A thiophene, furan or tetrahydrofuran triazolyl derivative having the general formula (I) or (II) :
and stereoisomers thereof, wherein R¹ is tertiary butyl, which may be optionally substituted with one or more halogen atoms; R² is the group :
-(CH₂)n-Z
or -CH=CH-X
or -C≡C-Z
where n is an integer from 0 to 3 and Z is an optionally substituted thiophene, furan, or tetrahydrofuran ring; and X is an optionally substituted thiophene or tetrahydrofuran ring; and R³ is hydrogen or an alkyl group containing from 1 to 4 carbon atoms; and salts, esters and metal complexes of the compound of formula (I) and (II) wherein R³ is hydrogen.