Potassium octacyanomolybdate(IV)
# Potassium octacyanomolybdate(IV)

Potassium octacyanomolybdate(IV) is the inorganic salt with the formula $K_4[Mo(CN)_8].$ A yellow light-sensitive solid, it is the potassium salt of the cyanometalate with the coordination number eight. The complex anion consists of a Mo(IV) center bound to eight cyanide ligands resulting in an overall charge of −4, which is balanced with four potassium cations. The salt is often prepared as its dihydrate $K_4[Mo(CN)_8]^.(H_2O)_2.$

## Preparation

The dihydrate $K_4[Mo(CN)_8]$ · 2 $H_2O$ can be prepared by the reduction of molybdate $(MoO_4^{2-})$ with potassium borohydride $(KBH_4)$ in a solution with potassium cyanide and acetic acid. Yields of 70% are typical and the method is suited for scale-up.

    4 $MoO_4^{2-}$ + 32 $CN^−$ + $BH_4^−$ + 25 $H^+$ → 4 $[Mo(CN)_8]^{4-}$ + 13 $H_2O$ + $H_3BO_3$

An alternative route starts from $MoCl_4(Et_2O)_2$ avoiding the need for reductants. The yield of this route is typically around 70%. This synthesis is convenient for lower batch sizes than the earlier method but the $MoCl_4(Et_2O)_2$ is typically less available than the molybdate.

    $MoCl_4(Et_2O)_2$ + 8 KCN → $K_4[Mo(CN)_8]$ + 4 KCl + 2 $Et_2O$

## Reactions

Octacyanomolybdate(IV) can be oxidized to the paramagnetic octacyanomolybdate(V).

The cyanide ligands in $[Mo(CN)_8]^{4-}$ remain basic. Strong acids lead to the hydrogen isocyanide complex $[Mo(CNH)_8]^{4+},$ in common with many cyanometalate complexes. These ligands can be substituted by others, for example $H_2O.$ The cyanide ligands also bind to other metals, leading to cages.

