3-Center-4-Electron Bonding in [(silox)₂MoN^tBu]₂(μ-Hg) Controls Reactivity while Frontier Orbitals Permit a Dimolybdenum π-Bond Energy Estimate

Na/Hg reduction of (silox)₂Cl₂MoN^tBu (3) afforded C_2h [(silox)₂MoN^tBu]₂(μ-Hg) (1₂-Hg), which consists of two distorted trigonal monoprisms with Hg at the each apex (d(MoHg) = 2.6810(5) Å). Calculations reveal 3c4e bonding in the linear MoHgMo linkage that renders 1₂-Hg susceptible to nucleophilic cleavage. Exposure to PMe₃ and pyridine rapidly (<5 min) affords (silox)₂(^tBuN)MoL_n (L = PMe₃, n = 1 (1-PMe₃); py, n = 2 (1-py₂)), while poorer nucleophiles (L = C₂H₄, 2-butyne) yield adducts (e.g., 1-C₂H₄ and 1-C₂Me₂) after prolonged heating. The HOMO and LUMO of 1₂-Hg are “stretched” π and π* orbitals from which four states arise:  ¹A_g (GS), ³B_u, ¹B_u, and ¹A_g. ΔE = E(¹B_u) − E(³B_u) = 2K, where K is the exchange energy. Magnetic studies indicate E(³B_u) − E(¹A_g) ≈ 550 cm^-1 (calcd 1744 cm^-1), and a UV−vis absorption at 10 000 cm^-1 is assigned to ¹A_g → ¹B_u, permitting K to be evaluated as 4725 cm^-1. With the π → π* transition in Schrock's [Mo(NAr)(CH₂^tBu)(OC₆F₅)]₂ (4) assigned at 528 nm, this estimation places its π-bond energy as {E(π² → ππ*¹ in 4) − E(¹A_g → ¹B_u in 1₂-Hg)} + E(¹A_g → ³B_u in 1₂-Hg) = 27 kcal/mol.