On bonding in bis(dimethylglyoximato)nickel(II)

DFT Investigation of the Molecular Properties of the Dimethylglyoximato Complexes [M(Hdmg)2] (M = Ni, Pd, Pt)

Important applications of the NiII, PdII and PtII complexes [M(Hdmg)2] (H2dmg = dimethylglyoxime) stem from their metal...metal stacked virtually insoluble aggregates. Given the virtual insolubility of the materials, we postulated that the rare reports on dissolved species in solution do not represent monomolecular species but oligomers. We thus studied the structural and spectral properties of the monomolecular entities of these compounds using density functional theory (DFT) and time-dependent DFT computations in dimethyl sulfoxide (DMSO) as a solvent. The molecular geometries, IR and UV-vis spectra, and frontier orbitals properties were computed using LANL2DZ ecp and def2TZVP as basis sets and M06-2X as the functional. The results are compared with the available experimental and other calculated data. The optimised molecular geometries proved the asymmetric character of the two formed O–H…O bonds which connect the two Hdmg‒ ligands in the completely planar molecules. Calculated UV-vis spectra revealed the presence of three absorptions in the range 180 to 350 nm that are red-shifted along the series Ni–Pd–Pt. They were assigned to essentially ligand-centred π−π* transitions in part with metal(d) to ligand(π*) charge transfer (MLCT) contributions. The notorious d‒p transitions dominating the colour and electronics of the compounds in the solid-state and oligomeric stacks are negligible in our monomolecular models strongly supporting the idea that the previously reported spectroscopic observations or biological effects in solutions are not due to monomolecular complexes but rather to oligomeric dissolved species.

Study on Thermal Decomposition Behavior, Gaseous Products, and Kinetic Analysis of Bis-(Dimethylglyoximato) Nickel(II) Complex Using TG-DSC-FTIR-MS Technique

The fiber-like bis-(dimethylglyoximato) nickel(II) complex, Ni(DMG)2 was successfully synthesized. The obtained samples were characterized by SEM-EDS, FT-IR, XRD, and XPS. The TG-DSC-FTIR-MS coupling technique was used to characterize the thermal decomposition behavior and evolved gas analysis of Ni(DMG)2. The non-isothermal decomposition reaction kinetic parameters were obtained by both combined kinetic analysis and isoconversional Vyazovkin methods. It was found that Ni(DMG)2 begins to decompose at around 280 °C, and a sharp exothermic peak is observed in the DSC curve at about 308.2 °C at a heating rate of 10 °C·min−1. The main gaseous products are H2O, NH3, N2O, CO, and HCN, and the content of H2O is significantly higher than that of the others. The activation energy obtained by the combined kinetic analysis method is 170.61 ± 0.65 kJ·mol−1. The decomposition process can be described by the random nucleation and growth of the nuclei model. However, it was challenging to attempt to evaluate the reaction mechanism precisely by one ideal kinetic model.

Structural, Electronic, and Spectral Properties of Metal Dimethylglyoximato [M(DMG)2; M = Ni2+, Cu2+] Complexes: A Comparative Theoretical Study

Dimethylglyoxime (DMG) usually forms thermodynamically stable chelating complexes with selective divalent transition-metal ions. Electronic and spectral properties of metal-DMG complexes are highly dependent on the nature of metal ions. Using range-separated hybrid functional augmented with dispersion corrections within density functional theory (DFT) and time-dependent DFT, we present a detailed and comprehensive study on structural, electronic, and spectral (both IR and UV-vis) properties of M(DMG)₂ [M = Ni²⁺, Cu²⁺] complexes. Ni(DMG)₂ results are thoroughly compared with Cu(DMG)₂ and also against available experimental data. Stronger H-bonding leads to greater stability of Ni(DMG)₂ with respect to isolated ions (M²⁺ and DMG^-) compared to Cu(DMG)₂. In contrast, a relatively larger reaction enthalpy for Cu(DMG)₂ formation from chemically relevant species is found than that of Ni(DMG)₂ because of the greater binding enthalpy of [Ni(H₂O)₆]²⁺ than that of [Cu(H₂O)₆]²⁺. In dimers, Ni(DMG)₂ is found to be 6 kcal mol^-1 more stable than Cu(DMG)₂ due to a greater extent of dispersive interactions. Interestingly, a modest ferromagnetic coupling (588 cm^-1) is predicted between two spin-¹/₂ Cu²⁺ ions present in the Cu(DMG)₂ dimer. Additionally, the potential energy curves calculated along the O-H bond coordinate for both complexes suggest asymmetry and symmetry in the H-bonding interactions between the H-bond donor and acceptor O centers in the solid-state and in solution, respectively, well corroborating with early experimental findings. Interestingly, a lower proton transfer barrier is obtained for the Ni(DMG)₂ compared to its Cu-analogue due to stronger H-bonding in the former complex. In fact, relatively weaker H-bonding in Cu(DMG)₂ results in blue-shifted O-H stretching modes compared to that in Ni(DMG)₂. On the other hand, qualitatively similar optical absorption spectra are obtained for both complexes with red-shifted peaks found for the Cu(DMG)₂. Finally, computational models for axial mono- and diligand (aqua and ammonia) coordinated M(DMG)₂ complexes are predicted to be energetically feasible and stable with relatively greater binding stability obtained for the ammonia-coordination.

High pressure: a complementary tool for probing solid-state processes

High pressure offers insight into the mechanisms of a wide range of solid-state phenomena occurring under atmospheric pressure conditions.

High pressure studies on bis(l-histidinate)nickel(II) monohydrate

Raman spectra of bis(l-histidinate)nickel(II) monohydrate crystal were obtained for pressures up to 9.5GPa. Our results show the disappearance of some of the Raman modes and the appearance of other modes. These modifications evidence that the sample undergoes phase transitions at around 0.8 and 3.2GPa. The role played by the Ni ions and hydrogen bonds in the dynamics of the phase transitions is discussed. Under decompression, down to atmospheric pressure, the original Raman spectra are recovered, showing that both phase transitions are fully reversible.