CONFORMATIONAL FLEXIBILITY OF THE SQUARE-PYRAMIDAL COORDINATION CAP IN A SERIES OF OCTAHEDRAL NICKEL(II) PENTAAMINE COMPLEXES - MAGNETOCHEMICAL CHARACTERIZATION OF THE SINGLY MU-CL-BRIDGED NICKEL(II) DIMER [(PYN(4))NI-CL-NI(PYN(4))](PF6)(3) [PYN(4) = ,6-BIS(1',3'-DIAMINO-2'-METHYLPROP-2'-YL)PYRIDINE]

The architecture of the tetrapodal pentaamine ligand 2,6-bis(1',3 '-di amino-2'-methylprop-2'-yl)pyridine (pyN(4), 1) allows it to coordinate to nickel(II) as a square pyramidal coordination cap. The pyridine ni trogen atom occupies an apical position of the coordination octahedron , while four equivalent pendent primary amino groups occupy the equato rial positions, with a sixth coordination site remaining for a monoden tate Ligand. Exchange of this Ligand is facile, and a series of comple xes [(1)NiX](n+) (X = OH2, OClO3, NCS, N-3, {Cl-Ni(pyN(4))}) has been prepared and characterised by elemental analysis, IR and UV/Vis spectr oscopies (as applicable), and X-ray structure determination. While the solid state structures show varying degrees of distortion of the Liga nd cap 1 from C-2v symmetry, a polynucleating coordination mode has no t been observed. The ligand enables the synthesis of dinuclear nickel( II) complexes containing a single bridging ligand, as exemplified by t he singly mu-chloro bridged complex [(1)Ni-Cl-Ni(1)] (PF6)(3). This co mplex has an antiferromagnetically coupled ground state of total spin S-T = 0, as determined from variable-temperature magnetic susceptibili ty measurements.