Potassium octacyanomolybdate(IV)

Potassium octacyanomolybdate(IV) is the inorganic salt with the formula K4[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 K4[Mo(CN)8].(H2O)2.

Preparation
The dihydrate K4[Mo(CN)8] · 2 H2O can be prepared by the reduction of molybdate (MoO42-) with potassium borohydride (KBH4) in a solution with potassium cyanide and acetic acid. Yields of 70% are typical and the method is suited for scale-up.


 * 4MoO42- + 32CN− + BH4− + 25H+ → 4 [Mo(CN)8]4- + 13H2O + H3BO3

An alternative route starts from MoCl4(Et2O)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 MoCl4(Et2O)2 is typically less available than the molybdate.
 * MoCl4(Et2O)2 + 8KCN → K4[Mo(CN)8] + 4KCl + 2Et2O

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 H2O. The cyanide ligands also bind to other metals, leading to cages.