PdCl(NO) – an iconic compound with corrugated Pd₄Cl₄ octagons built up by Pd₂Cl₂(NO)₂ moieties

Abstract

During outstanding research in the late 1950s in Munich, which led to the development of the Wacker process for converting ethene into acetaldehyde by catalysis of PdCl 2 , black insoluble nitrosyl-palladium chloride (PdCl(NO)) was obtained. More than sixty years after its first synthesis, its crystal structure was now determined by X-ray diffraction. PdCl(NO) (mP16, P2 1 /c, a = 10.2684( 5), b = 4.0737(2), c = 7.8456(4) Å, β = 111.125(1)°, wR2 = 0.0572) consists of distorted Pd 4 Cl 4 octagons in chair arrange-ment to which four distorted Pd 2 Cl 2 squares are annulated on every second edge. In this arrangement each of the two Pd atoms of the squares are connected to one NÀ O group, bonded alternatively up and down to the Pd atoms with a PdÀ NÀ O angle of 129°. Such a square has the composition of the dimer which was found in the mass spectrum at 343.6 m/e. The octagons with four squares are interconnected to corrugated layers in the b-c-plane as a two-dimensional polymer.

Bonding in PdCl(NO) and Related Nitrosylmetal Species of the Enemark–Feltham {MNO}10Type

“…For the Pd 8 species, the HOMO/LUMO transition is computed at 1.28 eV = 10322 cm À 1 and thus coincides with a marked increase of the absorption in the solid-state spectrum of PdCl(NO). [14] Hence, the small band gap of solid PdCl(NO) should be caused by transitions which we do not see in the low-molecular cutouts.…”

“…In 1, each of the symmetrically equivalent Pd atoms is surrounded by three triply bridging chlorido ligands and a terminal nitrosyl ligand. [14] Figure 1 (left) shows a layer of the two-dimensional PdCl coordination polymer which is wrapped by nitrosyl ligands below and above the layer. We see a pattern of linked binuclear Pd 2 Cl 2 (NO) 2 rhombs as a prominent building block.…”

“…Experimentally as well as computationally the colour goes along with a small band gap of only 0.3 eV of the solid which resembles a wavenumber of about 2400 cm À 1 in the IR region of the electromagnetic spectrum. [14] On increasing radiation energy, the absorption coefficient remains small in the low-energy region until the onset of more intense absorptions is reached at ca. 1.3 eV.…”

“…In an accompanying publication, Evers et al report the successful crystallisation and structural analysis of a key compound from this list, PdCl(NO) (1). [14] With Evers's work, the remarkably small number of structurally resolved nitrosylpalladium compounds has been enlarged. Restricting ourselves to terminally bonded nitrosyls, we are left, in terms of the CCDC data base, with only four analyses.…”

“…The procedure used by Evers et al was based on a reaction in aqueous solution and uses two electrons of two NO as the reductant: [14] 2PdCl…”