The former Cameroon international hasn't enjoyed the glittering career he had hoped for at Barca, winning a solitary Primera Division title in 2013, having joined the Spanish giants from Arsenal the year before.
Song played a bit-part role for the Catalans in Pep Guardiola's final season at the Camp Nou and failed to establish himself under new head coach Luis Enrique the following campaign.
The 28-year-old was subsequently loaned out to Premier League outfit West Ham, where he spent the next two seasons, although a loss of form and various injury problems led to the Hammers deciding not to sign him on a permanent deal.
Song now appears likely to quit the Blaugrana following confirmation from the Tatars that they had agreed a transfer with the champions of Spain.
"FC Rubin reports reaching a provisional agreement on the transition to the Kazan club of Alexandre Song," the Russian Premier League club said on Twitter.