Mercury Is a Transition Metal: The First Experimental Evidence for HgF4

On the highest oxidation states of the actinoids in AnO4 molecules (An = Ac - Cm): A DMRG-CASSCF study

Actinoid tetroxide molecules AnO₄ (An = Ac - Cm) are investigated with the ab initio density matrix renormalization group (DMRG) approach. Natural orbital shapes are used to read out the oxidation state (OS) of the f-elements, and the atomic orbital energies and radii are used to explain the trends. The highest OSs reveal a "volcano"-type variation: For An = Ac - Np, the OSs are equal to the number of available valence electrons, that is, Ac^III , Th^IV , Pa^V , U^VI , and Np^VII . Starting with plutonium as the turning point, the highest OSs in the most stable AnO₄ isomers then decrease as Pu^V , Am^V , and Cm^III , indicating that the 5f-electrons are hard to be fully oxidized off from Pu onward. The variations are related to the actinoid contraction and to the 5f-covalency characteristics. Combined with previous work on OSs, we review their general trends throughout the periodic table, providing fundamental understanding of OS-relevant phenomena.

Comment on "Realization of the Zn3+ oxidation state" by H. Fang, H. Banjade, Deepika and P. Jena, Nanoscale, 2021, , 14041-14048

Two zinc-boron clusters (ZnBeB₁₁(CN)₁₂ and ZnBeB₂₃(CN)₂₂) reported in a theoretical study by P. Jena and co-workers are reinvestigated using quantum chemistry calculations. The results prove that the zinc atoms in these two clusters retain a normal oxidation state of +2, overturning the conclusion reached in the previous study that a +3 oxidation state is present. The semi-empirical LOBA method points out this contrast, which is demonstrated via various wavefunction analysis approaches. No unpaired electrons are observed on zinc atoms nor is there a spin density difference distribution, revealing that the zinc atoms have a fully occupied 3d¹⁰ electron shell. Density of states studies give the same conclusion, and they further show that zinc atoms adopt an sp²-hybrid type during bonding. From the perspective of energy, we advise that the electron affinity energy is not a reliable way of evaluating the oxidation state. Instead, binding energy calculations and constrained DFT are applicable, and these also support the presence of Zn²⁺. The simulated XPS peaks are consistent with the experimental data for Zn(II) measured in ZnS. Lastly, the ETS-NOCV method is adopted to give insights into the bonding structures between zinc atoms and boron clusters. It is suggested that future theoretical research into similar problems is analyzed more cautiously to avoid potentially misleading other researchers.

Stable copernicium hexafluoride (CnF6) with an oxidation state of VI

As the heaviest group 12 element known currently, copernicium (Cn) often presents the oxidation states of I+, II+, and rarely IV+ as in its homologue mercury. In this work we systematically studied the stability of some oxides, fluorides, and oxyfluorides of Cn by two-component relativistic calculations and found that the CnF₆ molecule with an oxidation state of VI+ has an extraordinary stability. CnF₆ may decompose into CnF₄ by conquering an energy barrier of about 34 kcal mol^-1 without markedly releasing heat. Our results indicate that CnF₆ may exist under some special conditions.

Iridium(VII)-Corrole Terminal Carbides Should Exist as Stable Compounds

Scalar-relativistic DFT calculations with multiple exchange-correlation functionals and large basis sets foreshadow the existence of stable iridium(VII)-corrole terminal carbide derivatives. For the parent compound Ir[Cor](C), OLYP/STO-TZ2P calculations predict a short Ir-C bond distance of 1.69 Å, a moderately domed macrocycle with no indications of ligand noninnocence, a surprisingly low electron affinity of ∼1.1 eV, and a substantial singlet-triplet gap of ∼1.8 eV. These results, and their essential invariance with respect to the choice of the exchange-correlation functional, lead us to posit that Ir(VII)-corrole terminal carbide complexes should be isolable and indefinitely stable under ambient conditions.

