Allylation of cobalt-co-ordinated cyclo-octatetraene by the [fe{P(OMe)3}(NO)2(η3-allyl)] cation: Ring closure to 1,2,3,3a-tetrahydropentalenyl derivatives and the X-ray structure of [co(η5-C8H8R)(η-C5Me 5)][PF6] [r = CH2C(Me)=CH2]