This is an improved version of the first one, which used two relay contacts.
When the button is pressed the relay coil takes about 2,5 Volt (half voltage). By releasing button the coil takes full voltage and the Q2 transistor is supplied for a bit by the capacitor, so the relay turns on. By pressing button again the transistor Q1 is supplied and turns down the other one (through contact 11-14).
The relay coil is still energized (at 2,5 Volts) through the collector of Q1...
Momentary latching relay
Just for fun I designed and built this simple circuit which drives a monostable relay by a singol m