nouns (or adjectives) ending with -eën or -ën in plural form

[logicgate]

Is there a rule for the formation of the plural leading to something like en ending in -eën or -ën ?

E.g., bacterie -> bacteriën (and not bacterien or bacterieën)
idee -> ideeën (and not ideën or ideen)
etc.

Similarly for adjectives financieel -> financiële etc.

[LeSNT2]

There is a rule for the plural, it's about the emphasis.

Source: https://onzetaal.nl/taalloket/kolonien-epidemieen

English summary:

When the main stress of a word falls on the last syllable - as in 'epidemie' - the plural ending -en is added: 'epidemieën'. The diaeresis makes it clear that a new syllable starts at the last e. If the stress is not on the last syllable - as in 'kolonie' - only a plural n is added: 'koloniën'. Again, the diaeresis is needed to indicate that a new syllable starts at the e.

Most words that end in -ie have an s in the plural. So this problem does not arise there. There are far fewer plurals ending in -iën than plurals ending in -ieën (if you need to guess and don't have any possibilities to look on the internet or a dictionary).

The same rule applies for adjectives like 'financiële' or verbs like 'kopiëren'. Source: https://onzetaal.nl/taalloket/financieren-gefinancierd