Cloud Expansion Coefficient is the Racah interelectronic repulsion parameter, given the symbol B, that occurs when a transition-metal free ion forms a complex with ligands. And is denoted by B. Cloud Expansion Coefficient is usually measured using the Diopter for Wave Number. Note that the value of Cloud Expansion Coefficient is always negative. Typically, the value of Cloud Expansion Coefficient lies in the range from 0 to 100000.