Dihydrogen complexes significance

According to the valence bond order concept as applied to the case of stretched H2 complexes by Limbach, there is a significant H-H bond order phh even at /hh = 1.6 A, implying that stretched dihydrogen complexes are not usefully considered as compressed dihydrides . 

Dihydrogen complexes have frequently been suggested as possible intermediates in a variety of reactions of hydrogen at metal centers. Some of these reactions are steps in catalytic processes of significant commercial interest. As this hgand becomes more completely understood, the applications of its chemistry are likely to become extremely important. 

Table 7 lists the DQCC values calculated directly from the T j relaxation times measured for the (Dj) ligands of transition metal dihydrogen complexes in solution. These values have been calculated by eq. (3) with the assumption of Tj being zero or varying between 0 and 1. All these DQCC values are remarkably smaller than those of classical hydride systems (see Table 3 and 4). They also differ significantly from the values predicted by MO calculations (Table 6). Probably the DQCC values in Table 7 are reduced due to the presence of fast (D2) motions. According to H solid state NMR, the motions are present even at lowest temperatures in the solid state [22].

The presented data demonstrate the ambiguity in the interpretation connected with the (Dj) motions. Nevertheless they show that in terms of the deuterium behaviour, the properties of these (D2) ligands can significantly differ even in similar dihydrogen complexes. The direct comparison of the T, data obtained for OsD(D2)(CO)Cl(PPr 3)2 in solution and Os(D2)(CO)Cl2(PPr 3)2 in the solid state supports this conclusion. 

Two significant surveys are published in the sane edition of Coordination Chemistry Reviews. Oxygen donor-ligand derivatives of tri-osmium dodecacarbonyl are covered by Frauenhoff and the reactions of transition metal dihydrogen complexes (of which many are carbonyl-containing) are outlined by Jessop and Morris. The crucial role of matrix Isolation studies is well covered in this article. A shorter review of interest here has been published on the role of the phosphorus d-orbitals in M-P bonding. ... 

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