Compounds of general formula I wherein Fc and Fc′ may be the same or different and are substituted ferrocenyl moieties having at least one ring substituent selected from sulfur-containing groups, phosphorus-containing groups, iodo, chloro, silyl, fluoroalkyl groups containing two or more fluorine atoms, heteroaryl, substituted phenyl, and cyano, wherein if present as sole substituent the cyano group is located on the proximal cyclopentadienyl ring; X is a spacer, Y is a spacer, Z is a spacer; and R is a linker group. Compound I may be used to make labelled substrates, functionalised compounds for making labelled substrates and may be used as labels in an electrochemical assay.