Design of an Integrated Microfluidic Paper-Based Chip and Inspection Machine for the Detection of Mercury in Food with Silver Nanoparticles

For most of the fast screening test papers for detecting Hg2+, the obtained results are qualitative. This study developed an operation for the μPAD and combined it with the chemical colorimetric method. Silver nanoparticle (AgNP) colloids were adopted as the reactive color reagent to combine and react with the Hg standards on the paper-based chip. Then, the RGB values for the color change were used to establish the standard curve (R2 > 0.99). Subsequently, this detection system was employed for the detection tests of actual samples, and the detected RGB values of the samples were substituted back to the formula to calculate the Hg2+ contents in the food. In this study, the Hg2+ content and recovery rate in commercially available packaged water and edible salts were measured. The research results indicate that a swift, economical, and simple detection method for Hg2+ content in food has been successfully developed.

Process Optimization of Silver Nanoparticle Synthesis and Its Application in Mercury Detection

Silver nanoparticles (AgNPs) have stable reactivity and excellent optical absorption properties. They can be applied in various industries, such as environmental protection, biochemical engineering, and analyte monitoring. However, synthesizing AgNPs and determining their appropriate dosage as a coloring substance are difficult tasks. In this study, to optimize the process of AgNP synthesis and obtain a simple detection method for trace mercury in the environment, we evaluate several factors-including the reagent addition sequence, reaction temperature, reaction time, the pH of the solution, and reagent concentration-considering the color intensity and purity of AgNPs as the reaction optimization criteria. The optimal process for AgNP synthesis is as follows: Mix 10 mM of silver nitrate with trisodium citrate in a hot water bath for 10 min; then, add 10 mM of sodium borohydride to produce the AgNPs and keep stirring for 2 h; finally, adjust the pH to 12 to obtain the most stable products. For AgNP-based mercury detection, the calibration curve of mercury over the concentration range of 0.1-2 ppb exhibits good linearity (R2 > 0.99). This study provides a stable and excellent AgNP synthesis technique that can improve various applications involving AgNP-mediated reactions and has the potential to be developed as an alternative to using expensive detection equipment and to be applied for the detection of mercury in food.

Realization of the Zn3+ oxidation state

Due to unfilled d-shells, transition metal atoms exhibit multiple oxidation states and rich chemistry. While zinc is often classified as a transition metal, electrons in its filled 3d¹⁰ shell do not participate in chemical reactions; hence, its oxidation state is +2. Using calculations based on density functional theory, we show that the chemistry of zinc can fundamentally change when it is allowed to interact with highly stable super-electrophilic trianions, namely, BeB₁₁(CN)₁₂^3- and BeB₂₃(CN)₂₂^3-, which lie 15.85 eV and 18.49 eV lower in energy than their respective neutral states. The fact that Zn exists in +3 oxidation states while interacting with these moieties is evidenced from its large binding energies of 6.33 and 7.04 eV with BeB₁₁(CN)₁₂^3- and BeB₂₃(CN)₂₂^3-, respectively, and from a comprehensive analysis of its bonding characteristics, charge density distribution, electron localization function, molecular orbitals and energy decomposition, all showing a strong involvement of its 3d electrons in chemical bonding. The replacement of CN with BO is found to increase the zinc binding energy even further.

Electronic Structure and Spectroscopic Properties of Group-7 Tri-Oxo-Halides MO3X (M = Mn-Bh, X = F-Ts)

The 24 trioxide halide molecules MO₃X of the manganese group (M = Mn-Bh; X = F-Ts), which are iso-valence-electronic with the famous MnO_4^- ion, have been quantum-chemically investigated by quasi-relativistic density-functional and ab initio correlated approaches. Geometric and electronic structures, valence and oxidation numbers, vibrational and electronic spectral properties, energetic stabilities of the monomers in the gas phase, and the decay mode of MnO₃F have been investigated. The light Mn-3d species are most strongly electron-correlated, indicating that the concept of a closed-shell Lewis-type single-configurational structure [Mn⁺⁷(d⁰) O^-2(p⁶)₃ F^-(p⁶)] reaches its limits. The concept of real-valued spin orbitals φ(r)·α and φ(r)·β breaks down for the heavy Bh-6d, At-6p and Ts-7p elements because of the dominating spin-orbit coupling. The vigorous decomposition of MnO₃F at ambient conditions starts by the autocatalyzed release of n O₂ and the formation of Mn_mO_3m-2nF_m clusters, triggered by the electron-depleted "oxylic" character of the oxide ligands in MnO₃X.

