MOLECULAR-STRUCTURES OF A MONOVALENT AND A DIVALENT NICKEL CATENATE - COMPETITION BETWEEN METAL ORBITAL REQUIREMENTS AND GEOMETRICAL CONSTRAINTS IMPOSED BY THE LIGAND

Nickel(I) and nickel(II) catenates were crystallized, and their X-ray structures were solved. Ni(I) complex: [C68H68N4O12Ni+][ClO4-].C6H6.CH2Cl2, a = 17.328(7) angstrom, b = 14.965(6) angstrom, c = 14.675(6) angstrom, alpha = 97.78(6)-degrees, beta = 111.46(8)-degrees, gamma = 97.84(5)-degrees, V = 3481 angstrom3, triclinic P1BAR, Z = 2. Ni(II) complex: [C68H68N4O12Ni2+][BF4-]2. 1/2C6H6.1/2H2O, a = 20.472(10) angstrom, b = 24.255(8) angstrom, c = 15.405(8) angstrom, beta = 113.80(2)-degrees, V = 6999 angstrom3, monoclinic C2/c, Z=4. The complexes consist of two interlocking 30-membered rings with two 2,9-diphenyl-1,10-phenanthroline fragments as coordinating units, complexed to mono- or divalent nickel. The extremely strong stabilization of Ni(I) as determined by electrochemistry (Ni(II)/Ni(I): E-degrees = -0.18 V vs SCE in CH3CN) is reflected by both structures. The ligand system is perfectly well adapted to a tetrahedral geometry as preferred by monovalent nickel and in agreement with the molecular structure found. On the other hand, the divalent state leads to a strongly distorted structure, in accordance with a d8 configuration which is not likely to easily accommodate a tetrahedral environment. Rather, the geometry of the Ni(II) catenate is close to that of a square bipyramid lacking an axial position. Accordingly, the two chelate planes of the Ni(I) catenate being perpendicular to one another, no intramolecular stacking interactions between aromatic groups are observed, whereas in the Ni(II) case strong PI-PI interactions between phenyl rings and aromatic nuclei are present.