Synthesis, Crystal Structures, and Magnetic Properties of a Series of Linear Pentanickel(II) Complexes: [Ni(5)(&mgr;(5)-tpda)(4)X(2)] (X = Cl(-), CN(-), N(3)(-), NCS(-)) and [Ni(5)(&mgr;(5)-tpda)(4)(CH(3)CN)(2)]- (PF(6))(2) (tpda(2-) = the Tripyridyldiamido Dianion).