Exclusively Relativistic: Periodic Trends in the Melting and Boiling Points of Group 12

First-principles simulations can advance our understanding of phase transitions but are often too costly for the heavier elements, which require a relativistic treatment. Addressing this challenge, we recently composed an indirect approach: A precise incremental calculation of absolute Gibbs energies for the solid and liquid with a relativistic Hamiltonian that enables an accurate determination of melting and boiling points (MPs and BPs). Here, we apply this approach to the Group 12 elements Zn, Cd, Hg, and Cn, whose MPs and BPs we calculate with a mean absolute deviation of only 5 % and 1 %, respectively, while we confirm the previously predicted liquid aggregate state of Cn. At a non-relativistic level of theory, we obtain surprisingly similar MPs and BPs of 650±30 K and 1250±20 K, suggesting that periodic trends in this group are exclusively relativistic in nature. Ultimately, we discuss these results and their implication for Groups 11 and 14.

Understanding Periodic and Non-periodic Chemistry in Periodic Tables

The chemical elements are the "conserved principles" or "kernels" of chemistry that are retained when substances are altered. Comprehensive overviews of the chemistry of the elements and their compounds are needed in chemical science. To this end, a graphical display of the chemical properties of the elements, in the form of a Periodic Table, is the helpful tool. Such tables have been designed with the aim of either classifying real chemical substances or emphasizing formal and aesthetic concepts. Simplified, artistic, or economic tables are relevant to educational and cultural fields, while practicing chemists profit more from "chemical tables of chemical elements." Such tables should incorporate four aspects: (i) typical valence electron configurations of bonded atoms in chemical compounds (instead of the common but chemically atypical ground states of free atoms in physical vacuum); (ii) at least three basic chemical properties (valence number, size, and energy of the valence shells), their joint variation across the elements showing principal and secondary periodicity; (iii) elements in which the (sp)8, (d)10, and (f)14 valence shells become closed and inert under ambient chemical conditions, thereby determining the "fix-points" of chemical periodicity; (iv) peculiar elements at the top and at the bottom of the Periodic Table. While it is essential that Periodic Tables display important trends in element chemistry we need to keep our eyes open for unexpected chemical behavior in ambient, near ambient, or unusual conditions. The combination of experimental data and theoretical insight supports a more nuanced understanding of complex periodic trends and non-periodic phenomena.

The periodic table and the physics that drives it

Mendeleev's introduction of the periodic table of elements is one of the most important milestones in the history of chemistry, as it brought order into the known chemical and physical behaviour of the elements. The periodic table can be seen as parallel to the Standard Model in particle physics, in which the elementary particles known today can be ordered according to their intrinsic properties. The underlying fundamental theory to describe the interactions between particles comes from quantum theory or, more specifically, from quantum field theory and its inherent symmetries. In the periodic table, the elements are placed into a certain period and group based on electronic configurations that originate from the Pauli and Aufbau principles for the electrons surrounding a positively charged nucleus. This order enables us to approximately predict the chemical and physical properties of elements. Apparent anomalies can arise from relativistic effects, partial-screening phenomena (of type lanthanide contraction) and the compact size of the first shell of every l-value. Further, ambiguities in electron configurations and the breakdown of assigning a dominant configuration, owing to configuration mixing and dense spectra for the heaviest elements in the periodic table. For the short-lived transactinides, the nuclear stability becomes an important factor in chemical studies. Nuclear stability, decay rates, spectra and reaction cross sections are also important for predicting the astrophysical origin of the elements, including the production of the heavy elements beyond iron in supernova explosions or neutron-star mergers. In this Perspective, we critically analyse the periodic table of elements and the current status of theoretical predictions and origins for the heaviest elements, which combine both quantum chemistry and physics.

Pressure-Stabilized Zinc Trifluoride

By combining the particle swarm optimization algorithm with first-principles calculation, the high-pressure phase diagram of Zn-F binary compounds was established. An unexpected stoichiometry of ZnF₃ with space group Cccm is thermodynamically stable above 183 GPa. The new structure is fascinating with the appearance of Zn²⁺[F₃]^2- units. The stability of the new phase stems from the mixed ionic and covalent chemical bonding in ZnF₃. The electronic properties indicate that Zn has a tendency to form high oxidation states under higher pressure. Our work is an important step in understanding the bonding behavior of Zn under extreme conditions and provides a valuable reference for experimental synthesis and identification of ZnF₃.

