• Tungsten;
  • Metalation;
  • Kinetics;
  • Siloxides;
  • C–H activation


The symmetry-forbidden cyclometalation of (silox)2W=NtBu (1, silox = OSitBu3) to (silox)(tBuN)W(H)(κO,κC-OSitBu2CMe2CH2) (2) was investigated by kinetics [ΔH = 19.2(9) kcal/mol; ΔS = –23(3) eu; ΔG(25 °C) = 26.1(10) kcal/mol], isotopic labeling {[D54]1 [RIGHTWARDS ARROW] [D54]2; kH/kD = 2.7(4)}, and equilibrium studies [ΔH° = –6.7(3) kcal/mol; ΔS° = –12.1(8) eu; ΔG°(25 °C) = –3.1(4) kcal/mol]. The crystal structure of 2 reveals a pseudo-square-pyramidal structure that can be viewed as a distorted tetrahedron with the W–H and W–C bonds occupying one site. The addition of H2 (or D2) to 1 proceeds similarly to afford (silox)2(tBuN=)WH2 (3), and the addition of H2 to 2 also affords 3, but labeling experiments show that it proceeds via 1. Phosphane bases with cone angles < 160° trigger the cyclometalation of 1 to 2 in < 5 min, and PMe3 catalyzed the dihydrogen addition to 1. Quantum mechanics/molecular mechanics (QM/MM) calculations support the experimental findings and show that Lewis bases promote σ/π mixing. The experimentally observed intermediate (silox)2(tBuN=)WPMe3 (1-PMe3) has a (dyz)2 (i.e., π2) ground state in contrast to the (dmath image)2 (i.e., σ2) configuration of 1. The σ/π mixing circumvents the constraints of orbital symmetry for both cyclometalation and dihydrogen addition.