Bonding in M(NHBMe)2 and M[Mn(CO)5]2 complexes (M=Zn, Cd, Hg; NHBMe=(HCNMe)2B): divalent group 12 metals with zero oxidation state

Open questions on toxic heavy metals Cd, Hg and Pb binding small components of DNA and nucleobases. Are there any predictable trends?

In this perspective article, we provide a bibliographic compilation of experimental and theoretical work on Cd, Hg, and Pb, and analyze in detail the bonding of M²⁺ and CH₃M⁺ (M = Zn, Cd, Hg, Pb) with urea and thiourea as suitable models for larger biochemical bases. Through the use of DFT calculations, we have found that although in principle binding energies decrease according to ionic size (Zn²⁺ > Cd²⁺ > Pb²⁺), Hg²⁺ largely breaks the trend. Through the use of EDA (Energy Decomposition Analysis) it is possible to explain this behavior, which is essentially due to the strong contribution of polarization to the binding. This conclusion is ratified by the NEDA (Natural Energy Decomposition Analysis) formalism, showing that the charge transfer term is very large in all cases, but particularly in the case of the mercury-thiourea system. The general trends observed for the interactions with CH₃M⁺ monocations show however CH₃Hg⁺ binding energies systematically smaller than the CH₃Zn⁺ ones, likely because the relativistic contraction of the Hg orbitals is very much attenuated by the attachment to the methyl group. Finally, we have investigated the gas-phase reactivity between EtHg⁺ and uracil to compare it with that exhibited by CH₃Hg⁺ and n-ButHg⁺ previously described in the literature. This comparison gathers new information that highlights the importance of the length of the alkyl chain attached to the metal on the mechanisms of these reactions. For methyl mercury, only the alkyl transfer process is allowed; for butyl mercury, protonation is clearly favored, and for ethyl mercury, both paths are competitive experimentally.

Ligand Electrochemical Parameter Approach to Molecular Design. σ-Donation, π-Back Donation, and Other Metrics in Ruthenium(II) Dinitrogen Complexes

Using the density functional theory, [(N₂)Ru^IIL₅]ⁿ⁺ species are studied in silico. The properties of the Ru-N₂ bond are derived, including σ-donation, π-back donation, Ru-N and N-N bond lengths and bond orders, net charges and NN stretching frequencies, and so forth. These data are correlated using the ligand electrochemical parameter (E_L) theory, whereby the availability of electrons in the [RuL₅]ⁿ⁺ fragment is defined by its electron richness, which is the sum of the E_L parameters, ΣE_L(L₅). The objective is to better understand the binding of the N₂ ligand, leading to a molecular design whereby a specific species is constructed to have a desired property, for example, a particular bond length or charge. We supply cubic expressions linking ΣE_L(L₅) with these many metrics, allowing researchers to predict metric values of their own systems. The extended charge decomposition analysis is used. For the given target, N₂, σ-bonding does not vary greatly with the nature of ligand L, and π-back donation is the dominant property deciding the magnitudes of the various metrics. The E_L parameter provides the path to design the desired species. This contribution is devoted to dinitrogen, but the method is expected to be general for any ligand, including polydentate ligands.