Metal Hydride Vibrations: The Trans Effect of the Hydride

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(PiPr2CH2pyCH2PiPr2) provide M–H force constants and H–M–H interaction force constants that increase as FeII II III. DFT methods are used to determine, for the first time, the effect of the metal ion (MnI, ReI, FeII, RuII, OsII, CoIII, RhIII, IrIII, PtIV) and ligands on the gap in wavenumbers between the symmetric νsymH–M–H and antisymmetric νasymH–M–H vibrational modes of hydrides that are mutually trans in d6 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 trans-dihydrides, 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(PiPr2CH2pyCH2PiPr2) (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 (Δνt −235 cm–1) allows us to improve the simple equation reported previously, which allows a better estimate of νM–H.

反式二氢配合物(trans-Dihydride complexes)在诸多均相催化过程中具有重要意义。本研究首次采用振动光谱与密度泛函理论(density functional theory, DFT)方法，揭示出4d与5d金属相较于3d金属，能够更高效地传递配体对氢化物的反位效应，同时更有效地耦合反位氢化物的运动。该金属的这一特性与其更高的氢化物反应活性密切相关。

反式-FeH₂(dppm，双(二苯基膦)甲烷)₂、反式-RuH₂(PPh(OEt)₂)₄以及面式(mer)-IrH₃(PiPr₂CH₂pyCH₂PiPr₂)的红外(IR)与拉曼(Raman)光谱数据，给出了M–H键力常数与H–M–H相互作用力常数，其数值随FeⅡ、RuⅡ、OsⅡ的顺序依次递增。

本研究首次利用密度泛函理论方法，探究了金属离子（MnⅠ、ReⅠ、FeⅡ、RuⅡ、OsⅡ、CoⅢ、RhⅢ、IrⅢ、PtⅣ）与配体对d₆八面体配合物中互为反位的氢化物的对称νsymH–M–H与反对称νasymH–M–H振动模式之间的波数差的影响。该波数差的大小反映了这些振动模式间的耦合或相互作用程度，且被证实是金属离子的特征属性。

研究发现，极化率更高的4d与5d金属离子的平均波数差为246 cm⁻¹，而3d金属的平均波数差仅为90 cm⁻¹。这一结论已通过3d、4d及5d过渡金属反式二氢配合物的实验得到验证，相关红外与拉曼光谱均已测得：包括文献报道的反式-RuH₂(PPh(OEt)₂)₄，以及本研究中的反式-FeH₂(PPh₂CH₂PPh₂)₂与面式-IrH₃(PiPr₂CH₂pyCH₂PiPr₂)。

由于4d与5d金属离子往往是极性键底物氢化反应的更优催化剂，该波数差或许可作为催化剂动力学氢化物活性的基本决定因素。通过确定该波数差的大小，并新估算出较大的氢化物反位效应（Δνt = -235 cm⁻¹），我们得以优化此前报道的简易公式，从而实现对νM–H更为精准的估算。