Coordination number of  Cr  is  six .A  complex  with  $C_2{O}_4^{2-},$ en and  superoxide  ${O_2}^{-}$ will be in the ratio  to make  complex ${[Cr{\left(C_2{O}_4\right)}_x,{\left(en\right)}_y{\left(O_2\right)}_z]}^{-}$ , What is  x , y , z ?

$C{r}^{3+},C_2{O_4}^{2-},{\left(en\right)}^0,{O_2}^{-}$

$C_2{O_4}^{2-}and{\left(en\right)}^0$  bidentate ligand 

$CNofC{r}^{3+}=6,so,x=1,y=1,z=2$

$$\begin{gathered} CN=x\times 2+y\times 2+2z=6 \\ 1\times 2+1\times 2+2\times 1=6 \end{gathered}$$

Sum of  charges =Net  charge s 
 $$\begin{gathered} +3+(-2x)+0\left(y\right)+(-1\times z)=-1(x=1,y=1,z=2) \\ \therefore +3-2+9-1\times 2=-1 \end{gathered}$$
Sum  of charge = -1 ,Net  charges = -1