\ [:\ ddot {\ text {o}}-\ text {s} (=\ text {f}) _ {2} (\ text {=o}) _ {2} \ rightleftharpoons :\ ddot {\ text {o}} =\ text {s} (=\ text {f}) _ {2} (\ text {=o}) \ text {o}^-\]
\ [:\ ddot {\ text {o}}-\ text {s} (=\ text {f}) _ {2} (\ text {=o}) _ {2} \ rightleftharpoons :\ ddot {\ text {o}} =\ text {s} (=\ text {f}) _ {2} (\ text {=o}) \ text {o}^-\]