  It is now well known that a primordial chemistry, involving light elements
produced during the nucleosynthesis period, might develop during the hydrogen
post-recombination era. In particular, molecular ions and primordial molecules
such as H2, HD and LiH will be produced. We summarize this primordial chemistry
after the recombination epoch, and then present a simple gravitational collapse
model of a cloud. The potentiality of fragmentation of this collapsing
protoclouds through the thermal instability is also discussed. We suggest that
this study could also be extended to the CO molecule, because the carbon
reservoir molecule CO has already been observed in high redshifts objects.