Physical origin of chemical periodicities in the system of elements

The Periodic Law, one of the great discoveries in human history, is magnificent in the art of chemistry. Different arrangements of chemical elements in differently shaped Periodic Tables serve for different purposes. “Can this Periodic Table be derived from quantum chemistry or physics?” can only be answered positively, if the internal structure of the Periodic Table is explicitly connected to facts and data from chemistry. Quantum chemical rationalization of such a Periodic Tables is achieved by explaining the details of energies and radii of atomic core and valence orbitals in the leading electron configurations of chemically bonded atoms. The coarse horizontal pseudo-periodicity in seven rows of 2, 8, 8, 18, 18, 32, 32 members is triggered by the low energy of and large gap above the 1s and nsp valence shells (2 ≤ n ≤ 6 !). The pseudo-periodicity, in particular the wavy variation of the elemental properties in the four longer rows, is due to the different behaviors of the s and p vs. d and f pairs of atomic valence shells along the ordered array of elements. The so-called secondary or vertical periodicity is related to pseudo-periodic changes of the atomic core shells. The Periodic Law of the naturally given System of Elements describes the trends of the many chemical properties displayed inside the Chemical Periodic Tables. While the general physical laws of quantum mechanics form a simple network, their application to the unlimited field of chemical materials under ambient ‘human’ conditions results in a complex and somewhat accidental structure inside the Table that fits to some more or less symmetric outer shape. Periodic Tables designed after some creative concept for the overall appearance are of interest in non-chemical fields of wisdom and art.

Unexpected Molecular Structure of a Putative Rhenium-Dioxo-Benzocarbaporphyrin Complex. Implications for the Highest Transition Metal Valence in a Porphyrin-Type Ligand Environment

A combination of quantum chemical calculations and synthetic studies was used to address the possibility of very high (>6) valence states of transition metals in porphyrin-type complexes. With corrole as a supporting ligand, DFT calculations ruled out Re(VII) and Ir(VII) dioxo complexes as stable species. Attempted rhenium insertion into benzocarbaporphyrin (BCP) ligands on the other hand led to two products with different stoichiometries - Re[BCP]O and Re[BCP]O2. To our surprise, single-crystal structure determination of one of the complexes of the latter type indicated an ReVO center with a second oxygen bridging the Re-C bond. In other words, although the monooxo complexes Re[BCP]O are oxophilic, the BCP ligand cannot sustain a trans-ReVII(O)2 center. The search for metal valence states >6 in porphyrin-type ligand environments must therefore continue.

Beyond Oxides: Nitride as a Ligand in a Neutral IrIX NO3 Molecule Bearing a Transition Metal at High Oxidation State

Theoretical calculations utilizing relativistic ZORA Hamiltonian point to the conceivable existence of an IrNO₃ molecule in C_3v geometry. This minimum is shown to correspond to genuine nonavalent iridium nitride trioxide, which is a neutral analogue of cationic [IrO₄ ]⁺ species detected recently. Despite the presence of nitride anion, the molecule is protected by substantial barriers exceeding 200 kJ mol^-1 against transformations leading, for example, to global minimum (O=)₂ Ir-NO, which contains metal at a lower formal oxidation state.

Theoretical Search for the Highest Valence States of the Coinage Metals: Roentgenium Heptafluoride May Exist

We present here a relativistic density functional theory investigation of the penta- and heptavalent states of gold and roentgenium, employing the ZORA (zeroth order regular approximation to the Dirac equation) Hamiltonian, including spin-orbit coupling at the two-component level, and large all-electron relativistic Slater-type quadruple-ζ quadruple polarization (ZORA-STO-QZ4P) basis sets. Unsurprisingly, our calculations confirm the stability of the experimentally known complexes AuF₆^- and Au₂F₁₀ with respect to decomposition to trivalent Au products and F₂. The calculations also predict that RgF₆^- and Rg₂F₁₀ should be even more stable with respect to an analogous decomposition pathway. Like an earlier DFT study ( Inorg. Chem. 2007, 46 (13), 5338-5342), our calculations rule out the true heptavalent Au complex AuF₇ as a stable species, preferring instead a C_s AuF₅···F₂ formulation. Remarkably, our calculations confirm a D_{5 h} pentagonal-bipyramidal structure of RgF₇ as the global minimum, at an energy of approximately half an electron volt below the RgF₅···F₂ form.

