The Russians were the scariest enemies for the Turks they rarely won a war against them and in the 1800s if the ottoman didn't get French and British support they would have definitely taken ConstantinopleEnver pasha attacked them for their support of orthodox Russians. Even though Ottomans and Russia were technically on the same team the Ottomans viewed Russia as its greatest existential threat.
Also since like the 1700s the Russians were planning on taking back Constantinople from Islam and renaming it Tsargrad.