Investigation of Structural and Electronic Properties of [Tris(Benzene-1,2-Dithiolato)M] 3− (M = V, Cr, Mn, Fe and Co) Complexes: A Spectroscopic and Density Functional Theoretical Study

In this study, the first raw transition metals from V to Co complexes with benzene-1,2-dithiolate (L 2− ) ligand have been studied theoretically to eluci-date the geometry, electronic structure and spectroscopic properties of the complexes. Density Functional Theory (DFT) and Time-Dependent Density Functional Theory (TD-DFT) methods have been used. The ground state geometries, binding energies, spectral properties (UV-vis), frontier molecular orbitals (FMOs) analysis, charge analysis and natural bond orbital (NBO) have been investigated. The geometrical parameters are in good agreement with the available experimental data. The metal-ligand binding energies are 1 order of magnitude larger than the physisorption energy of a benzene-1, 2-dthiolate molecule on a metallic surface. The electronic structures of the first raw transition metal series from V to Co have been elucidated by UV-vis spectroscopic using DFT calculations. In accordance with experiment the calculated electronic spectra of these tris complexes show bands at 522, 565, 559, 546 and 863 nm for V 3+ , Cr 3+ , Mn 3+ , Fe 3+ and Co 3+ respectively which are mainly attributed to ligand to metal charge transfer (LMCT) transitions. The electronic properties analysis shows that the highest occupied molecular orbital (HOMO) is mainly centered on metal coordinated sulfur atoms whereas the lowest unoccupied molecular orbital (LUMO) is mainly located on the metal surface. From calculation of intramolecular interactions and electron delocalization by natural bond orbital (NBO) analysis, the stability of the complexes was estimated. The NBO results showed significant charge transfer from sulfur to central metal ions in the complexes, as well as to the benzene. The calculated charges on metal ions are also reported at various charge schemes. The calculations show encouraging agreement with the available experimental data.


Introduction
A metalloligand can be defined as a complex that contains several potential donor groups that enable coordination to a variety of metal ions. Among the well-established metalloligands, interest in the chemistry of dithiolene complexes has increased tremendously in the past five decades since their initial popularity in the 1960s [1]. Early interest focused primarily on the structural geometries [2], redox properties [3] [4] [5] and magnetic properties [6] of this class of complexes, which arise from the noninnocent property [7] [8] of dithiolenes. Ward and McCleverty [7] have pointed out that the term noninnocent is applied properly when it is referred to a particular combination of the metal and the ligand rather than to redox-active ligands only.
By taking into account the dithiolene ligands, two main classes of molecules can be distinguished. Such as the non-benzenoid dithiolenes, where the p-delocalization is confined in the dithiolene core [9] and benzenoid systems, where the p-delocalization is extended to the aromatic ring [10]. Dithiolene ligands often are referred to as noninnocent when coordinated to transition metals [11] [12]. Let us turn our attention to the chemistry of tris (benzene-1,2-dithiolato) complexes. In contrast to their well-studied redox behavior, bond-making and bond-breaking reactivities of tris(benzene-1,2-dithiolato) complexes have been paid less attention [4]. The term "dithiolene" (L) will be used for two classes of ligands as shown in scheme 1(a) and 1(b) of Figure 1 [13], irrespective of their "true" oxidation level as monoanionic radical (L•) 1− , or as dianionic, closed-shell (L Red ) 2− ligand or as neutral 1,2-dithioketone (L Ox ) 0 [14].
The bis(dithiolate) metal and tris(dithiolate) metal complexes have been extensively studied [2] and only very recently, the spectroscopic methods like S K-edge X-ray absorption spectroscopy have been developed to the extent that the presence of a benezene-1,2-dithiolate(1-) π radical in a coordination compound can be established beyond doubt [31].
In the present study, we have performed systematically a theoretical investigation on the structures, binding energies, spectroscopic and electronic properties Advances in Chemical Engineering and Science including V 3+ to Co 3+ is reported. These findings help us understand the thermodynamic behavior of each system as a function of the quantum chemistry descriptors.

Structural Analysis
After full optimization, the structural parameters such as bond distances and angles of the [tris(bdt)M] 3− (M = V 3+ , Cr 3+ , Mn 3+ , Fe 3+ and Co 3+ ) complexes are calculated. The calculated geometrical parameters as depicted in Figure 2 and  [37]. The DFT calculated average bond angle S-Co-S was found 90.51˚. Figure

Binding Energy
In this study we have investigated the metal-ligand binding energies, ΔE, of the trivalent metal ions considered and are presented in Figure 3 and Table 2  Interestingly, the previous study also reported that trivalent 1 st row transition metal coordination produced a 1 (one) order of magnitude increase in the cross-linking strength [39]. Figure 3 shows that the overall ΔE increases with increasing nuclear charge due to the increased electrostatic interaction with increasing nuclear charge. The decreasing ΔE with changing Cr to Mn arises from the Jahn-Teller distortion of the metal complex. The dip in ΔE observed for Fe is ascribed to the d 5 configuration of Fe (III). In this case, as all of the d orbitals are occupied, the ligand-to-metal charge transfer is inefficient resulting in a reduced ΔE. The peak at Cr can be understood by noting that the ligand field stabilization should be largest for the d 3 configurations of a high-spin complex. We have calculated the binding energies by including the zero-point energies (ZPEs), thermal energies, enthalpies and Gibbs free energies as summarized in Table 2.
The vibrational, thermal, and entropic contributions to ΔE values turned out to be small, presumably due to the covalent nature of the metal ligand binding. Regardless of the metal ion, the ZPE-, thermal-energy-, enthalpy-, Gibbs free energy corrected ΔE value were all within 6% of the uncorrected binding energies.

