A sample of CuSO4 is stirred in excess of water a pale-blue
solution is formed the specie responsible is [Cu(H2O)6]2+ ion. A
sample of NiSO4 gave green colour complex having formula
[Ni(H2O)6]2+. Why 6 water always shown?? can't they be 7 or 5?
The topic of complex ions is more complex than can be properly
understood at A levels. The simple A level version is : water
ligands are small, and hence 6 water ligands are able to surround
any hydrated metal cation. 6 is the ideal number of ligands,
because the more bonds formed, the more exothermic and
thermodynamically favourable, but too many ligands (eg. 7 or 8) can
cause steric overcrowding and van der Waals strain. So 6 is the
ideal number.