if a complex has either a mirror plane or centre of inversion, it cannot be chiral. if we look at the schematic complexes in 74, 75 and 76, we can see that neither 74 nor 75 has a mirror plane or a centre of inversion; so both are chiral (they also have no higher sn axis). Conversely, (75) has a plane of symmetry and hence is a chiral.(Although the CH2 groups in a dien ligand are not in the mirror plane, they oscillate rapidly above and below it)