Structural preferences of 20-electron bisindenyl complexes of Group 6 metals: a DFT study

The formally ‘20-electron’ [M(Ind)2L2] complexes (M=Group 6) are studied by means of DFT/B3LYP calculations for L=CO, H, and Cl. The carbonyl derivatives behave similarly for all metals and the preferred conformation of the two indenyl rings is staggered, with one η5- and one η3-indenyl, with the rings oriented in order to maximize back donation to the carbonyls. The calculated structures could be compared with experimentally determined ones for Mo and W, the agreement being very good. When the π-acceptor carbonyls are replaced by hydrides, strong σ-donors, the metal is more electron rich and the rings become the only π-acceptors in the system. As a consequence, the slippage is increased in the most favored conformation, with one η3-indenyl and the other ring having a hapticity between η5 and η3. In the case of the chloride complexes, the situation for Mo and W is intermediate between CO and H, as Cl is a π-donor, but electronegative. Interestingly, the most stable Cr complexes are paramagnetic tetrahedral species with four unpaired electrons, with an energy 63 kcal mol−1 lower than that of the only converged diamagnetic structure.