Define the following term: Coordination sphere.

(N/A) Coordination sphere: The central metal atom or ion and the ligands directly attached to it are enclosed within a square bracket,which is collectively referred to as the coordination sphere. For example,in the complex $K_{4}[Fe(CN)_{6}]$,the coordination sphere is $[Fe(CN)_{6}]^{4-}$,while $K^{+}$ acts as the counter ion. Counter ions are ionizable groups that are written outside the square bracket.