Effect of the metal⋯metal interactions on the absorption properties of Pt(II) and Pd(II) complexes of glyoxilic acid oxime in solution and solid state: Theoretical and experimental study

The electronic structures and vertical excitation energies of square-planar complexes of Pt(II) and Pd(II) with glyoxilic acid oxime (H2GAO) (cis-K[Pt/Pd(GAO)(HGAO)]) were investigated in gas phase, aqueous solution and solid state by combination of DFT/TD-DFT methods, experimental UV–vis measurement in solution and diffuse UV–vis study in solid state. The solid-state chain structures were simulated by means of dimer and trimer clusters modeling. The absorption properties of the complexes predicted for gas phase are slightly changed by the solvent and small bathochromic band shifts are found. The formation of chain structure of Pt(II) and Pd(II) complexes is discussed in relation with M⋯M dz2 orbitals interaction. According to the calculations complex aggregation due to M⋯M interactions occurs even in concentrate aqueous solution of Pt(II) complex producing change in the complex color: from yellow to orange-red. Upon aggregation the dz2 orbital interactions increase and the occupied MO involved in the first singlet excitation destabilizes due to the antibonding combination of the Pt/Pd dz2 contributions. As a result, the transition energies decrease producing bathochromic shift of the MLCT bands. For the Pt(II) complex the destabilization of HOMO is larger and produces larger bathochromic shift of the low-lying band (140 nm) as compared to that observed for the Pd(II) complex (90 nm). The larger bathochromic shift for the Pt(II) complex leads to absorption at ∼500 nm and color change from yellow (in diluted solution) to red for the crystalline Pt(II) complex. For Pd(II) trimer complex the low-lying excitation of MLCT character is below 500 nm in keeping with the yellow complex in the solid as it is in solution. It is established that the M⋯M orbital interaction strength is determined mainly by the M⋯M distances, the A(M⋯M⋯M) angle and the decisive role of the sum of VDW radii.