Orientation of H2 in the Kubas complex W CO 3 PR

One of the remarkable aspects of the structure of this complex is linked to the non-equivalence of the ligands other than H2 (PR3 or CO). The dihydrogen molecule is oriented parallel to the W—PR3 bonds, even though at least one other conformation, that with H—H parallel to the W—CO bonds (4-15b), could be imagined. Is there an electronic factor that causes this conformation preference Before we can answer this question, we have to analyse the orbital interactions that lead to the metal-H2 bond. 

We consider first a model d complex [ML5( -H2)] in which the five non-H2 ligands are identical and only have a interactions with the metal centre. We shall limit ourselves to the conformation in which the H—H bond eclipses two neighbouring M—L bonds (4-16). 

The first interaction, which leads to a transfer of electron density from dihydrogen to the metal, is of the donation type (4-17a), whereas the second, which involves the empty orbital on H2, is a metal dihydrogen back-donation interaction (4-17b). 

This bonding scheme is completely analogous to the Dewar-Chatt-Duncanson model for the interaction of an ethylene molecule with a metallic centre (Chapter 3, 3.4), the ncc and jr, MO being replaced here by the ohi and MO. 

Notice that both interactions weaken the H—H bond donation reduces the population of the bonding orbital Th2 and back-donation increases the population of the antibonding orbital. An increase of 

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