A Unique Two-Dimensional Silver(II) Antiferromagnet Cu[Ag(SO4 )2 ] and Perspectives for Its Further Modifications.

Copper(II) silver(II) sulfate crystallizes in a monoclinic CuSO4 -related structure with P21 /n symmetry. This quasi-ternary compound features Ag(SO4 )2 2- layers, while the remaining cationic sites may be occupied either completely or partially by Cu2+ cations, corresponding to the formula of (Cux Ag1-x )[Ag(SO4 )2 ], x=0.6-1.0. CuAg(SO4 )2 is antiferromagnetic with large negative Curie-Weiss temperature of -140 K and shows characteristic ordering phenomenon at 40.4 K. Density functional theory calculations reveal that the strongest superexchange interaction is a two-dimensional antiferromagnetic coupling within Ag(SO4 )2 2- layers, with the superexchange constant J2D of -11.1 meV. This renders CuAg(SO4 )2 the rare representative of layered Ag2+ -based antiferromagnets. Magnetic coupling is facilitated by the strong mixing of Ag d(x2 -y2 ) and O 2p states. Calculations show that M2+ sites in MAg(SO4 )2 can be occupied with other similar cations such as Zn2+ , Cd2+ , Ni2+ , Co2+ , and Mg2+ .

Ag(II) as Spin Super-Polarizer in Molecular Spin Clusters.

Using quantum mechanical calculations, we examine magnetic (super)exchange interactions in hypothetical, chemically reasonable molecular coordination clusters containing fluoride-bridged late transition metals or selected lanthanides, as well as Ag(II). By referencing to analogous species comprising closed-shell Cd(II), we provide theoretical evidence that the presence of Ag(II) may modify the magnetic properties of such systems (including metal-metal superexchange) to a surprising degree, specifically both coupling sign and strength may markedly change. Remarkably, this happens in spite of the fact that the fluoride ligand is the least susceptible to spin polarization among all monoatomic ligands known in chemistry. In an extreme case of an oxo-bridged Ni(II)2 complex, the presence of Ag(II) leads to a nearly 17-fold increase of magnetic superexchange and switching from antiferro (AFM)- to ferromagnetic (FM) coupling. Ag(II)─with one hole in its d shell that may be shared with or transferred to ligands─effectively acts as spin super-polarizer, and this feature could be exploited in spintronics and diverse molecular devices.

The fate of compound with AgF2:AgO stoichiometry-A theoretical study.

Metal oxyfluorides constitute a broad group of chemical compounds with a rich spectrum of crystal structures and properties. Surprisingly though, none of the ternary oxyfluorides contains a cation from group 11 of the periodic table. Intending to find one, we focused on the silver derivative, the Ag2OF2 system, which may be considered as the 1:1 "adduct" of AgF2 (i.e., an antiferromagnetic positive U charge transfer insulator) and AgO (i.e., a diamagnetic disproportionated negative charge transfer insulator). Here, possible crystal structures of the silver oxyfluoride were studied using evolutionary algorithms based on the density functional theory approach. We analyzed the oxidation states of silver in the low-energy structures, possible magnetic interactions, and energetic stability with respect to the available substrates. Our findings suggest that silver oxyfluoride, if obtained, may form a metastable crystal with cations in three different oxidation states of the same element. Due to the small energy difference, existence of a fully disproportionated metallic compound cannot be ruled out. Finally, we outlined a prospect for the synthesis of polytypes of interest using diverse synthetic approaches, starting from the direct fluorination of Ag2O.

Theoretical study of ternary silver fluorides AgMF4 (M = Cu, Ni, Co) formation at pressures up to 20 GPa.

