If you are in need of all syllables of Standard Mandarin (Putonghua) written in Pinyin you might come up with some seldom ones. The Xiàndài Hànyǔ Cídiǎn (现代汉语词典（第5版）商务印书馆, 北京 2005, ISBN 7-100-04385-9) for example lists syllables n and ng. The ISO 7098 norm for Pinyin then has syllables kei and rua.