Nonclassical Dihydrogen Complexes

The structure of W(CO)3(PPr 3)2(H2) has been established by a variety of methods, including a neutron diffraction study. The hydrogen molecule is present as a ligand with the H-H axis collinear to the P-M-P vector (eq. (2)). 

The complex W(CO)3(PPr 3)2(H2) is a true r-compIex, because its H2 ligand is bound by way of the 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]. 

After the advent of the nonclassical if-Vi.2 complexes, polyhydrides had to be reinterpreted [14]. Thus, RuH4(PPh3)3 is not a classical tetrahydride of coordination number seven at the ruthenium atom but a dihydride-dihydrogen complex Ru(H2)(H)2(PPh3)3 of octahedral geometry [25]. 

The number of recognized rf- 2 complexes increased rapidly [1]. Interestingly, however, there are also complexes with elongated H-H bonds 

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Fig. 15.11 Sinicture of MolCOMPiVilrH . the first nonclassical dihydrogen complex to be discovered. [From Kubas, G. J. Acc. Chem. Rts. 1988.21, 120-128. Reproduced wilh permission.]...

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... 

These complexes are sometimes called nonclassical hydrides because prior classical hydrides always had terminal M-H bonds or M-H-M bridges. Dihydrogen complexes are... 

The difficulties encountered in firmly establishing the structure of the first dihydrogen complex still have rot been entirely overcome. The H—D coupling constant experiment described above offers a method of distinguishing between the classical and nonclassical forms, but it is not applicable if the system is rapidly fluxional. 

Fig. 8.5. Complex-support interaction formation of classical hydrogen bonds (CFiB) and nonclassical dihydrogen bonds (DHB).

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