Electronic structure, spin-state, and geometrical relationships for a series of pseudotetrahedral Co(II) aryloxide, siloxide, arylthiolate, and silylthiolate complexes supported by the tris(phosphino)borate (BP3] ligands [PhBP3] and [PhBP3IPR] ([PhB(CH2PPh2)(3)](-) and [PhB((CH2PPr2)-Pr-i)(3)](-), respectively) are described. Standard H-1 NMR, optical, electrochemical, and solution magnetic data, in addition to low-temperature EPR and variable temperature SQUID magnetization data, are presented for the new cobalt(II) complexes [PhBP3]CoOSiPh3 (2), [PhBP3]CoO(4-'Bu-Ph) (3), [PhBP3]CoO(C6F5) (4), [PhBP3]COSPh (5), (PhBP3]COS(2,6-Me-2-Ph) (6), [PhBP3]CoS(2,4,6-1-Pr-i(3)-Ph) (7), [PhBP3]CoS(2,4,6-Bu-t(3)-Ph) (8), [PhBP3]CoSSiPh3 (9), [PhBP3]CoOSi(4-NMe2-Ph)(3) (10), (PhBP3]CoOSi(4-CF3-Ph)(3) (11), [PhBP3]CoOCPh3 (12), (PhBP3/Pt]-CoOSiPh3 (14), and [PhBP3/Pr]COSSiPh3 (15). The low-temperature solid-state crystal structures of 2, 3, 5-10, 12, and 15 are also described. These pseudotetrahedral cobalt(II) complexes are classified as featuring one of two limiting distortions, either umbrella or off-axis. Magnetic and spectroscopic data demonstrate that both S = 1/2 and S = 3/2 ground-state electronic configurations are accessible for the umbrella distorted structure type, depending on the nature of the X-type ligand, its denticity (n(1) versus n(3)), and the tripodal phosphine ligand employed. Off-axis distorted complexes populate an S = 1/2 groundstate exclusively. For those four-coordinate complexes that populate S = 112 ground states, X-ray data show two Co-P bond distances that are invariably shorter than a third Co-P bond. The pseudoletrahedral siloxides 2, 10, and 11 are exceptional in that they display gradual spin crossover in the solid state. The diamagnetic cobalt(III) complex [[PhBP3]CoOSiPh3}{BAr4} ({16}{BAr4 }) (Ar = Ph or 3,5-(CF3)(2)-C6H3) has also been prepared and structurally characterized. Accompanying electronic structure calculations (DFT) for complexes 2, 6, and {16}(+) support the notion of a close electronic structure relationship between these four-coordinate systems and octahedral, sandwich, and half-sandwich coordination complexes.