The ligand potassium hydrotris(3,4-diphenyl-5-methylpyrazolyl)borate (Tp) reacts with cobalt iodide to form a pseudotetrahedral (half-sandwich) complex TpCoI. The steric influence of the 3-phenyl group in Tp has been altered by the proximity of a bulky substituent at the 4-position, thereby preventing the formation of a bis-Tp complex. This is the first application of secondary steric effects on the coordination characteristics of the tris(pyrazolyl)borates.