“To ask the 'right' question is far more important than to receive the answer. The solution of a problem lies in the understanding of the problem; the answer is not outside the problem, it is in the problem.” 

― Jiddu Krishnamurti, The Flight of the Eagle

Hi, I am a researcher from inorganic chemistry,There

There are some rules for as coordination compounds. The first one is having coordinate covalent bond between metal and donor atoms. And also there must be an coordination sphere. The ligands are donor and metal is acceptor in this sphere. And they act in unison in solution. They don't leave after dissolving. But, potassium carbonate doesn't have these properties. There are no coordinate covalent bond between metal and donor atoms. İt doesn't have coordination sphere. Potassium carbonate separate after dissolving in water as cation and anion. So, potassium carbonate is a simple salt not coordination compound.

I hope you can benefit from my answer.

Take care

Fatma

The other important point is empty d orbitals. Potassium doesn't have any empty d orbitals that must close the valency electrons. 

What is the "carbonate complex anion?"

You have to follow the definitions used in chemistry which are recommended by IUPAC, Google "IUPAC Gold Book."

"coordination entity

An assembly consisting of a central atom (usually metallic) to which is attached a surrounding array of other groups of atoms (ligands)."

"complex

A molecular entity formed by loose association involving two or more component molecular entities (ionic or uncharged), or the corresponding chemical species. The bonding between the components is normally weaker than in a covalent bond. The term has also been used with a variety of shades of meaning in different contexts: it is therefore best avoided when a more explicit alternative is applicable. In inorganic chemistry the term 'coordination entity' is recommended instead of 'complex'."

Potassium carbonate In solid state can be formally called a coordination entity (J. Chem. Soc., Dalton Trans., 1973,0, 70-72 )

The structure of the hydrated potassium cation in water (coordination entity), see Inorg Chem. 2012 Jan 2; 51(1): 425–438. 

Is potassium carbonate a carbonate entity, is it having oxo,ligand carbon? Its confusing...

Sır,

You are right. Carbonate is a ligand in coordination chemistry. But, potassium is not transition metal. So potassium doesn't have any empty d orbitals. Potassium  doesn't compose complex with carbonate. 

Is potassium ion having 3s, 3p and 3d orbital epmpty?

Dear Fatma, plz read the IUPAC definitions

Is non metal like sulphur , nitrogen, carbon etc except halides and hydrides not act as central metal atom or ion (non metal) why?

“To ask the 'right' question is far more important than to receive the answer. The solution of a problem lies in the understanding of the problem; the answer is not outside the problem, it is in the problem.” 

― Jiddu Krishnamurti, The Flight of the Eagle