The synthesis, magnetic, and spectroscopic characteristics of the synthetically useful dimeric cobalt(II) silylamide complex [Co{N(SiMe3)(2)}(2)](2) (1) and several of its Lewis base complexes have been investigated. Variable-temperature nuclear magnetic resonance (NMR) spectroscopy of 1 showed that it exists in a monomer-dimer equilibrium in benzene solution and has an association energy (Delta G(reacn)) of -0.30(20) kcal mol(-1) at 300 K. Magnetic data for the polycrystalline, red-brown [Co{N(SiMe3)(2)}(2)](2) (1) showed that it displays strong antiferromagnetic exchange coupling, expressed as -2J(ex)S(1)S(2), between the two S = 3/2 cobalt(II) centers with a J(ex) value of -215(5) cm(-1), which is consistent with its bridged dimeric structure in the solid state. The electronic spectrum of 1 in solution is reported for the first time, and it is shown that earlier reports of the melting point, synthesis, electronic spectrum, and magnetic studies of the monomer "Co{N(SiMe3)(2)}(2)" are consistent with those of the bright green-colored tetrahydrofuran (THF) complex [Co{N(SiMe3)(2)}(2)(THF)] (4). Treatment of 1 with various Lewis bases yielded monomeric three-coordinated species-[Co{N(SiMe3)(2)}(2)(PMe3)] (2), and [Co{N(SiMe3)(2)}(2)(THF)] (4), as well as the previously reported [Co{N(SiMe3)(2)}(2)(py)] (3)-and the four-coordinated species [Co{N(SiMe3)(2)}(2)(py)(2)] (5) in good yields. The paramagnetic complexes 2-4 were characterized by electronic and H-1 NMR spectroscopy, and by X-ray crystallography in the case of 2 and 4. Magnetic studies of 2-5 and of the known three-coordinated cobalt(II) species [Na(12-crown-4)(2)] [Co{N(SiMe3)(2)}(3)] (6) showed that they have considerably larger chi T-M products and, hence, magnetic moments, than the spin-only values of 1.875 emu K mol(-1) and 3.87 mu(B) which is indicative of a significant zero-field splitting and g-tensor anisotropy resulting from the pseudo-trigonal crystal field. A fit of chi T-M for 2-6 yields a large g-tensor anisotropy, large negative D-values (between -62 cm(-1) and -82 cm(-1)), and E-values between +/- 10 cm(-1) and +/- 21 cm(-1).