Bonding, complex transition metal hydride

A second major route to metal-metal complexes, related to the salt-elimination method described above, is elimination of neutral molecules with concurrent formation of metal-metal bonded complexes. Transition metal hydrides readily undergo these dinuclear reductive elimination reactions. The oxidative addition/reductive elimination see Oxidative Addition and Reductive Elimination) reaction of molecular hydrogen is a key reaction in this area (equation 47). 

INTERMOLECULAR DIHYDROGEN BONDING IN TRANSITION METAL HYDRIDE COMPLEXES... 

There exist two broad families of complex transition metal hydrides. The first contains hydrogen bonded to transition elements only and has the general composition... 

The M-H bonds of transition-metal hydride complexes may be cleaved heterolyti-cally (H, H transfer) or homolytically (H transfer). AG for the transfer in Equation 1.1 is readily quantified by pKj measurements (see Chapter 3). Analogous measurements for H transfer, or hydricities , are difficult because the loss of generates a vacant coordination site. However, AG for Equation 1.2 can be determined indirectly, from electrochemical and pJG measurements in the appropriate solvent [1, 2], and we can thus compare the hydricities of various hydride complexes (see Chapter 3). The lowest values of AG. (corresponding to the complexes most eager to transfer H ) are found for second- and third-row transition metals [3], which is why those (relatively expensive) metals are good donors and effective catalysts for reactions like ionic hydrogenation [5-10],... 

Two closely related reactions, (a) and (b), illustrated by Eq. (12) (Rj = HPhj, Etj, Phj, CI3, CljPh) and (13), of silicon hydrides with transition metal complexes generate compounds with Si—M bonds with elimination of hydrogen (a) cleavage of metal-metal bonds and (b) reaction with transition metal hydrides. Reactions discussed in this section are relevant to... 

A Fe-H bond is generally polarized as Fe -H because H is more electronegative than Fe. However, iron hydride complexes impart much less negative charge to the hydride than early transition-metal hydride complexes. 

Because of their low intrinsic electronegativities, neutral late transition metals (bearing an abundance of lone pairs) can serve as good donors in nM— ctah interactions of the form (5.69a). Furthermore, transition-metal-hydride bonds (Section 4.4.1) often display sufficient covalency or polar-covalency (particularly in transition-metal cations) to serve as good acceptors in ns— ctmh interactions of the form (5.69b). In the present section we shall briefly examine the simple example of platinum dihydride (PtH2) as a water-mimic in binary H-bonded complexes with H20,... 

Since the nature of the hydride chemical shifts, particularly in transition metal hydride complexes, is not simple [32], there is no reliable correlation between Sh and the enthalpy of dihydrogen bonding. Nevertheless, the chemical shifts of hydride resonances and their changes with temperature and the concentration of proton-donor components, for example, can be used to obtain the energy parameters for dihydrogen bonding in solution. As earlier, the enthalpy (A/f°) and entropy (AS°) values can be obtained on the basis of equilibrium constants determined at different temperatures. Let us demonstrate some examples of such determinations. 

Table 7.1 lists energies of dihydrogen bonding and the H- H distances that have been calcnlated for transition metal hydride systems in the gas phase. As shown, the transition metal complexes calcnlated have different ligand environments and interact with different proton donors. 

ENERGY AND STRUCTURAL PARAMETERS OE INTERMOLECULAR DIHYDROGEN-BONDED COMPLEXES IN SOLUTIONS OE TRANSITION METAL HYDRIDES... 

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