Magneto‐Structural Study of an Oxamato‐Bridged Pd II Co II Chain: X‐ray Crystallographic Evidence of a Single‐Crystal‐to‐Single‐Crystal Phase Transition

Two new mononuclear oxamato‐containing palladium(II) complexes of formula K 2 [Pd(opba)] · 2H 2 O ( 1 ) and (PPh 4 ) 2 [Pd(opba)] · 2H 2 O ( 2 ) and the heterodimetallic palladium(II)–cobalt(II) chain {[Co(H 2 O) 2 Pd(opba)] · dmso} n ( 3 ) [opba = 1,2‐phenylenebis(oxamate), PPh 4 + = tetraphenylphosphonium cation and dmso = dimethyl sulfoxide] have been prepared, and the structures of two of them (compounds 2 and 3 ) were determined by X‐ray diffraction analysis of single crystals. The structure of 2 consists of discrete anions of [Pd(opba) 2 ] 2– and PPh 4 + cations. Each Pd II ion in 2 is surrounded by two oxamate nitrogen atoms and two carboxylate oxygen atoms in a square‐planar surrounding. Compound 3 is a neutral chain with regular alternating Pd II and Co II ions, the [Pd(opba)] 2– entity acting as a bis(bidentate) ligand towards trans ‐diaquacobalt(II) fragments, and dmso molecules of crystallization. Compound 3 exhibits a single‐crystal‐to‐single‐crystal phase transition between monoclinic C 2/ c (α phase) and triclinic P $\bar {1}$ (β phase) space groups. Within the α phase the dmso molecule of crystallization is disordered, but it becomes ordered below 215 K in the β phase. The ordering process of the dmso molecule is followed by a reduction in the unit cell volume. The magnetic properties of 3 are barely affected by the structural phase transition, and they indicate an important spin–orbit coupling of the high‐spin octahedral Co II ion [ α = 1.245, λ = –151 cm –1 , Δ = 494 cm –1 , the spin Hamiltonian being defined as H = – αλ LS + Δ( L z 2– 2/3) + βH (– α L + g e S )] with an almost negligible intra‐ [through the diamagnetic square‐planar Pd II centre] and interchain antiferromagnetic interactions ( θ = –0.32 K).