Bistetrazolium mercury salts

Treatment of tetrazolium salts (154) with mercuric acetate gives the bistetrazolium mercury salts (155) which represent essentially a metal-carbene trapping of the tetrazolium ylide species. Replacement of the mercury atom of (155) is readily achieved with halogens to give the 5-halotetrazolium compounds (156) (93CB1149). These electrophilic reactions at the tetrazole C-5 arise from the lability of the 5-CH proton and they show the necessity of generating carbanionic character at C-5 before electrophilic attack can occur on this strongly ir-deficient azole. 

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