This paper describes the conceptual density functional studies on (3+1) complexes of both Re(V) as well as Tc(V). The tridentate ligands have been varied in order to study the effect of the various combinations of N, O and S chelating sites on the overall stability of such complexes. The concept of stability as a measure of difference in energies of products and reactants as listed in Table 3 has been found reproducible. Some modifications in terms of choosing the monodentate ligand are however recommended to test the adaptability of the computational procedure. The ligand softness data were taken from reference no. 20. Modification of procedure so as to include the softness parameter from the HOMO-LUMO energy gap of the tridentate ligands as a whole is recommended for better coherence of the DFT study.
- The Re(V) and Tc(V) complexes with SSS ligand were taken as reference for the calculations and the relative energies were calculated by the equation:
∆E = [E(MOL2) + E(L1)] - [E(MOL1) + E(L2)]
Following the substitution reaction: MOL1 + L2 →MOL2 + L1
The procedure has been outlined very well in section III.2.
- The sections III.2. and III.3. elaborate on the “softness” parameters employed in this work. The collection of individual softness values of N, O, S, Re and Tc atoms and compiling them into one system is explained in these sections.
- The monodentate ligand selected for this study was Cl.
- All the calculations have been carried out using B3LYP/LANL2DZ combination.
- The estimation of substitution energies is fairly straightforward using the procedure outlined in section III.2.
- The softness parameters employed in this study are complex to compile from individual hardness values of atoms. The process is also time consuming and unnecessary for calculations for a large series of complexes.
- The variation of monodentate ligand to phenolate, thiolate etc. shows a similar trend in substitution energies.
- The choice of ST(Re/Tc)MIDI(other atoms) was found to be a better option to estimate the energies since the Re-S and Tc-S bonds show better reproducibility against experimental observations. The Re=O bond is indeed found to correlate better with experiments when LANL2DZ/B3LYP combination is employed.
- In order to save both time and computational cost, it is suggested that the overall hardness of the tridentate ligands is calculated in terms of global chemical reactivity indices.
- Phenolate/ thiolate ligands at monodentate binding sites serve as better descriptors of the usefulness of the study for experimental purposes as many of the (3+1) systems are investigated for radiopharmaceutical applications and their stabilities are of experimental interest.
- The authors mention in section III.2. “When OOS and OSO are compared, the destabilizing effect of the central oxygen overrules the stabilizing effect of the terminal S. In those cases where a N or O atom is central (ONS and OOS), O leads to further destabilization.”
This seems a direct result of choosing the LANL2DZ basis set for all atoms with results being tilted towards the of Re-O/ Tc-O bonding, but lacks the accuracy for coordination to both N and S. I would suggest using ST(Re/Tc)MIDI(other atoms) method for overall accuracy of results.