Ab initio molecular dynamics (MD) and relativistic density functional NMR methods were applied to calculate the one-bond HgC NMR indirect nuclear spin–spin coupling constants (J) of [Hg(CN)2] and [CH3HgCl] in solution. The MD averages were obtained as J(199Hg13C)=3200 and 1575 Hz, respectively. The experimental HgC spin–spin coupling constants of [Hg(CN)2] in methanol and [CH3HgCl] in DMSO are 3143 and 1674 Hz, respectively. To deal with solvent effects in the calculations, finite “droplet” models of the two systems were set up. Solvent effects in both systems lead to a strong increase of the HgC coupling constant. From a relativistic natural localized molecular orbital (NLMO) analysis, it was found that the degree of delocalization of the Hg 5dσ nonbonding orbital and of the HgC bonding orbital between the two coupled atoms, the nature of the trans HgC/Cl bonding orbital, and the s character of these orbitals, exhibit trends upon solvation of the complexes that, when combined, lead to the strong increase of J(HgC).