Russia and Ukraine have powerful ties of geography, history and energy. And when it comes to the geopolitics of the current crisis energy is a key factor. Ukraine has long profited from being the middle-man for Russian gas exports into Europe. Moscow is in the business of changing that reality. Stephen Sackur speaks to Yuri Vitrenko, CEO of Naftogaz, Ukraine's biggest energy company. As Kiev and Moscow face off, where does the economic leverage lie?