Amongst the ligands : 

${\eta }^2-{\mathrm{C}}_2{\mathrm{H}}_4;{\eta }^5-{\mathrm{C}}_5{\mathrm{H}}_5^{-};{\eta }^1-{\mathrm{C}}_8{\mathrm{H}}_5^{-};{\eta }^3-{\mathrm{C}}_5{\mathrm{H}}_5^{-};$

$\begin{array}{r}{\eta }^3-{\mathrm{C}}_2{\mathrm{H}}_5^{-};{\eta }^1-{\mathrm{C}}_3{\mathrm{H}}_5^{-};{\eta }^4-{\mathrm{C}}_4{\mathrm{H}}_6;{\eta }^7-{\mathrm{C}}_7{\mathrm{H}}_7^{+};\\ {\eta }^4-{\mathrm{C}}_4{\mathrm{H}}_4;{\eta }^4-{\mathrm{C}}_4{\mathrm{H}}_4^{2-};{\eta }^2-{\mathrm{C}}_4{\mathrm{H}}_4;{\eta }^6-{\mathrm{C}}_6{\mathrm{H}}_6\end{array}$

The number of species which are :

$2e^{-}\text{donors }=x$

$4e^{-}\text{donors }=y$

$6e^{-}\text{donors }=z$

Find the value of $2x-y+z$