Potassium octacyanomolybdate(IV) explained

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.

Preparation

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

Reactions

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]

References

  1. 10.1002/zaac.19744090310. The Preparation of Potassium Octacyanomolybdate(IV) Dihydrate. 1974. Leipoldt. J. G.. Bok. L. D. C.. Cilliers. P. J.. Zeitschrift für anorganische und allgemeine Chemie. 409. 3. 343–344.
  2. Book: Inorganic Syntheses. 156. 34. 2004. Cyano-Bridgeed M(II)9M(V)6 Molecular Clusters, M(II) = Mn, Co, Ni; M(V) = Mo, W. Federica . Bonadio . Joulia . Larionova . Mathias . Gross . Margret . Biner . Helen . Stoeckli‐Evans . Silvio . Decurtins . Melanie . Pilkington. 10.1002/0471653683.ch4.
  3. G. Handzlik, M. Magott, B. Sielucka, D. Pinkowicz, "Alternative Synthetic Route to Potassium Octacyanidoniobate(IV) and its Molybdenum Congener", Eur. J. Inorg. Chem., 2016, pp. 4872-77.
  4. M. Sellin, V. Marvaud, M. Malischewski, "Isolation and Structural Characterization of Eightfold Protonated Octacyanometalates [M(CNH)<sub>8</sub>]4+ (M=MoIV, WIV) from Superacids", Angew. Chem. Int. Ed., 2020 volume 59, pp. 10519-10522.