Electronegativity Seen as the Ground-State Average Valence Electron Binding Energy

We introduce a new electronegativity scale for atoms, based consistently on ground-state energies of valence electrons. The scale is closely related to (yet different from) L. C. Allen's, which is based on configuration energies. Using a combination of literature experimental values for ground-state energies and ab initio-calculated energies where experimental data are missing, we are able to provide electronegativities for elements 1-96. The values are slightly smaller than Allen's original scale, but correlate well with Allen's and others. Outliers in agreement with other scales are oxygen and fluorine, now somewhat less electronegative, but in better agreement with their chemistry with the noble gas elements. Group 11 and 12 electronegativities emerge as high, although Au less so than in other scales. Our scale also gives relatively high electronegativities for Mn, Co, Ni, Zn, Tc, Cd, Hg (affected by choice of valence state), and Gd. The new electronegativities provide hints for new alloy/compound design, and a framework is in place to analyze those energy changes in reactions in which electronegativity changes may not be controlling.

A Monoaryllead Trichloride That Resists Reductive Elimination

Transmetallation of Pb(OAc)₄ with R₂ Hg (1), followed by treatment with HCl in Et₂ O, provided RPbCl₃ (2), the first kinetically stabilized monoorganolead trihalide that resists reductive elimination under ambient conditions. The kinetic stabilisation relies on an intramolecularly coordinating O-donor substituent (R=6-Ph₂ P(O)-Ace-5-). The gram-scale preparation of 2 was key for the synthesis of unsymmetrically substituted diaryllead dichlorides RR'PbCl₂ (3 a, R'=Ph; 3 b, R'=4-MeOC₆ H₄ ; 3 c, R'=4-Me₂ NC₆ H₄ ).

Synthesis meets theory: Past, present and future of rational chemistry

Chemical synthesis has its roots in the empirical approach of alchemy. Nonetheless, the birth of the scientific method, the technical and technological advances (exploiting revolutionary discoveries in physics) and the improved management and sharing of growing databases greatly contributed to the evolution of chemistry from an esoteric ground into a mature scientific discipline during these last 400 years. Furthermore, thanks to the evolution of computational resources, platforms and media in the last 40 years, theoretical chemistry has added to the puzzle the final missing tile in the process of “rationalizing” chemistry. The use of mathematical models of chemical properties, behaviors and reactivities is nowadays ubiquitous in literature. Theoretical chemistry has been successful in the difficult task of complementing and explaining synthetic results and providing rigorous insights when these are otherwise unattainable by experiment. The first part of this review walks the reader through a concise historical overview on the evolution of the “model” in chemistry. Salient milestones have been highlighted and briefly discussed. The second part focuses more on the general description of recent state-of-the-art computational techniques currently used worldwide by chemists to produce synergistic models between theory and experiment. Each section is complemented by key-examples taken from the literature that illustrate the application of the technique discussed therein.

Removal of mercury by adsorption: a review

Due to natural and production activities, mercury contamination has become one of the major environmental problems over the world. Mercury contamination is a serious threat to human health. Among the existing technologies available for mercury pollution control, the adsorption process can get excellent separation effects and has been further studied. This review is attempted to cover a wide range of adsorbents that were developed for the removal of mercury from the year 2011. Various adsorbents, including the latest adsorbents, are presented along with highlighting and discussing the key advancements on their preparation, modification technologies, and strategies. By comparing their adsorption capacities, it is evident from the literature survey that some adsorbents have shown excellent potential for the removal of mercury. However, there is still a need to develop novel, efficient adsorbents with low cost, high stability, and easy production and manufacture for practical utility.

High-pressure stabilization of argon fluorides

On account of the rapid development of noble gas chemistry in the past half-century both xenon and krypton compounds can now be isolated in macroscopic quantities. The same does not hold true for the next lighter group 18 element, argon, which forms only isolated molecules stable solely in low temperature matrices or supersonic jet streams. Here we present theoretical investigations into a new high-pressure reaction pathway, which enables synthesis of argon fluorides in bulk and at room temperature. Our hybrid DFT calculations (employing the HSE06 functional) indicate that above 60 GPa ArF2-containing molecular crystals can be obtained by a reaction between argon and molecular fluorine.

