Comparison Of Chemical Mechanisms Of Heavier Vs. Lighter Elements

Much attention has been paid to the chemistry of heavier elements in recent years. The phenomena arising from the heavier atoms tend to be explained based on the theory established in the chemistry of lighter elements, especially in the viewpoint of the experimental chemists.

In such cases, one would often mistake superficial causes for the real one, since the mechanisms for the phenomena arising from the heavier atoms are sometimes quite different from those from the lighter atoms, even if the phenomena from the heavier atoms can be well explained, at first glance.

The mechanism for the significantly large indirect one–bond nuclear spin-spin couplings between Pb and C (¹J_obsd(Pb, C)) of –1030 Hz for Ph₃Pb^–, relative to that of Ph₄Pb (481 Hz), is a typical example of such a case. The ¹J(M, C) values are often used to evaluate the degrees of multiple-bond character between M and C of the M–C bonds since the values are considered to be mainly dependent on degrees of s-character in the bonds.

For example, ¹J(Si, C) of about 84 Hz in Brook-type silenes (Si=C) are larger than ¹J(Si, C) of 47–48 Hz between sp³-hydridized silicon and attached methyl carbon atoms. The observed results are consistent with the double-bond character in the bonds of the former. However, a similar mechanism seems unlikely for the significantly large ¹J_obsd(Pb, C) value of Ph₃Pb^–, relative to that of Ph₄Pb, due to the unfavorable contribution of the Pb–C double-bond character. We felt that this must arise from the large difference in the relativistic effect on ¹J(Pb, C) between Ph₃Pb^– and Ph₄Pb.

To elucidate the mechanism for ¹J(Pb, C), ¹J(M, C) are analyzed for Me₄M, Me₃M^–, Ph₄M, and Ph₃M^–, (M = Pb, Sn, Ge, Si, and/or C) based on the MO theory. The ¹J(M, C) values are evaluated under the zeroth-order regular approximation (ZORA) levels of the scalar ZORA relativistic (szr) and the spin-orbit ZORA relativistic (sozr) levels, together with the nonrelativistic (non) level, employing the Slater-type basis sets. Evaluated total values (¹J_TL(M, C)) reproduced well the observed values. Fermi contact terms (¹J_FC(Pb, C)) contribute predominantly to ¹J_TL(Pb, C) (≈ 99%).

Therefore, the mechanisms for the distinct relativistic effect on ¹J(Pb, C) are clarified, exemplified by mainly ¹J_FC:szr(Pb, C: Me₄Pb) and ¹J_FC:szr(Pb, C: Me₃Pb^–), of which contributions are decomposed into each occupied MO (ψ_i) and occupied MO to unoccupied MO transition (ψ_i→ψ_a). The ¹J_FC:szr (Pb, C: Me₃Pb^–) value contributed from ψ₅₅ (HOMO) is –619 Hz (see Figure 1), which is much more negative than the case of ¹J_FC:non(Pb, C: Me₃Pb^–) (–214 Hz). Larger contributions are also detected for ¹J_FC:szr (Pb, C: Me₃Pb^–) from ψ₄₅ (–136 Hz), y₄₆ (–182 Hz), and ψ₅₄ (–154 Hz), which amount to –472 Hz. As a result, a large negative value of –878 Hz is predicted for ¹J_FC:szr (Pb, C: Me₃Pb^–) in total. The ψ₅₅ (HOMO)→ψ₆₇ (LUMO+11) transition contributes much (–464 Hz) in ¹J_FC:szr (Pb, C: Me₃Pb^–) (see Figure 1). The large relativistic effect in ¹J_FC(Pb, C: Me₃Pb^–) is demonstrated to originate mainly from ψ₅₅ (HOMO), of which character is the s-type lone pair orbital of Me₃Pb^–.

The small value of ¹J_FC:szr(Pb, C: Me₄Pb) (–14 Hz) is the results of the canceling of two components with the opposite signs due to the symmetry, although the effect is large in magnitudes for the two. The distinct canceling between the contributions is not detected in ¹J_FC:szr (Pb, C: Me₃Pb^–), maybe due to typically one MO contributed to the value, corresponding to the s-type lone pair orbital, resulting in the large relativistic effect in ¹J_FC:szr(Pb, C: Me₃Pb^–). The mechanism for ¹J_FC:szr(Pb, C: Me₃Pb^–) makes us imagine that the s-type lone pair orbital plays a very important role in the distinct relativistic effect on ¹J(Pb, C: Ph₃Pb^–). The predicted results at the sozr level were very similar to those at the szr level, as a whole. Such treatment must be instructive for experimental chemists to analyze their own results, concerning the electronic structures around the M–C bonds for M of heavy atoms through ¹J(M, C), where the relationship between the s-character of M–C and ¹J(Pb, C) is not always justified.

This study, Relativistic Effect on 1J(M,C) in Me4M, Me3M−, Ph4M, and Ph3M− (M=Pb, Sn, Ge, Si, and/or C): Role of s-Type Lone Pair Orbitals in the Distinct Effect for the Anionic Species was recently published by Hayashi, T. Nishide, W. Nakanishi, and M. Saito in the journal ChemPhysChem.