Classical dihydrides

On the other hand, it is well known that a dihydrogen complex can be generated through direct proton attack on a hydride ligand or initial protonation of a metal center, leading to a new classical dihydride, [MH2]", which then converts to the dihydrogen complex [M(ti -H2)]. The latter is a thermodynamic product of the protonation reaction shown in Scheme 10.4 [16,17]. 

Over the past few years, there has been great interest in the reactions of organometallic species with dihydrogen, H2. The product of such a reaction may take one of two forms. Usually, irradiation by UV light causes loss of a CO ligand from the metal to be replaced by two individual M-H bonds. Thus, dihydrogen is added oxidatively to the metal, the H-H bond is broken, and a classical dihydride species results. 

In this case, there is no reason to suppose that hydrogen is coordinated in any way other than the classical dihydride manner (30) see Hydrides Solid State Transition Metal Complexes). However, similar experiments using Cr(CO)5 gave a product Cr(CO)5H2 for which strong circumstantial evidence pointed towards the nonclassical dihydrogen structure (31). Unfortunately, in low-temperature matrices... 

Interestingly, some systems, such as (1) itself, show a tautomeric equilibrium between an H2 complex and a classical dihydride form others show a stretched H2. In the first case we have a double minimum on the potential energy surface (PES) (H2 and dihydride) and in the other a single minimum. The difference appears to lie in the motion of the heavy ligands that produces a barrier to the dihydrogen/dihydride tautomerism and where there are no such heavy atom rearrangements, the barrier disappears and a stretched H2 becomes possible. 

The acidification of H2 may also be involved in hydrogenase action, where H2 is beheved to bind to an Fe(II) center. Isotope exchange between H2 and D2O is catalyzed by the enzyme see Nickel Enzymes Cofactors Nickel Models of Protein Active Sites Iron-Sulfur Proteins). Similar isotope exchange can also occur in H2 complexes. Oxidative addition to give a classical dihydride is also a common reaction. [W(H2)(CO)3(PCy3)2] is in equilibrium with about 20% of the dihydride in solution. This can lead to subsequent hydrogenolysis of M-C bonds as in the case of a cyclometallated phenylpyridine complex of Ir(III). ... 

The complex W(CO)3(PPr 3)2(H2) is a true electron pair forming the H-H bond. Consequently, it is the midpoint of the H2 ligand which occupies one of the octahedral coordination sites at the tungsten atom. The H-H distance in the complex is 0.82 A. It is only slightly longer than the H-H distance in the hydrogen molecule with 0.74 A, whereas for a classical dihydride an H-H distance of about 1.8 A would be expected. 

The nonclassical dihydrogen complex W(CO)3(PPr 3)2(H2) is in a slow and reversible equilibrium with the classical dihydride complex W(CO)3(PPr 3)2(H)2 (eq. (2)) which is seven-coordinate and stereochemically nonrigid. The activation energy for this oxidative addition is 16 kcal/mol [1, 22-24]. 

An example is ReH7[P(p-tolyl)3]2, for which a distance of 1.35 A for one of the H-H pairs has been documented [26]. Such a stretched intermediate between a classical dihydride and a nonclassical tf-W2 complex. 

Photolysis of LM(CO)4 (L = Cp or C9H7 (indenyl)) in polyethylene matrices under a high pressure of reactant gas provided a range of photoproducts. 13 Under helium, LM(CO)3 and LNb(CO)2 were observed while, under N2, LM(CO)3(N2) and LNb(CO)2(N2)2 were seen. Under H2, the classical dihydrides LMin(CO)3(H)2 were observed for Cp, whereas the non-classical dihydrogen complex (775-CgH7)NbI(CO)3(772-H2) was observed in the Nb-indenyl system. CpM(CO)3(Xe) was characterized in supercritical Xe solution at room temperature.614... 

The appreciation of H2 complexes was delayed by the notion that such complexes could not be stable relative to classical dihydrides. Even the theoretical basis for interaction of H2 and other o-bonds with a metal was still undeveloped at the time of the initial discovery. Ironically, a computational paper by Saillard and Hoffmann7 in 1984 on the bonding of H2 and CH4 to metal fragments such as Cr(CO)s was published only shortly after our publication of the W-H2 complex (without mutual knowledge). Such interplay between theory and experiment has continued to be an extremely valuable synergistic relationship.3,4,8 The innate simplicity of H2 was attractive computationally, but the structure/bonding/dynamics of H2 complexes turned out to be extremely complex and led to extensive study (>300 computational papers). 

An alternative pathway to eq 2-4 might be the classical dihydride route, eq 5-6. Here the oxidative addition of the dihydrogen ligand results in a seven-coordinate dihydride species. Reaction 6 has A=7/6 and might also result in the production of a hydridic-protonic bonded intermediate. 

The unusual H-H bonding mode in dihydrogen complexes is of the greatest interest, also from the point of view of DQCC measurements. MO calculations of a simple [Rb-Dj]" model have shown that a transformation of classical dihydride structures to dihydrogen complexes leads to a dramatic increase of DQCC from 50 up to 155 kHz [11]. In addition, the asymmetry parameter T) grows from 0.025 to 0.62 and the orientation of the major axis of EFG (eq ) is remarkably deviated from the D-D vector in the dihydrogen complex (Scheme 1). Similar data have also been obtained in MO calculations of transition metal hydrides (Table 6). 

The r term makes the relaxation rate very sensitive to the distance r. In classical dihydrides, this distance would never shorter than —1.6 A, leading to a relaxation time on the order of half a second. On the other hand, in unstretched molecular hydrogen complexes, this distance is —0.85 A and the relaxation time is tens of milliseconds at - 80 C. Figure 10.8 shows how the... 

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