Single-crystal optical absorption spectra for magnus-type salts of palladium and platinum

Abstract

Polarized single-crystal absorption spectra in the region 800-250 nm were recorded at 300 and 6 K for the 20 possible Magnus-type crystals containing the PtCl 4 2−, PtBr 4 2−, PdCl 4 2− and PdBr 4 2− anions, and the [Pt(NH 3) 4] 2+, [Pt(CH 3NH 2) 4] 2+, [Pt(en) 2] 2+, [Pd(NH 3) 4] 2+ and [Pd(en)] 2+ cations. Metal-metal distances were obtained from X-ray diffraction determination of crystal axes. A complete set of X-ray diffraction intensities was collected for 2659 reflections over four octants for crystals of Pt(en) 2PdCl 4. Triclinic cell parameters were: a: b: c: = 11.711(7):8.480(6):6.801(2) Å; α: β:γ = 96.10(4): 91.08(4): 106.74(8)°; V = 642.3(2) Å 3; Z = 2. However, only 20 very weak reflections occurred that would conflict with a body-centered space group. Refinement in I 1 yielded Pt, Pd, Cl and N positions satisfactory for spectra interpretation. C positions could not be satisfactorily established and refinement in P1, I1 or P 1 did not converge. The spectra at 6 K provided resolution into components not available at room temperature. The temperature dependence of intensities and in some cases resolution of vibrational structure in the bands was helpful in making transition assignments. For Pt(NH 3) 4PtCl 4 a very sharp electric dipole allowed band, not previously observed, was found just above the d-d transitions with x,y polarization (normal to the stacking direction). Similar bands occurred in the PtCl 4 2− salts with other Pt cations. The evidence is summarized for assignment of these bands as interionic electron transfers (anion- d xz,yz → cation- p z ). This evidence implies that the very intense absorption at ca 35,000 cm −1, z-polarized (along the stacking axis), is also an interionic electron transfer (anion- d Z 2 → cation- p z ). These results together with shifts of the intraionic d → d transitions suggest that the d z 2-orbital in the free MX 4 2- ions lies close in energy to the d xz,yz -orbitals. Spectra of PdCl 4 2− salts with the Pt cations reveal broad dipole-allowed transitions above the intramolecular d → d transitions. It appears likely that they are interionic electron transfers. However, their characteristics indicate they cannot be anion- d → cation- d nor anion- d → cation- p z , transitions. Hence they are most likely anion- Lπ → cation- dσ * transitions. The results indicated that Pt was more effective than Pd in shifting d-d transitions in the region of 3.25–3.55 Å. These shifts, except for the 1 A 1 g → 1 A 2 g transitions, were strongly dependent on the metal-metal distance with cations of the same metal. The collection of single-crystal absorption spectra that are now available permit some important conclusions about this interesting class of compounds. First of all, the planar nature of the ions provides a close contact of the metal atoms as these alternating cations and anions stack in one-dimensional arrays. There is some orbital overlap between the metal atoms in the cations and anions. However, there appears to be negligible covalent bonding or delocalization as occurs in the mixed-valence chains with the planar Pt(CN) 4 groups, which have much smaller PtPt distances and metallic conductivity. There is, however, sufficient overlap with the 3.25–3.5-Å metal spacing to permit interionic electron-transfer transitions and these account for a higher absorption of polarized light with the electric vector oriented in stacking direction that has been recognized as a characteristic of a number of these compounds. It is especially important to this assignment that the expected interionic electron transfer with polarization normal to the stacking direction can now be identified in some of the cases. The energies of a number of d-d transitions for the anions are highly sensitive to differences in the metal-metal spacings in the region of 3.25–3.6 Å. These differences in the band frequencies account for the anomalous colors which have drawn attention to these compounds. Controversy over where to place the d z 2 -orbital with respect to the other d-orbitals has simmered for the past 25 years. Our results provide, we believe, good but perhaps not over-whelming evidence for placing the d z 2 and the d xz,yz pair close together in the free MX 4 2− ion so the 1B 1 g -and 1 E g -bands are not resolved in aqueous solution or in the potassium salts. One general conclusion is that the valence shell orbitals of Pd are more compact than the corresponding orbitals of Pt. Thus the Pt cations gave greater shifts of aborption bands and higher interionic electron transfer probabilities for a given anion. The results were also consistent with a much lower d → p excitation energy for Pt compared to Pd. Finally, we believe that the earlier assigmnent of intermolecular electron-transfer transitions for the molecular compounds of Pt(en)Cl 2 and Pt(en)Br 2 should be reconsidered. 49,50 These nearly planar molecules stack face to face in one-dimensional arrays in orthorhombic crystals. Metal-metal spacings are 3.39 and 3.50 Å, respectively. Both have an intense absorption band polarized in the stacking direction. Polarized reflectance spectroscopy by Anex and Peltier 51 places the peak at 35,100 cm −1. Diffuse-reflectance spectra on the other hand showed maxima at 37,500 and 36,700 cm −1, respectively, for the chloride and bromide compounds. Thus they each have a strong band similar to those in the PtCl 4 2− and PtBr 4 2− Magnus-type salts. In view of the present work, it appears logical now to assign this transition as an intermolecular d z 2 → p z electron transfer rather than as the red-shifted intramolecular d z 2 → p z originally proposed. Although the neutral molecules would be expected to have greater ionization energy and electron affinity than the anion and cation, the formation of an ion-pair would greatly increase the lattice energy. The narrow band, indicated to arise from a dipole-allowed transition, polarized normal to the stacking direction at 33,100 and 33,500 cm −1 for the chloride and bromide would then correspond to intermolecular d yz → p z electron transfer rather than the intermolecular d yz → d xy * transition proposed. (In this case the axes were selected so that the d xy was the d-orbital involved in the σ-bonds to the ligands 50.) This transition occurred among the normal intramolecular d → d transitions. Although for these crystals the metal-metal distances permitted the intermolecular electron transfer transitions, they were sufficiently large that no significant red shift of the spin-forbidden d-d bands occurred as in the case of many of the Magnus-type salts with ethylenediamine cations.