Dihydrogen rhenium complexes

The cationic rhenium complexes [Re(NO)2(PR3)2] (R = Cy, Pr) [50] indeed show great potential for hydrolytic activation of dihydrogen. When [Re(NO)2(PR3)2l is treated with a mixture of H2 and D2 in toluene or chlorobenzene, hydrogen-deuterium scrambling is observed, and HD can be traced in the NMR spectrum of the reaction mixture [51]. The proposed mechanism for this catalytic exchange is illustrated in Scheme 4. 

Finally, insertion of alkynes into dihydrogen mononuclear cationic rhenium complexes, stabilized by the triphos (= l,l,l-tris(diphenylphosphinomethyl)ethane) ligand, leads to vinylidene derivatives which react with water or ethanol eliminating silanols or methane, Fig. 18. Solvents were acetone or THF no chromatography was used. 

About 400 compounds containing the T12-H2 ligand have been made that are stable. Almost all of them are octahedral with the metal in the d electron configuration, from Cr(0) to W(0) across the periodic table to Rh(lll) and Ir(III). There are a few dihydrogen complexes known with metals in other oxidation states. These are relatively rare. There are seven coordinate rhenium compounds like [Re(H2)(H)2(CO)(PMe2Ph)3]+ [8] and very unstable d systems like [Pt(ll2-H2)(PR3)2H]+ [9]. Neutral and cationic complexes are known but not anionic ones. Presumably anionic dihydrogen complexes are unstable because the metal s dTC electron richness promotes the oxidative cleavage of the H-H bond by d7t(M) —> o (H2) donation. 

Figure 18. Reaction of triphos-dihydrogen complexes of rhenium with alkynes, water, and alcohols.

Treatment of the Re(II) complex [NEt4]2[Re(Br)5(NO)J with the bulky alkyl phosphine (PiPrs and PCys) in ethanol, which acts as a solvent and as a reductant, yields for instance the dihydrogen mononitrosyl Re(I) complex [Re(Br)2(NO) (PR3)2( 7 -H2)] (16, R = /Pr a, Cy b). DPT calculation demonstrated propensity of Re(I) centers to bind H2. The interaction with the rhenium center is relatively strong showing an elongated dihydrogen ligand, which lies in plane with the P-Re-P axis (Scheme 15) [33]. 

UV irradiation of [cp Re (CO)3] (M = Mn, Re cp = rj -CsHs) in the presence of hydrogen yielded the non-classical dihydrogen complex [cp Re (CO)2 (H2)], but the classical dihydride [cpRe (CO)2 ra 5-H2]. The manganese complex possessed sufficient thermodynamic stability to arrest further oxidative addition at the metal centre. Formation of a rhenium trans-dihydhde complex must have occurred via a c/.s-intermediate. The latter is thermodynamically unstable and the researchers deduced the role of the hydrogen-rich environment in kinetically sustaining the mechanism [12]. 

Os(NH3)5(f/ -H2)] is stable to substitution for several hours in such donor solvents as acetonitrile, pyridine, and water (cf. the similar rhenium-dihydrogen complex mentioned in Section 8.2.4)/ [Os(NH3)5(f/ -C3H5)] readily undergoes addition at the allyl ligand.The mixed valence complex [(H3N)50s(pyrene)0s(NH3)5] is very labile (tj/2 approximately 1 sec at room temperature), in contrast to its benzene analog, whose lifetime under similar conditions is more than half an Much of Taube s kinetic and... 

---