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

Na/Hg reduction of (silox)2Cl2MoNtBu (3) afforded C2h [(silox)2MoNtBu]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)MoLn (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:  1Ag (GS), 3Bu, 1Bu, and 1Ag. ΔE = E(1Bu) − E(3Bu) = 2K, where K is the exchange energy. Magnetic studies indicate E(3Bu) − E(1Ag) ≈ 550 cm-1 (calcd 1744 cm-1), and a UV−vis absorption at 10 000 cm-1 is assigned to 1Ag1Bu, permitting K to be evaluated as 4725 cm-1. With the π → π* transition in Schrock's [Mo(NAr)(CH2tBu)(OC6F5)]2 (4) assigned at 528 nm, this estimation places its π-bond energy as {E2 → ππ*1 in 4) − E(1Ag1Bu in 12-Hg)} + E(1Ag3Bu in 12-Hg) = 27 kcal/mol.