Two organometallic complexes having cyclopentadienyldiazonium ligands have, been isolated and characterized by spectroscopy, X-ray crystallography, and electrochemistry Roth CoCp(eta(5)-C5H4N2)(2+) (2(2+)) and Mn(CO)(3)(eta(5)-C5H4N2)(+) (3(+)) undergo facile cyclopentadienyldiazonium ligand-based one-electron reductions which liberate dinitrogen and result in strong binding of the cyclopentachenyl ligand to a glassy carbon surface, similar to the processes well established for organic aryldiazonium salts The organometallic-modified electrodes are robust and have a thickness of approximately one monolayer (Gamma = (2-4) x 10(-10) mol cm(-2)). Their voltammetric responses are as expected for a cobaltocenium-modified electrode, [CoCp(eta(5)-C5H4-E)](+). where Cp = cyclopentachenyl and E = electrode, and a "cymantrene"-modified electrode Mn(CO)(3)(eta(5)-C5H4-E) The cobaltocenium electrode has two cathodic sui face waves The first (E-1/2 = -1 34 V vs ferrocene) is highly reversible, whereas the second (E-pc = -24 V) is not, consistent with the known behavior of cobaltocenium The cymantrene-substituted electrode has a partially chemically reversible anodic wave at E-1/2 = 0 96 V, also consistent with the behavior of its Mn(CO)(3)Cp parent Many of the properties of aryl-modified electrodes, such as "blockage" of the voltammetric responses of test analytes. are also seen for the organometallic-modified electrodes Surface-based substitution of a carbonyl group by a phosphite ligand. P(OR)(3), R = Ph or Me, was observed when the cymantrene-modified electrode was anodically oxidized in the presence of a phosphite Inland The successful grafting of organometallic moieties by direct bonding of a cyclopentadienyl ligand to electrode surfaces expands the chemical and electrochemical dimensions of diazonium-based modified electrodes