Paramagnetic hydrides

Few paramagnetic hydrides are known and even fewer have been studied by Electron Paramagnetic Resonance. For example, [TiH2Cp2] and TaH2(dmpe)2Cl2 have been examined in this way. 

Habboush DA, Osteryoung RA (1984) Paramagnetic hydride complexes of niobimn(lV) and tantalum(IV). Inorg Chem 23 1726-1730... 

In this chapter, we shall first briefly survey the methods used for the synthesis or generation of paramagnetic hydride complexes, subsequently survey the known classes of stable open-shell hydride complexes, then review our current knowledge on the fundamental properties of the M-H bond in open-shell compounds, and finally examine the various established decomposition modes. No review articles specifically focusing on paramagnetic hydride compounds and their chemical reactivity appear to have been previously published. With few exceptions, this chapter will be limited to the analysis of monometallic species of the d elements. The f elements are treated in a separate chapter in this... 

Since, as a general rule, paramagnetic hydride complexes are thermally sensitive compounds and are unstable toward a variety of decomposition pathways (see section 6.5), all methodologies for their preparation/generation should in principle be carried out under mild conditions. Certain products, however, have proven sufficiently stable to be isolated under normal laboratory conditions (see section 6.3). 

For the purpose of this section, the attribute stable is not restricted to those compounds that are sufficiently inert to be isolated and fully characterized, but is also extended to sufficiently long-lived products in solution under any kind of experimental conditions (including low temperature) to allow an unambiguous spectroscopic characterization. Most of the reported stable paramagnetic hydride complexes have a 17-electron configuration, while those having 19, 16 and 15 electrons are much less represented. Paramagnetic hydride complexes of the f elements are not considered in this chapter. 

Compounds ReH(PPh3)2(acac)X2 (X = Cl, Br, I) are apparently the first isolated and stable paramagnetic hydride compounds ever reported [56], They were obtained by oxidation of ReH2(PPh3)3(acac), see Scheme 4. Besides elemental analyses, they have been characterized by magnetic susceptibility and IR spectroscopy. In addition, further oxidation or protonation reactions leas to the liberation of H2. 

In many cases, the reactivity of paramagnetic hydrides can be traced to changes in the fundamental properties of the metal-hydrogen bond relative to the 18-elec-tron compounds from which they are prepared or generated in situ. Before examining the various reactivity pathways of paramagnetic hydride complexes, it is therefore necessary to review our current knowledge in this area. 

When 17-electron complexes generated by oxidation decompose by deprotonation, the overall stoichiometry is highly dependent on the nature of the base capturing the proton, on the stability of the proton transfer products, and on the rate of oxidation. Equation 14 shows proton capture by an external base (e.g. pyridine or lutidine, often used for this kind of studies). The resulting 17-electron deprotonated radical may in principle evolve by either dimerization (equation 15), or by reaction with the paramagnetic hydride precursor, (equation 16), or by subsequent oxidation, which is usually assumed to be preceded by solvent coordination (equations 17-18) [86]. The oxidation potential of M(S) may be less positive than that of the MH precursor, resulting in an overall two-electron process for the oxidation of MH. 

Steric protection has been shown to increase the inertness of paramagnetic hydrides not only toward deprotonation, but also toward disproportionation. While the one electron oxidation of Cp MoH3(dppe) in a cyclic voltammetric experiment in MeCN is chemically reversible only a rates greater than 0.1 V s , the same process for (C5Et5)MoH3(dppe) remains chemically reversible even at a rate of 10 mV s [109]. 

When a paramagnetic hydride is obtained by a one-electron reduction process starting fi om a saturated precursor, the electron density on the hydride ligand increases, resulting in a decrease of its acidity and an increase of its basicity... 

I am grateful to the students and postdocs who have contributed to advancing the area of paramagnetic hydride chemistry in my laboratory. My activities in this area have mostly been carried out at the University of Maryland at College... 

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