Coordination number of metals in complexes

Question

The co-ordination number of a metal is fixed in which way?

My chemistry textbook says, " The secondary valence is equal to the coordination number and is fixed for a metal."

Obviously, the coordination number for a metal is not constant in all its compounds. So does it mean to say that it is constant in just the isomers of the compound? Or,does it mean that it is constant in all compounds with the same central atom/ion, anions and ligands as seen for Co in CoCl3.6NH3 , CoCl3.5NH3 and CoCl3.4Nh3(=6)? So all complex compounds with Co, NH3 and Cl only(in various proportions)will have coordination number of Co equal to 6?

Answer 1

It's usually constant but there's always an compound that shows exception. What I have been taught is that the coordination number is in most cases is the number of ligands directly bonded to the CMA. So for something like [PtCl6]-2 the coordination will be 6, as 6 ligands are directly attached to the CMA, also keep note the coordination number for a particular metal atom won't be more than it's valance electrons. Similarly [PtCl4] will have coordination number of 4.

To summarize, it is not constant, but due to most CMA showing same CN over and over it is generally said to be so.