Can the onyomi be longer than 2 morae?


I started learning japanese recently, and I looked back at the kanji I've
learned, and I simply can't think of any kanji with an onyomi longer than 2
morae.

Is there really no kanji with an onyomi longer than 2 morae, and if so, why?




Recall that on-yomi are derived from the Chinese pronunciation of characters,
which are (with [few
exceptions](http://languagelog.ldc.upenn.edu/nll/?p=3330)) monosyllabic. The
basic structure of a syllable in [Middle
Chinese](http://en.wikipedia.org/wiki/Middle_Chinese#Phonology) is as follows:

  * An initial consonant
  * A glide
  * A vowel
  * A final – either another glide or a consonant, but not both

For instance, 關 (resp. 怪, 脚) is _kwæn_ (resp. _kwai_ , _kjak_ ) in Middle
Chinese and has the kan-on クヮン (resp. クヮイ, キャク), but there are no syllables
like _kwain_. Thus there is no way a trimoraic on-yomi would arise.




Recall that on-yomi are derived from the Chinese pronunciation of characters,
which are (with [few
exceptions](http://languagelog.ldc.upenn.edu/nll/?p=3330)) monosyllabic. The
basic structure of a syllable in [Middle
Chinese](http://en.wikipedia.org/wiki/Middle_Chinese#Phonology) is as follows:

  * An initial consonant
  * A glide
  * A vowel
  * A final – either another glide or a consonant, but not both

For instance, 關 (resp. 怪, 脚) is _kwæn_ (resp. _kwai_ , _kjak_ ) in Middle
Chinese and has the kan-on クヮン (resp. クヮイ, キャク), but there are no syllables
like _kwain_. Thus there is no way a trimoraic on-yomi would arise.

