Double bond polarization

For silatrane, however, containing a penta-coordinate silicon atom, such a resonance structure is improbable. Both the enhanced cr-donor (an increase in 5C ) and lowered Tt-acceptor (a decrease in 6Cp) effects of the silatranyl grouping result m not only a decrease of double bond polarization (SC ) but also in a chang of the sign. The 8C p values for trimethylsilylvinylsilatrane and the model triethoxysilane as calculated by an additive scheme with the use of 5Qp = 11.0 ppm for 1-vinylsilatrane... 

The ease of partial double-bond polarization has the opposite order. 

S >1600) whereas heteroatoms shield the selenium nueleus. A simple qualitative rationalization of this finding is the eoncept of a baek-donation from the heteroatom X (mesomeric form II) and a C=Se double bond polarization (mesomerie form III) which both increase the electron density at Se (Scheme 24). Note that the Se chemical shifts of analogous germaneselones (52 and 53 RR Ge=Se) are more than 1000 ppm lower. 

The NBO analysis then revealed that both the o and the n U-C interactions are mainly developed on the C atom (80.7% C and 19.3% U in the interaction of o symmetry, 82.9% C and 17.1%U in the n interaction). It is however interesting to note that the U contribution is done via hybrid 5f/6d orbitals and that the lower energy of the 5f orbitals results in a major contribution of them in the hybrid (52.6% 5f, 37.0% 6d in the contribution to the o bond, and 59.0% 5f and 40.9% 6d in the contribution to the n bond), despite their smaller radial expansion compared to the 6d. The bonding scheme points a U=C double bond polarized toward the carbon atom (NBO calculated charges qc = —1.52 and qu = 0.98). DFT calculations were then performed by the groups of Liddle and Cavell on their U(IV) complexes, featuring... 

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