Elongated Dihydrogen Complexes

In elongated H2 complexes, experimental and computational studies indicate that the 7 -H2 ligand is greatly delocalized and caimot be envisaged as a fixed, rigid unit [7]. Rapid motion of two hydrogen atoms occurs on a flat potential energy surface with a shallow minimum at the neutron-diffraction determined position of 1.2 A for trans-[OsCl(H2)(dppe)2]+. 

The potential energy surface for the H-H vibrational stretch is so flat for some complexes that the stretch of this bond can traverse the entire distance range 

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The elongated dihydrogen complex OsCl2(r 2-H2)(CO)(P,Pr3)2 reacts with phenylacetylene to give the dichloro-carbene derivative OsCl2(CHCH2Ph)(CO) (P Pr3)2 (Eq. 6), which has been characterized by X-ray diffraction analysis.35... 

The elongated dihydrogen complex OsCh -fFXCOXP1 Pr3)2 also reacts with 2-methyl-l-buten-3-yne. In toluene at room temperature, the reaction affords the alkenylcarbene derivative OsCl2 CHCH=CMe2 (CO)(P,Pr3)2 in 50% yield, after 1 h (Eq. 7). 

As noted above, a continuum of values for dHH is now available, ranging from very short as in the original Kubas complexes to conventional dihydride complexes with 1.5 A. A relatively small number of complexes with intermediate values of dHH known as elongated dihydrogen complexes have been reported.14 An example of this type of complex is provided by [Cp Ru(dppm)(H2)]+. A value of dHH = 1.10 . 03 A was reported using neutron diffraction by Morris, Koetzle, and coworkers in 1994 (Figure 5.2).122... 

Elongated Dihydrogen Complexes Extraordinarily Delocalized Dynamic Systems... 

Elongated dihydrogen complexes Concluding remarks References... 

From a theoretical point of view, electronic calculations have also systematically failed to provide correct H-H distances for the elongated dihydrogen complexes. We performed theoretical calculations in the [Ru(H—HXC5H5)(H2PCH2PH2)] complex, taken as a model of the permethylated dppm compound [69]. Its optimised geometry is depicted in Figure 15. 

For both elongated complexes our results also predict a dependence of the measured H-H distance on the isotopic substitution. That is, given the high anharmonicity of the calculated surfaces in the H-H direction, the H-H distance is predicted to become noticeably shorter when heavier isotopes (deuterium or, even better, tritium) are used in the dihydrogen ligand. Recent experimental results have verified this prediction for the [Cp Ru(dppm)(H2)] elongated dihydrogen complex [71]. 

A new elongated dihydrogen complex of iridium, [Cp Ir(dmpm)H2] , has been synthesized by Pons and Heinekey ° and characterized by the use of NMR data which also included Vhp couplings. It has been established that the compound exists in the form of cis and trans arranged dihydride isomers Vhp = 6 Hz for the trans and Vhp = 20 Hz for the cis isomer have been found. The obtained data are in agreement with those reported earlier for the related [CpOs(dppm)H2] complex. ... 

Heinekey DM, Lledos A, Lluch JM (2004) Elongated dihydrogen complexes what remains of the H-H Bond Chem Soc Rev 33 175-182... 

Complexes with H-H distances between 1.0 and 1.5 A are often referred to as elongated or stretched dihydrogen complexes [57]. The properties of these complexes make it difficult to justify their classification as either trae dihydrogen complexes (dH-H<10A) or ds-dihydride complexes (dH-H>l-5A). Other authors further categorize these complexes as true elongated dihydrogen complexes (dn-H < 10-1.3 A) and compressed bis-hydrides (dn-H < 13-1.5 A) [46]. 

There is no consensus on the nature of elongated dihydrogen complexes and how they should best be described. While electronic structure calculations generally provide accurate H-H distances for true ff -Hz complexes and true ds-dihydride complexes, they often fail to replicate the H-H distances observed in the molecular structures determined by single-crystal neutron diffraction for... 

The H-H distance in elongated dihydrogen complexes, as determined by the value of Jhd, is often dramatically affected by small changes in temperature [63-65]. This result provided strong experimental evidence that the potential energy surface is flat with respect to changes in the H-H distance. 

HaU and coworkers used density functional theory calculations to assign the inelastic neutron scattering derived vibrational spectrum of the elongated dihydrogen complex, (Tp )Rh(H)2(/ -H2) [66], They conclude that the H-H distance derived from neutron diffraction for the (Tp )Rh(H)2( / -H2) complex may in fact correspond to the average of the H-H distances of the tetrahydride and bis-hydride/// -H2 species. 

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