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.

Potassium octacyanomolybdate(IV)

Names
IUPAC name Potassium octacyanidomolybdate(IV)
Other names Potassium octacyanomolybdate(IV)
Identifiers
CAS Number	anhydrous: 17456-18-7 dihydrate: 17457-89-5
3D model (JSmol)	anhydrous: Interactive image monohydrate: Interactive image dihydrate: Interactive image
PubChem CID	monohydrate: 71308809 dihydrate: 165360158
CompTox Dashboard (EPA)	dihydrate: DTXSID901336495
InChI monohydrate: InChI=1S/8CN.4K.Mo.H2O/c8*1-2;;;;;;/h;;;;;;;;;;;;;1H2/q8*-1;4*+1;+4; Key: JJVZTQPHXSHIOV-UHFFFAOYSA-N dihydrate: InChI=1S/8CN.4K.Mo.2H2O/c8*1-2;;;;;;;/h;;;;;;;;;;;;;2*1H2/q8*-1;4*+1;+4;; Key: GRZGTFZNVZNYOQ-UHFFFAOYSA-N
SMILES anhydrous: [K+].[K+].[K+].[K+].N#C[Mo-4](C#N)(C#N)(C#N)(C#N)(C#N)(C#N)C#N monohydrate: O.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.O.[K+].[K+].[K+].[K+].[Mo+4]
Properties
Chemical formula	K4[Mo(CN)8]
Molar mass	460,47 g/mol (anhydrous) 496.5 g/mol (dihydrate)
Appearance	yellow powder
Melting point	>300 °C
Hazards
GHS labelling:
Pictograms
Signal word	Warning
Hazard statements	H302, H315, H319, H335
Precautionary statements	P261, P305+P351+P338
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). Infobox references

Preparation

The dihydrate K₄[Mo(CN)₈] · 2 H₂O can be prepared by the reduction of molybdate (MoO₄^2-) 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 MoO₄^2- + 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

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