Metal hydride ions, bond energies

Figure 4. Comparison of theoretical and experimental bond dissociation energies for first row diatomic metal hydride ions. Data from reference 27.

Finally, the proton affinities of several atomic metal anions, M" (M -V, Cr, Fe, Co, Mo, and W), have been determined by bracketing methods (47). Combining these data with measured electron affinities of the metals yielded homolytic bond energies for the neutral hydrides, D (M-H). The monohydride bond energies compare favorably with other experimental and theoretical data in the literature and were used to derive additional thermodynamic properties for metal hydride ions and neutrals. 

There is a lively controversy concerning the interpretation of these and other properties, and cogent arguments have been advanced both for the presence of hydride ions H" and for the presence of protons H+ in the d-block and f-block hydride phases.These difficulties emphasize again the problems attending any classification based on presumed bond type, and a phenomenological approach which describes the observed properties is a sounder initial basis for discussion. Thus the predominantly ionic nature of a phase cannot safely be inferred either from crystal structure or from calculated lattice energies since many metallic alloys adopt the NaCl-type or CsCl-type structures (e.g. LaBi, )S-brass) and enthalpy calculations are notoriously insensitive to bond type. 

Various modes of termination of anionic polymerization can be visualized. The growing chain end could split out a hydride ion to leave a residual double bond. This is, however, a high activation energy process and has not as yet been reported in the cases where alkali metal cations are present. It is important in systems involving Al—C bonds, however (73). A second possibility is termination through isomerization of the carbanion to an inactive anion. Proton transfer from solvent, polymer, or monomer would also cause termination of the growing chain. Lastly, the carbanion could undergo an irreversible reaction with solvent or monomer. The latter three types have been shown or postulated as termination or transfer reactions. 

This section briefly considers the proton H , the hydride ion H , the hydrogen molecule ion H2", the triatomic 2-electron species H3+ and the recently established cluster species The hydrogen atom has a high ionization energy (1312kJmol ) and in this it resembles the halogens rather than the alkali metals. Removal of the Is electron leaves a bare proton which, having a radius of only about 1.5 x 10 pm, is not a stable chemical entity in the condensed phase. However, when bonded to other species it is well known in solution and in... 

FIGURE 22 First-row (closed symbols) and second-row (open symbols) transition metal ion hydride bond energies versus the atomic metal ion promotion energy to an sd" spin-decoupled state the lines are linear regression fits to the first- and second-row data excluding Pd. Reproduced with permission from Armentrout and Georgiadis (1988). Copyright 1988 Elsevier. 

The tendency of a transition metal hydride to transfer H to a substrate is called hydricity [ 12]. It is possible to determine the Gibbs free energy of the splitting of the covalent polar M-H bond to afford a metal cation and the hydride ion in solution. The hydricity is not parallel to the polarity of the M-H IxMid, nor can it be predicted on the basis of the electronic structure of the metal atom. It is a complex property that can be modeled for transition metal hydrides using multiparameter approaches. The hydricity concept applies to the interaction of M-H bonds with CO2 as well [13]. The reactivity of M-H bonds toward CO2 is linked to reactions that may have industrial interest, such as the hydrogenation of CO2 to afford formic acid (4.2) and the electrochemical reduction of CO2 to other Cl or C1+ molecules (4.3). 

In ionic compounds with certain metals, hydrogen exists as the hydride ion, H . Determine the electron affinity of hydrogen that is, for the process H(g) -I- e H (g)- To do so, use data from Section 12-7 the bond energy of H2(g) from Table 10.3 —812kJmoP for the lattice energy of NaH(s) and —57kJmoP NaH for the enthalpy of formation of NaH(s). 

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