The bonds occur between carbonyl and amine groups in the amide repeat unit. These anions can be protonated to give the corresponding metal carbonyl hydrides. Esters react with nucleophiles at the carbonyl carbon. The Barbier reaction involves nucleophilic addition of a carbanion equivalent to a carbonyl. Carbonyls are coordination complexes between transition metals and carbonyl ligands. There are several nickel carbonyl cluster anions formed by reduction from nickel carbonyl. The acetyl group contains a methyl group single-bonded to a carbonyl. The molecule is tetrahedral, with four carbonyl (carbon monoxide) ligands. Nickel carbonyl (Ni(CO)) and other metal carbonyls present an interesting case. Infrared spectroscopy is a sensitive probe for the presence of bridging carbonyl ligands. Some of the carbonyl ligands are highly labile. Sweetness may be connected to aldehydes and ketones, which contain a carbonyl group. Cobalt carbonate is a precursor to cobalt carbonyl and various cobalt salts. It is often connected to aldehydes and ketones, which contain a carbonyl group. It is important to distinguish the aldol condensation from other addition reactions of carbonyl compounds.