Direct measurement of 1J(13C13C) in coordinated double bonds

¹J(¹³C¹³C) has been determined in naturally abundant samples of η¹,η² -alkenyl-η⁵ -cyclopentadienylnickel complexes; for the coordinated double bonds ¹J(CC) lies between the value for free ethylene and cyclopropane, and therefore gives direct insight into the relevant bonding situation.