Heterocyclic chelating agents

A compound of formula I ##STR21## wherein X represents a bond, an oxygen or sulphur atom or a group of formula CHR.sup.1 or NR.sup.3 ;

• each R.sup.1 which may be same or different represents a hydrogen atom, a group of formula OR.sup.3 or NR.sup.3 R.sup.3 or a C.sub.1-8 alkyl or (C.sub.1-8 alkoxy)-C.sub.1-8 alkyl group optionally substituted by a hydroxyl group or by a group of formula NR.sup.3 R.sup.3 or CONR.sup.3 R.sup.3 ;
• each R.sup.2 which may be the same or different represents a hydrogen atom or a C.sub.1-8 alkyl or C.sub.1-8 alkoxy group optionally mono- or poly-substituted by hydroxyl or C.sub.1-8 alkoxy groups;
• each R.sup.3 which may be the same or different represents a hydrogen atom, an optionally mono-or poly-hydroxylated C.sub.1-8 alkyl group or a group of formula CH.sub.2 Y;
• Y represents a group of formula COZ, CON(OH)R.sup.4 POZ.sub.2 or SO.sub.2 Z;
• Z represents a group of formula OR.sup.4, NR.sup.4 R.sup.4, or ##STR22## where each R.sup.11 which may the same or different is a hydrogen atom, a hydroxyl group or an optionally hydroxylated C.sub.1-8 alkyl group,
• s is 0, 1 or 2, and
• W is a group CHR.sup.11, NR.sup.11 or an oxygen atom;
• and
• each R.sup.4 which may bet he same or different represents a hydrogen atom or an optionally mono- or poly-hydroxylated C.sub.1-8 alkyl, (C.sub.1-8)alkoxy)-C.sub.1-8 alkyl or poly-(C.sub.1-8 alkoxy)-C.sub.1-8 alkyl group;
• with the provisos that where is O then in the resultant 5 membered heterocyclic ring W is a CHR.sup.11 group, and that where X represents a bond or a group CHR.sup.1 at least one group R.sup.1 or R.sup.2 represents other than a hydrogen atom or an unsubstituted C.sub.1-8 -alkyl group, or a salt thereof.