Coordination versus spodium bonds in dinuclear Zn(II) and Cd(II) complexes with a dithiophosphate ligand

The important role of spodium bonding (SpB) in Zn(II) and Cd(II) complexes is highlighted in this feature article. SpB has been recommended to distinguish coordination bonds (high covalent character) from non-covalent contacts. Here, two new d10-metal dithiophosphate complexes, [M(L)2]2 {where M = Zn (1) and Cd (2); L = O,O′-bis(4-tert-butylphenyl) phosphorodithioate} have been synthesized in purely aqueous media and characterized by elemental and spectral analyses. The X-ray crystallography analyses indicate that both complexes are dinuclear, in which each metal atom is coordinated with four sulfur atoms in a distorted tetrahedral environment containing two four-membered chelate rings joined to a central eight-membered ring through metal (Zn/Cd) atoms. In complex 1 the eight-membered ring possesses a “saddle” conformation located on a two-fold axis of symmetry, C2, while in complex 2 it possesses a centrosymmetric “twisted chair” conformation, Ci. In addition, the oxygen and sulfur atoms belonging to the bridging ligands form intramolecular SpBs with Zn(II) in 1 and Cd(II) in 2. The existence of intramolecular SpBs in both complexes is evidenced by DFT calculations by using the Bader's Quantum Theory of Atoms-in-Molecules (QTAIM) combined with the non-covalent interaction plot (NCI plot) index that is based on the reduced density gradient (NCI-RDG) method. A Hirshfeld surface study has been carried out to understand the intermolecular contacts in the crystal lattice.