The energy transfer between single Cr³⁺ ions and N₂ pairs in ruby is interpreted by energy diffusion on single ions. It is shown that the superexchange is responsible for this diffusion. The probability of transfer between single ions and the diffusion coefficient are evaluated. Experimental results on the transfers from fourth next-neighbour pairs to single ions and to third next-neighbour pairs are given. It appears that the diffusion is weak on the fourth NN pairs. The temperature dependence of the transfer probabilities are determined and interpreted.