Only several compounds bearing the Ag(ii) cation and other paramagnetic transition metal cations are known experimentally. Herein, we predict in silico stability and crystal structures of hypothetical ternary silver(ii) fluorides with copper, nickel and cobalt in 1 : 1 stoichiometry at a pressure range from 0 GPa up to 20 GPa employing the evolutionary algorithm in combination with DFT calculations. The calculations show that AgCoF4 could be synthesized already at ambient conditions but this compound would host diamagnetic Ag(i) and high-spin Co(iii). Although none of the compounds bearing Ag(ii) could be preferred over binary substrates at ambient conditions, at increased pressure ternary fluorides of Ag(ii) featuring Cu(ii) and Ni(ii) could be synthesized, in the pressure windows of 7-14 and 8-15 GPa, respectively. All title compounds would be semiconducting and demonstrate magnetic ordering. Compounds featuring Ni(ii) and particularly Co(ii) should exhibit fundamental band gaps much reduced with respect to pristine AgF2. The presence of Cu(ii) and Ni(ii) does not lead to electronic doping to AgF2 layers, while Co(ii) tends to reduce Ag(ii) entirely to Ag(i).

PtX as the limit of high oxidation states in oxide-nitride species.

A series of pentaatomic species has been investigated theoretically with relativistic DFT using the M06-L functional with both ZORA scalar relativistic correction, and including spin-orbit coupling effects. The distorted quasi-octahedral local minima for PtNO3+, PtN2O2 and PtN3O- corresponding to decavalent Pt were found to be unstable with respect to the elimination of O2, NO or N2. However, barriers surrounding these minima suggest that these species could be achieved under low-temperature conditions, similar to what was predicted for PtO42+ dications. Decavalent platinum sets the upper limit for high oxidation states of the chemical elements.

The high covalence of metal-ligand bonds as stability limiting factor: the case of Rh(IX)O4+ and Rh(IX)NO3.

Rhodium, a 4d transition metal and a lighter analogue of iridium, is known to exhibit its highest VIth oxidation state in RhF6 molecule. In this report, the stability and decomposition pathways of two species containing rhodium at a potentially formal +IX oxidation state, [RhO4]+ and RhNO3, have been investigated theoretically within the framework of the relativistic two-component Hamiltonian calculations. Possible rearrangement into isomers featuring lower formal oxidation numbers has been explored. We found that both species studied are metastable with respect to elimination of O2 or NO. However, the local minima containing Rh(IX) are protected by sufficient energy barriers on the decomposition pathway, and they could in principle be prepared. The analysis of a broader set of compounds containing group 8 and 9 metals in high formal oxidation states that correspond to the group number showed that, in contrast to a standard trend, the limits of formally attainable oxidation state correlate with high level of covalent bonding character in the complexes studied.

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 C3v 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.

Classical and Reverse Substituent Effects in Substituted Anthrol Derivatives.

The substituent effect in 1-, 2-, and 9-anthrols is studied by means of B3LYP/6-311++G(d,p) computation, taking into account substituents (X): NO2, CN, OH and NH2 located in all positions except the adjacent ones. The substituent effect is characterized by approaches based on quantum chemistry: The charge of the substituent active region (cSAR), substituent effect stabilization energy (SESE) and the charge flow index (CFI) describing flow of the charge from X to the fixed group (or vice versa) as well as substituent constants σ. Changes in properties observed in the fixed group (OH) are described by cSAR(OH). Mutual interdependences are found between these descriptors. The HOMA index is used to describe an effect of a substituent on aromaticity of an anthrol hydrocarbon skeleton and of individual rings. In all cases, the classical (influence of X on the properties of OH) and reverse (influence of OH on the properties of X) substituent effects are studied. The latter is clearly documented by the cSAR approach.

Toward the Physical Interpretation of Inductive and Resonance Substituent Effects and Reexamination Based on Quantum Chemical Modeling.

An application of a charge of the substituent active region concept to 1-Y,4-X-disubstituted derivatives of bicyclo[2.2.2]octane (BCO) [where Y = NO2, COOH, OH, and NH2 and X = NMe2, NH2, OH, OMe, Me, H, F, Cl, CF3, CN, CHO, COMe, CONH2, COOH, NO2, and NO] provides a quantitative information on the inductive component of the substituent effect (SE). It is shown that the effect is highly additive but dependent on the kind of substituents. An application of the SE stabilization energy characteristics to 1,4-disubstituted derivatives of BCO and benzene allows the definition of inductive and resonance contributions to the overall SE. Good agreements with empirical approaches are found. All calculations have been carried out by means of the B3LYP/6-311++G(d,p) method.