The reactivity of (C6Me5)2Sm(THF)2 with bis(2-pyridyl)ethene including the synthesis of [(C5Me5)2Sm]2(.mu.-.eta.2:.eta.2-pyCHCHpy) from [(C5Me5)2Sm]2[.mu.-.eta.3:.eta.3-1,2,3,4-(py)4C4H4] by reductive C-C bond cleavage