Two nickel (II) complexes with side chain isomeric ligands: L-leucine and L-isoleucine to study non-covalent interactions and metal-ligand bonding

Two new Ni(II) complexes with isomeric α-amino acid ligands were prepared and studied from Synchrotron X-ray powder diffraction data. In Complex I, trans-diaqua-(L-leucinate- κ² N,O) nickel(II), the two L-leucinate ligands display symmetrical (κ² N,O) bites, with Ni···Ni distances of 4.842(2) Å. In complex II, cis-diaqua-(L-isoleucinate- κ² N,O) nickel(II), the L-isoleucinate ligands adopt (κ² N,O) crossbites to the metal center. A redistribution of the hydrophobic and bulkier ethyl groups occurs that balances the H···H interactions in the L-isoleucinate's tails and increments Ni···Ni distances to 5.822(5) Å. In complex I, Hirshfeld surfaces show that two C—H···O hydrogen bonds act as stacking forces during molecular aggregation. The electron localization analysis (ELF), quantum theory of atoms in molecules (QTAIM), and natural bond orbital (NBO) results suggest that all the coordination sphere's bonds have weak covalent character. In hydrogen bond formation, the more polarized Ni—N coordination bond makes the N a stronger H-donor than related O—H donors from coordination water molecules. Net electron transfer from the N(2s,2p) and O(2s,2p) orbitals to the 4s and 4p Ni orbitals, with net atomic charges over the Ni atom being less than the expected +2 value by more than 54%, confirm the weak covalent character of these bond.