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Experimental spectroscopic and magnetic data for Co 2+ (3d 7) ions in various systems are reviewed and critically examined. The focus is on Co 2+ ions with the electronic spin S = 3/2, properties of which may be interpreted using the spin Hamiltonian with the effective S̃ = 3/2 or the fictitious 'spin' S (S′) = ½. Possible distinct ground states of Co 2+ (3d 7) ions arising from crystal field energy levels are discussed. Distinctions between the concepts of the effective spin S̃ and the fictitious 'spin' S′ are outlined to clarify the terminological confusion encountered in literature. Sample cases of the ground state assignments and options for the 'spin' S′ = ½ origin are considered for better understanding of the Co 2+ ions local environment in various systems, including low symmetry cases. Present study is motivated by potential applications of Co 2+ (S̃ = 3/2) complexes exhibiting very large or moderate zero-field splitting as molecular nanomagnets.

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