Intramolecular Hg⋯π interactions of d-character with non-bridging atoms in mercury–aryl complexes

The late transition metal mercury, known for its propensity to not share 5d electrons, actually does so in short-range π-bonding and medium- to long-range π-interactions.

Experimental and theoretical identification of the Fe(vii) oxidation state in FeO4−

Two isomers of iron tetraoxygen anion, dioxoiron peroxide [(η²-O₂)FeO₂]⁻ and tetroxide FeO₄⁻ were characterized by experiment and theoretical calculations, with heptavalent Fe(vii) oxidation state identified in the later.

Investigation of Praseodymium Fluorides: A Combined Matrix-Isolation and Quantum-Chemical Study

The chemistry of the lanthanides is mostly dominated by compounds in the oxidation state +III. Only few compounds of Ce, Pr, and Tb are known with the metal in the +IV oxidation state. Removal of the last f-electron on praseodymium +IV would lead to a closed-shell system with formal oxidation state V. In this work we investigated the stability of the PrF5 molecule by theory and matrix-isolation techniques through the reaction of laser-ablated praseodymium atoms with fluorine in excess of neon, argon, krypton, or neat fluorine. Besides the known PrF3 molecule, unreported IR bands for PrF4 could be observed, and there is evidence for the formation of PrF and PrF2 but not for the formation of PrF5.

Relativistic and Solvation Effects on the Stability of Gold(III) Halides in Aqueous Solution

The redox stability of gold halide complexes in aqueous solution has been examined quantum-chemically by a systematic comparison of scalar- and nonrelativistic pseudopotential calculations, using both COSMO and D-COSMO-RS solvent models for water. After a computational benchmarking of density-functional methods against CCSD(T) results for the gas phase decomposition AuX4(-) → AuX2(-) + X2, B3LYP calculations have been used to establish solvent contributions. While relativity clearly enhances the stability of AuX4(-) (X = F, Cl, Br, I) complexes against X2 elimination, solvation favors the lower oxidation state. Solvation and relativity are nonadditive, due to the relativistic reduction of bond polarity. At scalar relativistic D-COSMO-RS level, the reaction AuX4(-) ⇌ AuX2(-) + X2 is computed to be endergonic, except for X = I, where it is slightly exergonic. Under the chosen conditions, partial hydrolysis of AuCl4(-) to AuCl3OH(-) is exergonic. The latter complex in turn is stable against Cl2 elimination. The disproportionation 3 AuCl2(-) ⇌ AuCl4(-) + 2 Au(s) + 2 Cl(-) is clearly exergonic. All of the computed reaction energies at scalar relativistic D-COSMO-RS level agree well with the observed speciation in dilute pH-neutral solutions at ambient temperatures.

Mercury under Pressure acts as a Transition Metal: Calculated from First Principles

The inclusion of Hg among the transition metals is readily debated. Recently, molecular HgF4 was synthesized in a low-temperature noble gas but the potential of Hg to form compounds beyond a +2 oxidation state in a stable solid remains unresolved. We propose high-pressure techniques to prepare unusual oxidation states of Hg-based compounds. Using an advanced structure search algorithm and first-principles electronic structure calculations, we find that under high pressure Hg in Hg-F compounds transfers charge from the d orbitals to the F, thus behaving as a transition metal. Oxidizing Hg to +4 and +3 yielded the thermodynamically stable compounds HgF4 and HgF3. The former consists of HgF4 planar molecules, a typical geometry for d(8) metal centers. HgF3 is metallic and ferromagnetic owing to the d(9) configuration of Hg, with a large gap between its partially occupied and unoccupied bands under high pressure.

Molecular CsF5 and CsF2+

D5h star-like CsF5 , formally isoelectronic with known XeF5 (-) ion, is computed to be a local minimum on the potential energy surface of CsF5 , surrounded by reasonably large activation energies for its exothermic decomposition to CsF+2 F2 , or to CsF3  (three isomeric forms)+F2 , or for rearrangement to a significantly more stable isomer, a classical Cs(+) complex of F5 (-) . Similarly the CsF2 (+) ion is computed to be metastable in two isomeric forms. In the more symmetrical structures of these molecules there is definite involvement in bonding of the formally core 5p levels of Cs.