In the case of the silver complex, the bond is not from one atom of the CC
unit to the silver ion, but from the p center such that two electrons are
transferred from the alkene to the metal ion.89 Ethene has two p-electrons
and is a dihapto or Z2 ligand, as are other simple alkenes. Similarly, benzene
has six p-electrons and is a hexahapto or Z6 ligand. Ferrocene (10) has
two cyclopentadienyl ligands (each is a five-electron donor or an Z5 ligand),
and ferrocene is properly called bis(Z5-cyclopentadienyl)iron(II). This
system can be extended to compounds in which only a single s bond
connects the organic group to the metal, for example, C6H5–Li (a monohapto
or Z1 ligand), and to complexes in which the organic group is an ion,
for example, p-allyl complexes, such as 11, in which the allyl ligand is
trihapto or Z3. Note that in a compound such as allyllithium, where a s
bond connects the carbon to the metal, the allyl group is referred to as
monohapto or Z1.