The design of organometallic complexes is increasingly trending towards multimetallic species, especially those in which the metals are in close contact. These systems gain unique chemical and electronic properties through heterometallic interactions, mixed valency, and metalmetal bonding, which are not available in monometallic systems. As a result, multimetallic systems have potential in various processes, including molecular catalysis, separations, small molecule activation, and magnetic applications. Previous studies detail how charge transfer between metals influences the electronic structure of these sites, giving rise to unique chemical properties and reaction pathways. Multimetallic systems can also exhibit unique properties via the cooperative binding of different metal sites to a substrate. In many cases, transition in the UV-vis region has been widely characterized for homo-and heteronuclear bimetallic complexes with carbonyl ligands. Square planar d 8 -d complexes, which lack a formal metal-metal bond due to full occupancy of both the σ and σ* orbitals, nevertheless exhibit a characteristic dσ* → pσ transition in the UV-vis region. In many cases, however, the σ → σ* transition is challenging to characterize experimentally and thus characteristics are instead inferred indirectly from changes in metal-ligand or ligand-metal charge transfer bands. 36 In this work, we describe the electronic structure and bonding in four dicobalt complexes supported by a PNNP expanded pincer ligand (Figure ). Organometallic Co species have applications in hydrosilylation chemistry for organic and polymer syntheses, and thus the nature of Co-Si and Co-H bonding is of interest in these areas. Mono-and dinuclear Co complexes have been studied in this context, with some proposed reaction mechanisms involving cooperative binding of multiple metal sites to reaction intermediates. The PNNP ligand platform enables tight binding of two metals in close proximity with tunable coordination geometry and metal-metal distance, thus making it well-suited to study the effect of Co-Co and Co-ligand interactions on the electronic properties of silane-bound dicobalt species. In the compounds depicted in Figure , Co is in the nominal 2+ oxidation state. However, in 3 and 4, there is uncertainty regarding the oxidation states of Co, whose electronic structures may be significantly perturbed by charge transfer between the metals or with the ligands. The PNNP ligand is neutral in 1 and 3, and mono-anionic in 2 and 4. Compound 4 is derived from 3 by a single deprotonation of the PNNP ligand, but it is unclear whether deprotonation results in an internal redox process between Co and the PNNP ligand. Compounds 3 and 4 have 17-electron Co centers, suggesting a single bond between the metals to satisfy the 18-electron rule. However, the direct Co-Co or multicenter nature of such a bond is unknown. In addition to understanding the electronic properties specific to these species, this set of complexes is well-suited to a broader investigation of multiple factors that affect Co-Co interactions, including coordination geometry, ligand-field strength, and Co-Co distance. Recently, two structures similar to 3 and 4 have been reported in the literature by the groups of Deng and Nakajima. In both cases, the complexes are supported by mononucleating N-heterocyclic carbene (NHC) ligands and feature multiple bridging silane moieties. Based on the Co-Co distances and diamagnetic nature of these molecules, the authors propose that a Co-Co bond is present (Nakajima et al.) or absent (Deng et al.). However, the nature of the Co-Co interaction and the role of the coordinating ligands in mediating this interaction are not described.