A nonanuclear mixed-valent coordination cluster, isolated as [Co^III₃Co^II₆(µ₆-O)(piv)₇(thmp)₄(MeCN)₂(H₂O)]·4(MeCN) (1), forms in a one-pot reaction of cobalt(II) pivalate and 1,1,1-tris(hydroxymethyl)propane (H₃thmp) in acetonitrile solution. Single-crystal X-ray diffraction analysis reveals an unusual {Co₆O₁₉} Lindqvist-type core capped by three Co sites. High-resolution ESI-mass spectrometry studies confirm the integrity of this core structure in solution, exhibiting a series of peaks corresponding to the intact cluster species. Magnetic studies indicate that the magnetic behavior in 1 is dominated by six high-spin Co(II) ions in octahedral ligand fields exhibiting weak intra-cluster antiferromagnetic exchange coupling.