Reaction of FeBr2 with Li(N=CtBu2) (0.5 equiv) and Zn0 (2 equiv) results in the formation of the formally mixed-valent cluster [Fe4Br2(N=CtBu2)4] (2.1) in moderate yield. Subsequent reaction of 2.1 with Na(N=CtBu2) results in formation of [Fe4Br(N=CtBu2)5] (2.2), also in moderate yield. Both 2.1 and 2.2 were characterized by zero-field 57Fe Mössbauer spectroscopy, X-ray crystallography, and SQUID magnetometry. Their tetrahedral [Fe4]6+ cores feature short Fe-Fe interactions (ca. 2.50 Å). Additionally, both 2.1 and 2.2 display S = 7 ground states at room temperature and slow magnetic relaxation with zero-field relaxation barriers of Ueff = 14.7(4) and 15.6(7) cm-1, respectively. Moreover, AC magnetic susceptibility measurements were well modelled by assuming an Orbach relaxation process. In order to study the effect of ketimide ligand substituents on magnetic coupling strength, a series of ketimide-bridged Fe(III) dimers were studied next. Reaction of Fe(acac)3 with 3 equiv of Li(N=C(R)Ph) (R = Ph, tBu) results in formation of the [Fe2]6+ complexes, [Fe2(μ-N=C(R)Ph)2(N=C(R)Ph)4] (R = Ph, 3.1; tBu, 3.2), in low to moderate yields. Reaction of FeCl2 with 6 equiv of Li(N=C13H8) (HN=C13H8 = 9-fluorenone imine) results in formation of [Li(THF)2]2[Fe(N=C13H8)4] (3.3), in good yield. Subsequent oxidation of 3.3 with ca. 0.8 equiv of I2 generates the [Fe2]6+ complex, [Fe2(μ-N=C13H8)2(N=C13H8)4] (3.4), along with free fluorenyl ketazine. Complexes 3.1, 3.2, and 3.4 were characterized by 1H NMR spectroscopy, X-ray crystallography, 57Fe Mössbauer spectroscopy, and SQUID magnetometry. The Fe–Fe distances in 3.1, 3.2, and 3.4 range from 2.803(7) to 2.925(1) Å, indicating that no direct Fe–Fe interaction is present in these complexes. The 57Fe Mössbauer spectra for complexes 3.1, 3.2, and 3.4 are all consistent with the presence of symmetry-equivalent, high-spin Fe3+ centers. Finally, all three complexes exhibit a similar degree of antiferromagnetic coupling between the metal centers (J = -23 to -29 cm-1), as ascertained by SQUID magnetometry. Next, there was an effort to expand the small list of known homoleptic Fe(IV) complexes by utilizing the phenyl-tert-butylketimide ligand. Reaction of 4 equiv of Li(N=C(tBu)Ph) with FeIICl2 results in isolation of [Li(Et2O)]2[FeII(N=C(tBu)Ph)4] (4.1), in good yields. Reaction of 4.1 with 1 equiv of I2 leads to formation of [FeIV(N=C(tBu)Ph)4] (4.2), in moderate yields. 57Fe Mössbauer spectroscopy confirms the Fe(IV) oxidation state of 4.2, and X-ray crystallography reveals that 4.2 has a square planar coordination geometry along with several intramolecular H···C interactions. Furthermore, SQUID magnetometry indicates a small magnetic moment at room temperature, suggestive of accessible S = 1 state. Both density functional theory and multiconfigurational calculations were done to elucidate the nature of the ground state. Consistent with the experimental results, the ground state was found to be a closed-shell S = 0 state with an S = 1 excited state close in energy. Reaction of [CuH(PPh3)]6 with 1 equiv of Tl(OTf) results in formation of [Cu6TlH6(PPh3)6][OTf] ([5.1]OTf]), which can be isolated in good yields. Variable-temperature 1H NMR spectroscopy, in combination with density functional theory (DFT) calculations, confirms the presence of a rare Tl–H orbital interaction. According to DFT, the 1H chemical shift of the Tl-adjacent hydride ligands of [5.1]+ includes 7.7 ppm of deshielding due to spin-orbit effects from the heavy Tl atom. This study provides valuable new insights into a rare class of metal hydrides, given that [5.1][OTf] is only the third isolable species reported to contain a Tl–H interaction. Lastly, new ways to synthesize Ni and Co nanoclusters were developed in an effort to expand an extremely small catalog of first-row transition metal nanoclusters. Treatment of [Ni23Se12Cl3(PEt3)10] (6.1) with excess Me3SiX (X = Br, Cl) results in formation of [Ni23Se12X3(PEt3)10] (X = Br, 6.2; X = I, 6.3) in good yields. 6.2 and 6.3 are exceptionally rare examples of atomically-precise nickel nanoclusters (APNCs). Their syntheses represent rare examples of post-synthetic modification of a 3d metal APNC. Both 6.2 and 6.3 were characterized by X-ray crystallography, NMR spectroscopy, and ESI-mass spectrometry. Clusters 6.2 and 6.3 are isostructural: both feature a compact [Ni13]7+ kernel capped by a [Ni10(μ-Se)9X3] shell. Additionally, SQUID magnetometry suggests that 6.2 exhibits an S = ½ ground state at low temperatures. Cluster 6.2 could also be isolated cleanly via a bottom-up synthetic approach, via reaction of [Ni(1,5-cod)2] and PEt3 with SePEt3 and small amounts of [NiBr2(PEt3)2]. Under these conditions, it could be isolated in 30% yield. In contrast, reaction of [Ni(1,5-cod)2] and PEt3 with SePEt3 and [NiI2(PEt3)2] results in formation of [Ni3(μ3-Se)2I2(PEt3)4] (6.6) as the only isolable product. Cluster 6.3 is only formed in trace amounts under these conditions. These results highlight the challenges inherent in the bottom-up synthesis of Ni APNCs, and demonstrate the value of post-synthetic modification in the synthesis of 3d metal APNCs.