High-valent oxygenated transition metal complexes are implicated in a variety of synthetic and biological transformations, including O–O bond-formation and C–H functionalization. These intermediates span a range of transition metals, supporting ligands, and oxygen binding modes. As such, there has been a wide body of research dedicated to exploring the relationship between their structure and reactivity. Insights from isolated well-defined oxygenated transition metal complexes have allowed for the rational design of catalysts for the above-mentioned important transformations. However, the reactivity of these compounds can make their isolation and detailed study challenging. This thesis focuses on the synthesis and reactivity of compounds supported by strongly-donating tris(imidazol-2-ylidene)phenylborate ligands to stabilize unusual high-valent oxygenated transition metal species. Previous works have demonstrated that tris(imidazol-2-ylidene)phenylborate ligand scaffolds stabilize unusual high-valent Fe and Co complexes, such as transition metal-oxo and -imido species. However, analogous chemistry with Ni has yet to be explored, and the chemistry of Co systems is only nascent. Chapter 2 of this thesis focuses on the isolation and characterization of several Ni complexes that feature these tris(imidazol-2-ylidene)phenylborate ligands. These complexes serve as a platform to explore further oxidative reactivity. Additionally, the donor strength of this ligand scaffold is exemplified by the terminal Ni-methyl complexes PhB(RIm)NiMe, which are distorted from pseudo-tetrahedral to seesaw geometries. Chapter 3 of this thesis explores the oxidative reactivity of these precursor complexes, where dioxygen activation by a NiII-chloride precursor yields an unprecedented binuclear NiIII2-(μ-1,2-peroxo) complex. The isolation of this complex confirms the viability of such a species as an intermediate in water oxidation by Ni(oxy)hydroxide materials. While this complex is thermally unstable, we demonstrate its reactivity with both nucleophiles and electrophiles at low temperatures which suggest possible mechanistic paradigms in nickel-based water oxidation materials. Chapter 4 of this thesis explores the X–H bond activation reactivity of a terminal CoIII-oxo complex supported by tris(imidazol-2-ylidene)phenylborate ligands. Previous work has demonstrated that the rate of C–H bond activation by the terminal CoIII-oxo complex PhB(tBuIm)3CoO correlates with substrate acidity, rather than substrate bond dissociation energy. Mechanistic studies have demonstrated that this selectivity is imparted by the bacisity of the CoIIIO complex, giving rise to a basic asynchronous concerted proton electron transfer mechanism, wherein the transition state adopts more proton transfer character. In this chapter, the C–H bond activation reactivity of the adamantyl congener PhB(AdIm)3CoO is demonstrated to be even more sensitive to substrate acidity, ultimately resulting in mechanistic crossover from a concerted mechanism to a stepwise proton transfer-electron transfer mechanism in a series of substituted phenol substrates. This observation underscores that a multitude of factors, other than reactant-product bond dissociation energies, play a role in the selectivity, mechanism, and rate of X–H bond activation.