Potassium octacyanomolybdate(IV) is the inorganic salt with the formula K<sub>4</sub>[Mo(CN)<sub>8</sub>]. 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 âÂÂ4, which is balanced with four potassium cations. The salt is often prepared as its dihydrate K<sub>4</sub>[Mo(CN)<sub>8</sub>]<sup>.</sup>(H<sub>2</sub>O)<sub>2</sub>.
The dihydrate K<sub>4</sub>[Mo(CN)<sub>8</sub>] ÷ 2 H<sub>2</sub>O can be prepared by the reduction of molybdate (MoO<sub>4</sub><sup>2-</sup>) with potassium borohydride (KBH<sub>4</sub>) in a solution with potassium cyanide and acetic acid. Yields of 70% are typical and the method is suited for scale-up.
An alternative route starts from MoCl<sub>4</sub>(Et<sub>2</sub>O)<sub>2</sub> 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<sub>4</sub>(Et<sub>2</sub>O)<sub>2</sub> is typically less available than the molybdate.
Octacyanomolybdate(IV) can be oxidized to the paramagnetic octacyanomolybdate(V).
The cyanide ligands in [Mo(CN)<sub>8</sub>]<sup>4-</sup> remain basic. Strong acids lead to the hydrogen isocyanide complex [Mo(CNH)<sub>8</sub>]<sup>4+</sup>, in common with many cyanometalate complexes. These ligands can be substituted by others, for example H<sub>2</sub>O. The cyanide ligands also bind to other metals, leading to cages.