How can [Mo^IV(CN)₆]²⁻, an apparently octahedral (d)²complex, be diamagnetic? Insights from quantum chemical calculations and magnetic susceptibility measurements

Abstract

Quantum chemical calculations are employed to elucidate the origin of a puzzling diamagnetism for a hexacyanomolybdate(IV) anion, [Mo(CN)6](2-), which was previously reported by Szklarzewicz et al. [Inorg. Chem., 2007, 46, 9531-9533]. The diamagnetism is surprising because for the octahedral (d)(2) complex one would rather expect a (paramagnetic) triplet ground state, clearly favored over a (diamagnetic) singlet state by an exchange interaction between two d electrons in the t2g orbitals. Nevertheless, the present calculations reveal that the minimum energy structure of isolated [Mo(CN)6](2-) is not an octahedron, but a trigonal prism; the latter geometry allows maximization of a σ-donation from the cyanides to the electron-deficient Mo(iv) center. Unlike for the octahedron, for the trigonal prism structure the singlet and triplet spin states are close in energy to within a few kcal mol(-1). Although the actual relative energy of the two spin states turns out to be method-dependent, the complete active space calculations (CASPT2; with the appropriate choice of the IPEA shift parameter) can reproduce the singlet ground state, in agreement with the experimentally observed diamagnetism. Moreover, magnetic measurements reveal a slight increase of the magnetic susceptibility with the increase of temperature from 100 to 300 K, suggesting an admixture of a thermally induced paramagnetism (possibly due to Boltzmann population of the low-energy triplet state) on top of the dominant diamagnetism. Our prediction that the geometry of [Mo(CN)6](2-) should significantly deviate from the ideal octahedron, not only in the gas phase, but also in a periodic DFT model of the crystalline phase, as well as the experimentally confirmed diamagnetic properties, does not agree with the previously reported ideal octahedral structure. We suggest that this crystal structure might have been determined incorrectly (e.g., due to overlooked merohedral twinning or superstructure properties) and it should be re-investigated.

Synthesis, characterization and antibacterial study of Co(II) and Cu(II) complexes of mixed ligands of piperaquine and diclofenac

“…In a similar pattern, Pip, a nitrogen-based ligand, shows coordination to the metal ions through the amine N and the Cl as evidenced by the disappearance of the band 3286 cm -1 attributable to the amine N with Δ value of 147 cm -1 , and also the hypsochromic shift of C-Cl band at 650 cm -1 in the ligand to 662 cm -1 in the metal complexes, indicating the possibility of coordination through the Cl group. These rationally suggest the ligand to act as a bidentate ligand (16) The electronic spectra, magnetic moment, and elemental analysis results of the synthesized metal complexes are reported in Table 3 (17,30,31). The presence of the ligands in the complexes as proposed (Schemes 1 & 2) was supported with good consistency between the CHNS elemental analysis results (Supplementary file) and the suggested molecular formulas.…”

Spin-State Energetics of Fe(III) and Ru(III) Aqua Complexes: Accurate ab Initio Calculations and Evidence for Huge Solvation Effects

“…For instance, the BP86 and B3LYP results differ by only 0.1 kK; the B3LYP and B3LYP* results differ by 0.3 kK. We recently observed a comparably small dependence of spin-state energetics on the choice of functional for a cyanido complex of Mo­(IV) . We anticipate that this may be typical to 4d-electron transition metal complexes, by contrast to their 3d-electron analogues, and it may be partly explained by smaller values of exchange integrals for 4d than for 3d orbitals .…”

Photochemistry of Fe:H2O Adducts in Argon Matrixes: A Combined Experimental and Theoretical Study in the Mid-IR and UV–Visible Regions

“…To avoid intruder states, the real level shift was applied in all the CASPT2 calculations, with a value of 0.4 au. The ionization potential-electron affinity (IPEA) shift was not used here as there is some controversy in the literature concerning its application. − The validation of the different CASPT2 parameters was performed by determining the 5 D 4 – 5 F 5 energy splitting of the iron atom with different IPEA and level shifts. For this, 12 states were calculated, corresponding to the 5 D (3d 6 4s 2 ) and 5 F (3d 7 4s 1 ) configurations.…”