A theoretical study of the formation and reactivity of substituted cyclohexadienyliron complexes. The structures and reactivities of tricarbonyl(2-methoxycyclohexadienyl)iron cation and tricarbonyl(1-methyl-4-methoxycyclohexadienyl)iron cation and tricarbonyl(1-methyl-4-methoxycyclohexadienyl)iron c