Hapticity of a ligand

The hapticity of a ligand is the number of atoms that are directly bonded to the metal centre. Structures 1.1a and 1.1b show two representations of an [η⁵-C₅H₅]^- (cyclopentadienyl, Cp^-) ligand. For clarity in the diagrams in this chapter, we adopt 1.1b and similar representations for π-ligands such as η³-C₃H₅ and η⁶-C₆H₆.

(1.1a)          (1.1b)

Bridge ligand

Counting electrons provided by bridging ligands, metal–metal bonds and net charges requires care. When bridging between two metal centres, an X˙ (X = Cl, Br, I) or R₂P˙ ligand uses the unpaired electron and one lone pair to give an interaction formally represented by structure 1.26, i.e. one electron is donated to M, and two to M’. In a doubly bridged species such as(CO)₂Rh(μ-Cl)₂Rh(CO)₂, the μ-Cl atoms are equivalent as are the Rh atoms, and the two Cl bridges together contribute three electrons to each Rh. A bridging H˙ provides only one electron in total, shared between the metal atoms it bridges, e.g. in [HFe₂(CO(₈]^-, (1.2). Example 1.2 also illustrates that the formation of anM^_Msingle bond provides each M atom with one extra electron; an M¼M double bond contributes two electrons to each metal.

(1.2)