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CN- is a strong field ligand. Strong-field ligands are those that cause a large splitting of the d-orbitals in a metal complex. This splitting is due to the interaction of the ligand's electrons with the metal's d-electrons. CN- is a strong field ligand because it has a high electronegativity, which means that it can attract electrons away from the metal. This causes the d-electrons to be more strongly repelled by the ligand electrons, which in turn leads to a larger splitting of the d-orbitals.