Potassium octacyanomolybdate(IV) is the inorganic salt with the formula K4[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 −4, which is balanced with four potassium cations. The salt is often prepared as its dihydrate K4[Mo(CN)<sub>8</sub>].(H2O)2.
The dihydrate K4[Mo(CN)<sub>8</sub>] · 2 H2O can be prepared by the reduction of molybdate (MoO42-) with potassium borohydride (KBH4) in a solution with potassium cyanide and acetic acid.[1] [2] Yields of 70% are typical and the method is suited for scale-up.
4MoO42- + 32CN− + BH4− + 25H+ → 4 [Mo(CN)<sub>8</sub>]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%.[3] 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)<sub>8</sub>] + 4KCl + 2Et2O
Octacyanomolybdate(IV) can be oxidized to the paramagnetic octacyanomolybdate(V).
The cyanide ligands in [Mo(CN)<sub>8</sub>]4- remain basic. Strong acids lead to the hydrogen isocyanide complex [Mo(CNH)<sub>8</sub>]4+, in common with many cyanometalate complexes.[4] These ligands can be substituted by others, for example H2O. The cyanide ligands also bind to other metals, leading to cages.[2]