hydrides more effectively. This property of the metal is linked to higher hydride reactivity. The IR and Raman spectra of trans-FeH2(dppm)(2), trans-RuH2(PPh(OEt)(2))(4), and mer-IrH3(P(i)Pr(2)CH(2)pyCH(2)P(i)Pr(2)) provide M-H force constants and H-M-H interaction force constants that increase as Fe-II < Ru-II < Ir-III. DFT methods are used to determine, for the first time, the effect of the metal ion (Mn-I, Re-I, Fe-II, Ru-II, Os-II, Co-III, Rh-III Ir-III Pt-IV) and ligands on the gap in wavenumbers between the symmetric nu(H-M-H)(sym) and antisymmetric nu(H-M-H)(asym) vibrational modes of hydrides that are mutually trans in d(6) octahedral complexes. The magnitude of this gap reflects the degree of coupling of, or interaction between, these modes, and this is shown to be a distinctive property of the metal ion. The more polarizable 4d and 5d metal ions are found to have an average gap of 246 cm(-1), while the 3d metals have only 90 cm(-1). This has been verified experimentally for 3d, 4d, and 5d transition-metal transdihydrides, where both the IR and Raman spectra have been measured: trans-RuH2(PPh(OEt)(2))(4) (from the literature) and trans-FeH2(PPh2CH2PPh2)(2) and mer-IrH3(P(i)Pr(2)CH(2)pyCH(2)P(i)Pr(2)) (this work). Because the 4d and 5d metal ions tend to be better catalysts for the hydrogenation of substrates with polar bonds, this gap may be a fundamental determinant of the kinetic hydricity of the catalyst. Finding the magnitude of this gap and a new estimate of the large hydride trans-effect (Delta nu(t) 235 cm(-1)) allows us to improve the simple equation reported previously, which allows a better estimate of nu(M-H).