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

Indirect one‐bond nuclear spin–spin couplings between M and C [1J(M,C)] in Me4M, Me3M−, Ph4M, and Ph3M− (M=Pb, Sn, Ge, Si, C) are analyzed with consideration of the relativistic effect and by employing Slater‐type basis sets. The evaluated total values 1JTL(M,C) reproduced the observed values with some systematic calculation errors. Fermi contact terms 1JFC(M,C) contribute predominantly to 1JTL(M,C) (≈99 %). A distinct relativistic effect on 1J(Pb,C) is predicted for Me3Pb− and Ph3Pb−. The mechanisms for the distinct effect are elucidated by using the comparison between Me3Pb− and Me4Pb as an example. The contributions to 1JFC(M,C) [or 1JSD+FC(M,C), where SD denotes the spin–dipolar term] are decomposed into those of occupied orbitals and occupied‐to‐unoccupied transitions. The s‐type lone‐pair orbitals are demonstrated to contribute to the distinct relativistic effect on 1J(Pb,C) of Me3Pb− (and Ph3Pb−). The results are in sharp contrast to the cases of 1J(M,C) for M atoms lighter than Pb, such as Si, and are explained by the s character of the M−C bonds. This treatment enables visualization and clear recognition the origin of the nuclear couplings for the species exhibiting a relativistic effect. Relativistically coupled: Indirect one‐bond nuclear spin–spin couplings 1J(M,C) in R4M and R3M−(M=Pb, Sn, Ge, Si, C; R=Me, Ph) are analyzed with consideration of relativistic effects. A large contribution of the s‐type lone‐pair orbital of Me3Pb− (see figure) is found at the scalar ZORA relativistic level of theory with 1JFC(Pb,C)=−873 Hz, but only 1JFC(Pb,C)=−214 Hz at the nonrelativistic level of theory.