Hapto number

Hapto number Formal charge Electrons donated... 

For the elimination to proceed, a vacant coordination site at the metal centre is required. In the example quoted the square planar platinum complex has only a 16-electron configuration, so it is coordinatively unsaturated. In 18-electron compounds this site has to be provided by dissociation or by change of hapto number of a ligand. 

Electrons from the ligands this depends, naturally on the ligands. For hydrocarbon ligands, the number is equal to the hapto number. Single-bonded ligands (hydride, halide etc) count as 1 (although a bridging halide counts as 2 - a lone-pair donor), while carbenes and carbynes count as 2 and 3, respectively. Lone-pair donors, such as phosphines and CO, count as 2. 

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