Tuning the gap: Doubly Si-bridged ansa-tungstenocene, a proposed intermediate for C–H activation

The doubly Si-bridged ansa -tungstenocene, [W(η 5 -C 5 H 3 ) 2 (SiMe 2 ) 2 ], ( a -Si 2 Cp 2 W) is calculated to have singlet and triplet states of comparable energy, in contrast to other tungstenocenes which have triplet ground states. Therefore, on an SCF energy surface there is no barrier to formation of a sigma complex with methane, and formation of a methyl hydride is exo-energetic by −125 kJ mol −1 . Orbital analysis of the tungstenocenes shows that the singlet ground state is favoured by a ring conformation that stabilizes the 2b 1 orbital by back donation, and that a double ansa bridge promotes this ring conformation.