Studies on the metalamide bond. XIX. A comparison of molecular distortions in the crystal structures of [N,N′-bis(2′-pyridinecarboxamido)-1,2-benzene] nickel(II) with its 6′-methyl-substituted Analogue

[N,N′-Bis(pyridine-2′-carboxamide)-1,2-benzene]nickel(II) monohydrate, C₁₈H₁₄N₄O₃Ni, crystallizes in the monoclinic space group C2/c with a = 14.240(4), b = 20.071(3), c = 16.275(2) Å,β = 97.25(2)^o, Z = 12 and its crystal structure has been refined to R = 0.033 for 3597 diffractometer data. [N,N′-Bis(6′-methylpyridine-2′-carboxamide)-1,2- benzene]nickel(II) monohydrate, C₂₀H₁₈N₄O₃Ni, crystallizes in the orthorhombic space group Pbca with a = 10.14(2) b = 17.12(2), c= 21.11(5) Å, Z = 8 and its crystal structure has been refined to R = 0.088 for 1979 photographic data. In both structures the nickel atoms are four coordinate with the ligands acting as N₄ tetradentates. For the first mentioned complex the structure consists of two independent molecules one of which is constrained, by space group requirements, to have C₂ (2) symmetry. These two molecules are closely similar and both exhibit nearly planar molecular arrangements with a small tetrahedral twist of up to 4^o at the nickel atoms. In the second complex the methyl substitution at the 6′-pyridyl positions causes severe steric strain in the molecule which gives rise to a 14.9^o tetrahedral twist at the nickel atom and approximately 25% pyramidal distortion at both amide nitrogen atoms. The resulting methyl-methyl separation of 3.26(1) Å is considerably less than the sum of the van der Waals radii for two such groups. This close separation leads to carbon-acid character for the methyl group protons which are shown to exchange for deuterons in NMR studies. A full analysis of the out-of-plane distortions and torsion angles of the two structures and a comparison with the previously reported analogous copper structures are made.