Manganate
The manganate ion is MnO42 and a manganate is any compound that contains the MnO42 group.  These salts, e.g. potassium manganate, are the conjugate base of the hypothetical species manganic acid, H2MnO4.  Manganate salts are intermediates in the industrial synthesis of potassium permanganate.  Occasionally permanganate and manganate salts are confused, but they behave quite differently.

# Structure
MnO42 is tetrahedral, being structurally related to other tetrahedral oxides such permanganate and osmium tetroxide.  Salts of MnO42, SO42, and CrO42 are usually isostructural or nearly so, which means that they form solid solutions and have similar solubilities.  Unlike sulfate and chromate, however, MnO42 is paramagnetic.

# Synthesis
See potassium manganate
Manganate ion is stable in water only under strongly basic conditions. Consequently, salts are found with electropositive metals, such as Na+, K+, and Ba2+ that do not form insoluble hydroxides.  Manganates can be prepared by oxidation of MnO2 or by reduction of permanganate.
Note that O2 is insufficiently oxidizing to further convert Mn(VI) to Mn(VII).  Thus, the Mn(VI)/Mn(VII) oxidation is effected using stronger oxidants including O3 or Cl2.

# Reactions
MnO42 is the conjugate base of the hypothetical manganic acid, H2MnO4, which is too unstable to be isolated.  The half-deprotonated acid has, however, been examined.  Thus, the pKa of HMnO4 has been determined as 7.1.  MnO42 is about 100 times more basic than the acetate ion.  At lower pH's, the manganate ion will disproportionate to permanganate ion and manganese dioxide:

# Permanganate, manganate, hypomanganate, and manganite
The above reactions illustrate the close chemical relationship between the two principal soluble manganese oxides, permanganate and manganate.  Two other, rarer members of the series are known.  Hypomanganate, MnO43, is a blue-colored ion of Mn(V) that forms when MnO42 is treated with manganite MnO44 at pH>12.  It has also been generated by treatment of KMnO4 in 10M KOH with H2O2.  The pKa of HMnO42 is 13.7, which indicates that solutions of hypomanganate contain significant amounts of the protonated form even at high pH's.  Oxidations of alkenes by permanganate are proposed to proceed via the intermediacy of "esters" of hypomanganate, that is, species of the type MnO2(OR)2 where R is an alkyl group.
The brown-colored manganite ion, a Mn(IV) species, in turn is produced by dissolving its parent acid anhydride MnO2 in strong alkali.  Alternatively, MnO44 can be generated by reduction of MnO43.

# Uses
BaMnO4 and K2MnO4 are used to oxidize primary and secondary alcohols.  Primary alcohols are oxidised to aldehydes, then carboxylic acids. Secondary alcohols are oxidised to ketones. Tertiary alcohols cannot be oxidised in this way. Its use is similar to that for the Jones reagent.

# Literature cited
- ^ Nyholm R.S. Woolliams P.R. (1986). ""Manganates(VI)"". Inorganic Syntheses. XI: 5661..mw-parser-output cite.citation{font-style:inherit}.mw-parser-output q{quotes:"\"""\"""'""'"}.mw-parser-output code.cs1-code{color:inherit;background:inherit;border:inherit;padding:inherit}.mw-parser-output .cs1-lock-free a{background:url("")no-repeat;background-position:right .1em center}.mw-parser-output .cs1-lock-limited a,.mw-parser-output .cs1-lock-registration a{background:url("")no-repeat;background-position:right .1em center}.mw-parser-output .cs1-lock-subscription a{background:url("")no-repeat;background-position:right .1em center}.mw-parser-output .cs1-subscription,.mw-parser-output .cs1-registration{color:#555}.mw-parser-output .cs1-subscription span,.mw-parser-output .cs1-registration span{border-bottom:1px dotted;cursor:help}.mw-parser-output .cs1-hidden-error{display:none;font-size:100%}.mw-parser-output .cs1-visible-error{display:none;font-size:100%}.mw-parser-output .cs1-subscription,.mw-parser-output .cs1-registration,.mw-parser-output .cs1-format{font-size:95%}.mw-parser-output .cs1-kern-left,.mw-parser-output .cs1-kern-wl-left{padding-left:0.2em}.mw-parser-output .cs1-kern-right,.mw-parser-output .cs1-kern-wl-right{padding-right:0.2em}
- ^ Lee D.G. Chen T. (1993). "Reduction of Manganate(VI) by Mandelic Acid and Its Significance to Development of a General Mechanism for Oxidation of Organic Compounds by High-Valent Transition Metal Oxides". Journal of the American Chemical Society. 115: 1123111236. doi:10.1021/ja00077a023.
- ^ Rush J.D. Bielski B.H. (1995). "Studies of Manganate(V), -(VI), and -(VII) Tetraoxyanions by Pulse Radiolysis. Optical Spectra of Protonated Forms". Inorg. Chem. 34: 58328. doi:10.1021/ic00127a022.

# Other references
- G. Procter, S. V. Ley, G. H. Castle Barium Manganate  in Encyclopedia of Reagents for Organic Synthesis (Ed: L. Paquette) 2004, J. Wiley & Sons, New York. DOI: 10.1002/047084289.
- Holleman, A. F.; Wiberg, E. "Inorganic Chemistry" Academic Press: San Diego, 2001. ISBN 0-12-352651-5.
See category for a list.