Computational and Experimental Determination of the Electronic Structure and Optical Properties of Three-Dimensional Zn[M(CN)₄] Tetracyanates (M = Ni, Pd, and Pt)

Abstract

The electronic structure of cyanide-based Zn­[M­(CN)4] (M = Ni, Pd, and Pt) coordination polymers is studied by means of spectroscopic techniques and DFT-based computational calculations. The observed different ν­(CN) band shifts to higher frequencies when the inner metal from the tetracyanate moiety [M­(CN)4] changes from Ni to Pt and Ni to Pd as a consequence of the charge distribution produced by the π back-bonding phenomena and the competition between the polarization powers from M and Zn. This is evidenced by infrared, Raman, and UV–vis spectroscopic techniques in conjunction with hybrid HSE06 calculations. The sample characterization was completed from XPS spectra and HR-TEM images. The electronic structure was also studied by the computed lm-decomposed density of states and band dispersion diagrams. The nature of the valence band top and conduction band bottom is described by d-M, p-M, p-nitrogen, and c-carbon hybridized orbitals. The electronic behavior of the former solids strongly differs from that of the isolated square-planar tetracyanates, but the HOMO–LUMO electronic transitions are still dominated by the tetracyanate [M­(CN)4] and π–π* interactions in the three cases. Band gap energy values are reported for the three studied semiconductors, and the internal metal effect is analyzed. The indirect nature of the electronic transitions associated with the gap is discussed, and the values of the effective and reduced masses are reported.

A first principles study of the electronic structure and optical response of heterobimetallic M–dicyanoaurate-based coordination polymers (M = Mn, Co, Ni, Zn and Cd)

“…This is clear evidence of the greater impact that structural changes have on the electronic structure compared to minor changes such as metal exchange in isostructural systems. 42 This behavior can also be observed from the comparison of the optical band gap energies ( E Dir g and E Ind g ) in Table 1.…”

“…This is clear evidence of the greater impact that structural changes have on the electronic structure compared to minor changes such as metal exchange in isostructural systems. 42 This behavior can also be observed from the comparison of the optical band gap energies (E Dir g and E Ind g ) in Table 1. Additionally, it must be considered that Raman signals are very sensitive to Au(I)-Au(I) interactions, 47 which certainly have an influence on the Raman n(CN) and d(Au-CN-M) energies, in addition to the polarizing power of M cations.…”

“…In this sense, the band shift can be considered as an indirect probe to quantify the strength of the Lewis acids, on the electrostatic strength scale, through their relation with the polarizing power. 42 This parameter, calculated according to Z / r 2 , in which Z corresponds to the charge of the metal ion and r is the ionic radius, allows a better understanding of the metal–ligand interaction in terms of the metal's ability to subtract charge from cyano ligands. The polarizing power of the metals studied herein follows the order Ni > Co > Mn > Cd, considering their divalent nature and the crystal ionic radii reported.…”

“…The hybrid HSE06 density functional has been extensively employed to calculate the electronic properties of coordination complexes obtaining good results with respect to experimental data. 35,36,42 Additionally, for comparison, the general gradient approximated PBE exchange-correlation density functional was also used to calculate the electronic band gap energies (Table 1). From the computed results, HSE06 produces a very good approximation of the allowed direct transitions (E Dir g ), with energy deviation values o0.3 eV (with the only exception of Mn-DAu).…”

“…5) evidence that electronic transitions have a MLCT nature. 42,49 In this sense, the logical transitions promoted by the absorption process occur from the hybridized filled gold d, s orbitals to the empty ligand-based 2p u orbital, according to the molecular orbital theory. Therefore, the excitations derived from the DAu moiety correspond to 2s + g (z 2 ) -2p u , 2p g (xz,yz) -2p u and D g (xy,x 2 À y 2 ) -2p u .…”