Different electronic structure of phosphonyl radical adducts of N-heterocyclic carbenes, silylenes and germylenes: EPR spectroscopic study and DFT calculationsDedicated to Professor Wataru Ando, a pioneer of silicon chemistry, on the occasion of his 75th birthday.Electronic supplementary information (ESI) available: Cartesian coordinates and energies of stationary points in the calculations of 2a, 3a, 4a, 5a, 6a and Fig. S1-S7. See DOI: 10.1039/c0sc00143k

Stable N -heterocyclic carbenes and germylenes were allowed to react with a phosphonyl radical, ( i -PrO) 2 (O)P&z.rad; ( 7 ), generated by photolysis of [( i -PrO) 2 (O)P] 2 Hg. The products were identified by EPR spectroscopy. An unsaturated carbene ( 1 ) and germylene ( 3 ) react with 7 at the divalent atom to give unstable radical products ( τ ½ = 0.2 s). A benzo-annulated carbene ( 4 ) and a saturated germylene ( 6 ) react with 7 to give more active radicals. An unsaturated ( 2 ) and a saturated silylene ( 5 ) undergo rapid reaction (in the dark) with [( i -PrO) 2 (O)P] 2 Hg to yield unusual silyl phosphites. In these cases only secondary radicals were observed. DFT (PBE0/TZVP//B3LYP/6-31+G(d)) calculations of the radical adducts of the different (C, Si, Ge) unsaturated N -heterocyclic divalent species with the phosphonyl radical show that the unpaired electron is delocalized over the five-membered ring; the spin density on the central atoms decreases in the order C, 39% > Si, 14% > Ge, 2%. These trends can be understood in terms of a zwitterionic structure of the radical adducts. The calculations of the radical adducts of 4 , 5 and 6 with 7 indicate larger spin density on the central atom, 47%, 58% and 42% on C, Si, Ge, respectively. The geometry, spin density distribution and reactivity of radical adducts of stable N -heterocyclic carbenes, silylenes and germylenes demonstrate a strong dependence on the nature of the heterocycle and on the central atom.