Compositions and methods for electrochemical detection of an analyte comprising a transition metal compound ##STR00001## wherein M is a metallic element that can form a coordinate bond to nitrogen; R and R' are coordinated to M at their nitrogen atoms; L is a linking ligand; Z is chlorine or bromine; m can be from 1 to 6 and X is an anion, or combination of anions, that balances the charge m. Also provided are electrochemical tags and methods of detection.