Spectroscopic Data
In order to assign the electronic absorption bands of the complexes, TD-DFT calculations have been carried out on these complexes in gas phase optimized geometries at the CAM-B3LYP [34]/6-311 + G(d,p) level of theory. TD-DFT is a useful method for studying excitation energies, and its application has increased in the recent years. The electronic absorption spectra of complexes formed by V, Cr, Mn, Fe and Co have been calculated. The calculated electronic absorption spectra of the tris complexes formed by V to Co are presented in Figure 4 and    [15]. The calculated spectra of the model compound [Cr(bdt) 3 ] 3− are 442 nm and two d-d transitions with relatively low intensities in the visible region at 565 and 687 nm, as typical for chromium (III) octahedral complex. The electronic structure of this complex is best described as [Cr III ( 3,5 L S,S ) 3 ] 3− with a chromium (III) central ion (d 3 , S = 3/2) and three closed-shell dithiolate-(2-) ligands [19]. The electronic spectra of these tris(dithiolene)chromium complexes resemble closely those reported previously for the corresponding tris(dioxolene) chromium complexes [19].

Electronic Properties
The electronic structure parameters are very important for understanding the molecular interactions with other species. We have calculated the electronic energy gap Eg, electron affinity A, electronegativity χ, chemical hardness η and chemical softness S for all the considered complexes. The results are presented in Table 4. The energetics of frontier molecular orbitals (FMOs), i.e., the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energies are calculated. HOMO, which can be considered of as the outermost orbital containing electrons, tends to give these electrons and acts as an electron donor. On the other hand LUMO can be considered as the innermost orbital containing free places and accepts electrons. The difference between HOMO and LUMO provides the electronic gap Eg as depicted in Figure 5. Our investigation shows that Eg for the considered complexes increases in the following direction Mn 3+ > V 3+ > Cr 3+ > Co 3+ > Fe 3+ . The electron affinity and electro negativity values also follow the same trend (Table 4). From MOs plots ( Figure 5), it can be concluded that the HOMO is mainly centered on metal coordinated sulfur atoms (which acts as a donor atoms) whereas LUMO is mainly located on the metal surface. The Energy gap characterizes the molecular chemical stability, and it is a critical parameter in determining molecular electrical transport properties because it is a measure of electron conductivity. The energy gap between HOMO and LUMO (E g ) also plays important role in predicting the polarizability of a molecule. Smaller the energy gap, the more polarizable the molecule is. The chemical stability of molecule can be also studied by the calculation of the chemical hardness η and chemical softness S as shown in Table 4   [tris(benzene-1,2-dithiolato)Fe] 3− complex is the hardest, followed by those of Co 3+ , Cr 3+ , V 3+ and Mn 3+ respectively. The soft molecules are more polarizable than the hard ones as they need small energy for excitation.

Atomic Charges
The atomic charge on the metal ion has been calculated and summarized in Ta

NBO Analysis
In the natural bond orbital (NBO) [45] analysis, electronic wave function is ex- In Equation (2), q is the donor orbital occupancy, ε i and ε j are diagonal elements (orbital energies) of donor and acceptor NBOs respectively and ˆi   [41]. b MK(Merz-Singh-Kollman) [42]. c CHelpG (CHarges from ELectrostatic Potentials using a Grid based method) [43]. d CHelp methods to fit the electrostatic potential method [44]. effect. The strongest interaction in V-complex is LP S12 → LP V, in Cr-complex is LP S37 → LP Cr , in Mn-complex is LP S12 → LP Mn . In Fe-complex the strongest interaction is LP S37 → LP Fe and in Co-complex is LP S36 → LP Co . The results of NBO analysis reflect generally charge transfer from lone pair orbitals located on the donor atoms S to the central metal ions. NBO analysis provides the most accurate possible "natural Lewis structure".

Conclusion
The structural, energetic, spectroscopic and electronic properties of [M(benzene-1,2-dithiolato) 3 ] 3− (M = V 3+ , Cr 3+ , Mn 3+ , Fe 3+ and Co 3+ ) complexes have been investigated theoretically using quantum chemical calculations based on HF/DFT hybrid approach. The calculated geometrical parameters of the tris (benzene-1,2-dithiolato) complexes are in good agreement with the experimental measurement values. The binding energies of the tris complexes range from 467.19 -497.20 kcal•mol −1 which are 1 (one) order higher than physically adsorbed on metallic surface. TD-DFT calculations have been successively employed to simulate the electronic spectra of the V, Cr, Mn, Fe and Co complexes and facilitated a transition assignment. With the electronic structure of this tris-complexes, we have begun to make meaningful comparisons to experimental spectra for investigation into many tris(dithiolato) complexes of early transition metals. The investigation of electronic parameters including frontier molecular orbital energies suggests that Fe containing complexes should have the largest band gap and therefore being the hardest among all. The results of NBO analysis reflect charge transfer from lone pair orbitals located on the donor atoms to the central metal ions. The present metal-ligand binding energies, structures, stability and atomic charges of the metal-benzene-1,2-ditholato (bdt 2− ) complexes will serve as a cornerstone for such modeling using molecular dynamics or Monte Carlo simulations.