What's the grammatical function of not and to in this sentence?
It is legitimate for Slovenia not to allow the merger.
How do I analyse the verb phrase? Allow is the headword, but what are not and to?
"to" is part of the "to allow" construction, the infinitive of the verb. The "not" simply negates (i.e. says "do the opposite") to this.
It would be equivalent to say this:
to is a preposition. However, in this case to simply acts as a marker of the infinitive verb to allow.
not is usually considered an adverb. In this case it negates the infinitive to allow.