Synthesis, Spectroscopic Characterization, and Determination of the Solution Association Energy of the Dimer [Co{N(SiMe 3 ) 2 } 2 ] 2 : Magnetic Studies of Low-Coordinate Co(II) Silylamides [Co{N(SiMe 3 ) 2 } 2 L] (L = PMe 3 , Pyridine, and THF) and Related Species That Reveal Evidence of Very Large Zero-Field Splittings