The reaction of [CoH{P(OEt), Ph},] with RN,+ (R = 4-CH,C,H4) affords the paramagnetic cationic hydride [CoH{P(OEt),Ph},] +, with NO+ the dinitrosyl cation [Co( NO),{P(OEt),Ph},] +, and with HBF,-Et,O the cationic dihydride [CoH,{P(OEt),Ph},] +. Characterization of the complexes b y Lr., e.s.r., and variable-temperature H and 31P-{1H} n.m.r. spectra is described. Variable-temperature .I H n.m.r. T, measurements at 80,200, and 400 MHz for both [CoH{P(OEt),Ph},] and [CoH,{P(OEt),Ph},] BPh, are also reported. The reactivity of the cationic hydrides with CO, RNC, NO, I , , and RN,+ was investigated and the new cations [Co(CNR),{P(OEt),Ph},1+ (R = 4-CH,C,H4, 4-CH,0C,H4, or 4-CIC,H4) and [CoH (I){P(OEt),Ph},] + synthesized. The unstable intermediate [CoH(NO){P(OEt),Ph},] + [v(NO) at 1 680 cm-'1 was also detected. Arenediazonium cations are reported to react with transitionmetal hydrides to give aryldiazene or arylhydrazido complexes ' by insertion of RN,' into the M-H bond. Aryldiazenido derivatives have also been obtained by this reaction, generally by deprotonation of the initial diazene intermediate.'e*f~2 However, no data are yet available on the reactivity of cobalt hydrides towards arenediazonium cations.Previous reports from our laboratory have dealt with studies on reactions of iron(I1) and di-hydrides, with RN,', and on the synthesis of the first aryldiazenido complexes of cobalt(~).~ We have now extended these studies to include the reactivity of the cobalt@ hydride [CoH(P(OEt),Ph},] towards arenediazonium cations: instead of insertion or substitution reactions, we found oxidation of the complex to cobalt(r1). The results of these studies are reported here, including those related to the properties of the cobalt(I1) hydride obtained. Furthermore, the use of other electrophilic reagents, such as NO' and H', was tested on [CoH(P(OEt),Ph),], in order to compare the results with those obtained with RN2+ and, in the case of H', to verify whether protonation leads to the formation of q2-H2 complexes or to oxidative addition of H + to give a new cobalt(m) hydride species.