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Advanced Bonding Theories for Complexes
Published in Ashutosh Kumar Dubey, Amartya Mukhopadhyay, Bikramjit Basu, Interdisciplinary Engineering Sciences, 2020
Ashutosh Kumar Dubey, Amartya Mukhopadhyay, Bikramjit Basu
The feasibility of LS or HS configuration depends on the transition metal under consideration, the charge/valence on the same (usually Δ increases with the oxidation state) and, more importantly, the type of ligands that form the complex. In this context, the ligands which lead to a relatively smaller value for the crystal field splitting energy (CFSE or the Δ) are known as “weak field ligands,” which usually leads to HS configuration. In contrast, the ligands that produce a larger crystal field splitting (or value of Δ) are known as “strong field ligands,” which usually leads to LS configuration. The ligands are arranged in the increasing order of CFSE; they are expected to form a complex with a given transition metal (and given transition metal oxidation state) in a series known as the “spectrochemical series” absorption spectra of transition-metal complexes). A part of the “spectrochemical series” series (which is usually included in the more complicated ligand field theory), listing ligands from small to large Δ is: O22− < I− < Br− < S2− < SCN− (for bonding with S) < Cl− < N3− < F− < NCO− < OH− < ONO− < C2O42− ≈ H2O < NCS− (for bonding with N) < CH3CN < NH3 < NO2− < PPh3 < CN− < CO. The transition metal ions can also be tentatively arranged in a form similar to the above “spectrochemical series” of the ligands, namely, in the order of increasing CFSE or Δ (for a fixed ligand type); as Mn2+ < Ni2+ < Co2+ < Fe2+ < V2+ < Cu2+ < Fe3+ < Cr3+ < V3+ < Co3+ < Rh3+ < Ir3+ < Pt4+. Overall, the range of Δ is quite significant, with Δ for octahedral complexes (more specifically denoted as Δo) is ∼100 kJ/mol for Ni(H2O)62+, ∼104 kJ/mol for Co2+ in CoF2, and as high as ∼520 kJ/mol in the Rh(CN)63− ion (just as a few examples).
Separation of Pt and Pd from chloride solutions by liquid–liquid extraction using Alamine 308 and analysis of their mechanism: A possible recovery from spent auto catalysts
Published in Geosystem Engineering, 2022
In general, S or N atoms in a ligand can coordinate with a metal atom, either in the hydrated metal ions or as the metal complex. The mechanism of Pt stripping by NaSCN or Pd stripping by (NH2)2CS could be regarded as the coordination-substitute of the stripping agents to and complex, respectively. In the stripping reaction the Cl−ions of and complex, complexes are substituted with (NCS−) and NH2CSNH−, respectively. The spectrochemical series for ligands reflects a sequence: Cl−< F−< OH− ~ ONO−< C2O42− < H2O < NCS−, and for metals it follows Pd(II) < Ir(III) < Pt(IV; Gerloch & Constable, 1994). Hence, the interaction of S(NCS−) with Pt(IV) is much stronger than that of Cl – which facilitates the formation of complex. Thus, NaSCN can selectively strip Pt as into the aqueous phase. Where only Pd(II) complexes are present in the organic phase, the Pd is extracted by a coordination-substitution reaction between NH2CSNH−and Cl–. In the spectrochemical series, Cl–< NH2CSNH−, hence NH2CSNH− substitutes Cl− and forms a more stable complex such as Pd(Cl) 2((NH)2CS)2 leading to Pd stripping. When both metal complexes are present in the organic phase, the substitution reaction of NH2CSNH−with Cl – takes place competitively, which depends upon the concentration of the metal ion complexes formed and the concentration of the stripping agent used. In the competitive substitution reaction, the Cl – in the the complex is substituted by NH2CSNH− faster than , which is the cause for preferential Pd stripping.