Here's what rhymes with phosphonium.

Consider these alternatives

diazonium / chromium macrocyclic / heterocyclic alkoxide / dioxide xanthate / state glycosyl / diglycosyl thiocyanate / late hydrazone / shown chelate / late chelated / created hydroxymethyl / level hydroxylamine / mean phosphonate / late carboxylate / late dihydrate / hydrate intermediacy / immediacy