Define : (i) Co-ordination numbers(ii) Ambidentate ligand(b) Write a note on Clemmensen reduction of propanone.

Answer:

(a) (i) Coordination number: the number of ligand donor atoms directly attached (by coordinate bonds) to the central metal atom or ion in a complex, e.g. 6 in [Co(NH₃)₆]³⁺, 4 in [Ni(CO)₄].

(ii) Ambidentate ligand: a monodentate ligand that has two different donor atoms and can bind to the metal through either of them (but only one at a time), e.g. NO₂^– (through N: nitro; through O: nitrito, –ONO), SCN^– (through S: thiocyanato; through N: isothiocyanato).

(b) Clemmensen reduction: the carbonyl group of aldehydes and ketones is reduced to a methylene (–CH₂–) group by heating with zinc amalgam and concentrated hydrochloric acid. Propanone gives propane:

CH₃–CO–CH₃ + 4[H] \(\xrightarrow{\text{Zn–Hg},\ \text{conc. HCl}}\) CH₃–CH₂–CH₃ + H₂O

The reaction is used for compounds stable to acid (for base-sensitive compounds, Wolff–Kishner reduction is used instead).