Neutral labeling reactants and conjugates derived thereof

A chelate or chelating agent of formula (I)

• wherein, —A— is a linker;
• R is —CONH2, —CONHR1 or —CONR1R2 where R1 and R2, same or different are formed from one to ten moieties, each moiety being selected from the group consisting of phenylene, alkyl containing 1-12 carbon atoms, ethynediyl (—C≡C—), ethylenediyl (—C═C—);
• ether (—O—), thioether (—S—), amide (—CO—NH— and —NH—CO— and —CO—NR′ and —NR′—CO—), carbonyl (—CO—), ester (—COO— and —OOC—), disulfide (—SS—), diaza (—N═N—) or a tertiary amine (—NR′—), where R′ represents an alkyl containing less than 5 carbon atoms;
• X is a reactive group for conjugation of the chelate to a biospecific binding reactant, wherein said reactive group X is selected from amino, aminooxy, haloacetamido, isothiocyanato, 3,5-dichloro-2,4,6-triazinylamino, maleimido, a thioester or an active ester of a carboxylic acid,
• and M is a metal or M is not present.