Potassium octacyanomolybdate(IV)

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Potassium octacyanomolybdate(IV)
K4Mo(CN)8 chemdraw.png
K4Mo(CN)8 avogadro1.png
IUPAC name
Potassium octacyanidomolybdate(IV)
Other names
Potassium octacyanomolybdate(IV)
3D model (JSmol)
  • dihydrate: InChI=1S/8CN.4K.Mo.H2O/c8*1-2;;;;;;/h;;;;;;;;;;;;;1H2/q8*-1;4*+1;+4;
  • anhydrous: O.[K+].[K+].[K+].[K+].N#C[Mo-4](C#N)(C#N)(C#N)(C#N)(C#N)(C#N)C#N
  • dihydrate: [C-]#N.[C-]#N.[C-]#N.[C-]#N.[C-]#N.[C-]#N.[C-]#N.[C-]#N.O.[K+].[K+].[K+].[K+].[Mo+4]
Molar mass 460,47 g/mol (anhydrous)
496.5 g/mol (dihydrate)
Appearance yellow powder
Melting point >300 °C
GHS labelling:
GHS07: Exclamation mark
H302, H315, H319, H335
P261, P305+P351+P338
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).

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.


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.[1][2] Yields of 70% are typical and the method is suited for scale-up.

4 MoO42- + 32 CN + BH4 + 25 H+ → 4  [Mo(CN)8]4- + 13 H2O + 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 + 8 KCN → K4[Mo(CN)8] + 4 KCl + 2 Et2O


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.[4] These ligands can be substituted by others, for example H2O. The cyanide ligands also bind to other metals, leading to cages.[2]


  1. ^ Leipoldt, J. G.; Bok, L. D. C.; Cilliers, P. J. (1974). "The Preparation of Potassium Octacyanomolybdate(IV) Dihydrate". Zeitschrift für anorganische und allgemeine Chemie. 409 (3): 343–344. doi:10.1002/zaac.19744090310.
  2. ^ a b Bonadio, Federica; Larionova, Joulia; Gross, Mathias; Biner, Margret; Stoeckli‐Evans, Helen; Decurtins, Silvio; Pilkington, Melanie (2004). Cyano-Bridgeed M(II)9M(V)6 Molecular Clusters, M(II) = Mn, Co, Ni; M(V) = Mo, W. Inorganic Syntheses. Vol. 34. p. 156. doi: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.doi:10.1002/ejic.201600669
  4. ^ M. Sellin, V. Marvaud, M. Malischewski, "Isolation and Structural Characterization of Eightfold Protonated Octacyanometalates [M(CNH)8]4+ (M=MoIV, WIV) from Superacids", Angew. Chem. Int. Ed., 2020 volume 59, pp. 10519-10522.doi:10.1002/anie.202002366