Hydration dynamics gives the distinctive brown color in the "brown ring" nitrate test

The brown ring test is one of the most popular and visually appealing reagent tests, commonly known to chemistry undergrads and familiar even to school students. The exact composition, mechanism and structure of the complex has been investigated for nearly a century. Recent studies have elucidated its UV-vis, EPR and Mossbauer spectra, mechanistic details and kinetics, followed by crystallization and structure determination in solid state. Nonetheless these studies were unable to address the aspects of solution structure and dynamics of the brown ring complex. We have conducted ab initio molecular dynamics simulations of the classic brown ring complex in aqueous solution. In the process from the simulation trajectory, we have identified that the classically established pseudo-octahedral [Fe(H2O)5(NO)]2+ complex is in chemical equilibrium with the square-pyramidal [Fe(H2O)4(NO)]2+ complex through the exchange of one of the coordinated H2O molecules. The dynamics in aqueous solution between the penta-aqua and tetra-aqua complexes in the brown ring system has to our knowledge never been suggested earlier. Interestingly we find, using ab initio multi-reference quantum chemical methods i.e. CASSCF/NEVPT2 and CASPT2 calculations, that the mixture of these two complexes is what gives the distinctive brown coloration to the brown ring test. We show that its UV-vis spectrum can be theoretically reproduced only by accounting these two species, and not solely the classically established [Fe(H2O)5(NO)]2+ complex. The energetics of the penta-aqua and tetra-aqua complexes is also investigated at the level of multi-reference quantum chemical methods.

Evaluation of the effect of solution ageing on the DGT speciation of Rh and Pt

Synthetic solutions of Rh and Pt at near neutral pH, as well as spiked natural waters are subjected to an ageing effect, so that over time, due to aquation and hydrolysis, the speciation of the PGEs changes to various hydroxy and aqua complexes.

Synthesis, UV-visible spectroelectrochemistry and theoretical characterization of new polypyridyl Ru(ii) complexes containing 2,4,6-tris(2-pyridyl)-1,3,5-triazine as precursors for water oxidation catalysts

New chloro and aqua complexes of Ru(ii) with 2,4,6-tris(2-pyridyl)-1,3,5-triazine as precursors for water oxidation catalysts at pH = 1 using Ce(iv).

Allan White and Solvento/Aqua Complexes: ScIII Solvation

Addition of various oxygen-donor ligands (L, all dipolar, aprotic solvents), to a solution of hydrated scandium picrate in weakly coordinating solvents (S), has yielded several crystalline adducts of the form Sc(pic)3(L)m(·nS) in which all ligands L are coordinated, subsequently characterised by a series of single crystal X-ray studies. For L=dmso, m=3 and the picrate anions are all bound as phenoxide-O donor units, while for nmp, dma, tmp, and hmpa, m=2 and one of the bound picrates becomes bidentate through phenoxide- and nitro-O donation. For L=ompa, m=2 and two picrate ligands are bound through phenoxide-O while one is displaced from the primary coordination sphere. All complexes contain six-coordinate ScIII, confirming that this coordination number is a consequence of ligand bulk.

INTERACTION OF CIS-Pd(NH3)2Cl2 WITH DIPHOSPHONIC ACIDS IN SOLUTIONS AT PHYSIOLOGICAL CHLORIDE-IONS CONCENTRATION

The complex formation of cis-Pd(NH3)2Cl2 with 1-hydroxyethylidene-1,1-diphosphonic (HEDP, H4L1), 3-amino-1-hydroxypropylidene-1,1-diphosphonic (AHPrDP, H4L2), and 1-aminopropylidene-1,1-diphosphonic (APrDP, H4L3) acids in aqueous solutions with the concentration CKCl=0.15 mol/L, which corresponds to the concentration of chloride ions in the intercellular fluid, has been studied by spectrophotometry and pH potentiometry. The results of studying the interaction between cis-Pd(NH3)2Cl2 and diphosphonic acids have been interpreted taking into account the equilibrium concentration distribution of complexes forming in Pd(NH3)2Cl2 solutions at a chloride ion concentration of 0.15 mol/L. It has been found that when Pd(NH3)2Cl2 is dissolved in 0.15 mol/L KCl, ammonia molecules are substituted by chloride ions and a water molecule in the pH range of 2 – 4 to form chloro-aqua complexes [PdCl4]2- and [PdCl3(H2O)]-. In the case of complex formation of Pd(II) chloro-aqua complexes with HEDP and AHPrDP, complexes with [Pd 2OPO3 2Cl] chromophore with bidentate coordination of ligands by two oxygen atoms of phosphonic groups are formed in the acidic pH range. At pH>5, a [Pd(L1)(NH3)2]2- complex (lgβ=30.55(5)) is formed in the cis-Pd(NH3)2Cl2:HEDP=1:1 system, and at pH>6, a [Pd(HL2)(NH3)2]- complex (lgβ=40.29(2)) is formed in the cis-Pd(NH3)2Cl2:AHPrDP=1:1 system. The formation of complexes with [Pd 2OPO3 2Namine] chromophore takes place with the displacement of chloride ions from the coordination sphere of complexes with [Pd 2OPO3 2Cl] chromophore by ammonia molecules. In the system cis-Pd(NH3)2Cl2:APrDP=1:1, the ligand is coordinated to Pd(II) in a bidentate fashion by the nitrogen atoms of the amine group and oxygen atoms of the phosphonic group to form a [Pd(H2L3)Cl2]2- complex with [Pd Namine OPO3 2Cl] chromophore in the acidic pH range. When pH is increased to 5 and then to 7, a sequential substitution of chloride ions by ammonia molecules takes place to form a [Pd(HL3)(NH3)Cl]2- complex (lgβ=38,84(4)) with [Pd 2Namine OPO3 Cl] chromophore and a [Pd(HL3)(NH3)2]- complex (lgβ=43,14(2)) and [Pd(L3)(NH3)2]2- complex (lgβ=34.91(2)) with [Pd 3Namine OPO3] chromophore.