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Recently in the Google news feed, I discovered this news:

Gold stabilized in very rare oxidation state +II

Missing golden link found: Divalent gold complex isolated for the first time in a pure form; new fundamental insight also provides possible explanation of the mechanism of action of cytostatic gold(+III) porphyrins

Date: August 8, 2017

Source: Johannes Gutenberg Universitaet Mainz

Summary: A team of chemists has been able to isolate and analyze gold in the very rare oxidation state +II. This provides the missing links in the homologous series of the coinage metal ions copper(+II), silver(+II), gold(+II), and in the 'relativistic' triad of platinum(+II), gold(+II), and mercury(+II).

And there are loads of similar news- Here, here and here.

Now, I started digging for more information and found out that gold(II) complexes were known previously and has been synthesized but they are unstable and prone to disproportionation.

  1. Coordination chemistry of gold(II) complexes (October 1999)

    The number of complexes with gold in a formal oxidation state of two have increased considerably and nowadays this oxidation state can be considered nearly as a common state in gold chemistry. This review covers the evolution of the knowledge of this oxidation state from the first dithiocarbamate derivatives $\ce{[Au2(S2CNR2)2X2]}$, stable only at very low temperature, to the room temperature stable dinuclear gold complexes $\ce{[Au2(L–L)2X2](L–L=CH2PPh2CH2, CH2PPh2S, C5H4PPh2-2; X=halogen}$). Although a dinuclear structure with a gold(II)–gold(II) bond supported by two equal bridging ligands were the first and most common, this review shows the present diversity as mononuclear, $\ce{[Au([9]aneS3)2](BF4)2}$, dinuclear with two different bridging ligands, $\ce{[Au2(S2CNR2)(CH2PPh2CH2)X2]}$, with a non-supported metal–metal bond, $\ce{[Au2(Ph2PC8H6PPh2)2Cl2](PF6)2}$, and polynuclear gold(II) complexes either with chains of only gold centers or with other metals, such as $\ce{[{(C6F3H2)Au(CH2PPh2CH2)2Au}2Au2(CH2PPh2CH2)2]ClO4}$ or $\ce{[Au2Pt(CH2PPh2S)4X2]}$

  2. Disproportionation of Gold(II) Complexes. A Density Functional Study of Ligand and Solvent Effects (July 13 2006)

    For the disproportionation of $\ce{Au(II)L3}$ complexes with neutral ligands, disproportionation is highly endergonic in the gas phase. Calculations imply that for synthesis of a monometallic $\ce{Au(II)}$ complex, a nonpolar solvent is preferred. With the exception of $\ce{[Au(CO)3]^2+}$, disproportionation of $\ce{Au(II)L3}$ complexes to $\ce{Au(I)L}$ and $\ce{Au(III)L3}$ is exergonic in solution phase for the ligands investigated.

  3. Structural Studies of Gold(I, II, and III) Compounds with Pentafluorophenyl and Tetrahydrothiophene Ligands(February 28 2007)

    A golden opportunity was seized to prepare an unbridged, dinucleur gold(II) compound, $\ce{[Au2(C6F5)4(tht)2]}$ (tht=tetrahydrothiophene), without any stabilizing chelating ligands (see picture; Au orange, S yellow, F green, C black) and to crystallographically characterize the gold(I) and gold(III) complexes participating in this conversion. A unique ligand scrambling of the latter gold(III) compound occurs in solution.

Gold(II) complexes has been intensly studied previously. They have been trying to synthesize a stable complex but failed miserably. The stable ones are only present in low temperature and only one was found to be stable in room temperature. So, although gold(II) complexes were previously discovered and (synthesized?), why scientists at University of Mainz and those who discovered it in 2017 claims to have synthesized gold(II) complexes for the first time?

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    $\begingroup$ The claim is isolating it, which is not the same as just synthesizing it. You can synthesize $\ce{HNO_2}$ by adding sodium nitrite to hydrochloric acid in water solution, but can you isolate or even concentrate it? $\endgroup$ – Oscar Lanzi Dec 25 '17 at 13:44
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    $\begingroup$ AuXe4 (2+) was known for for quite some time. $\endgroup$ – Mithoron Jul 3 '18 at 22:28
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    $\begingroup$ The claim might be clarified by inserting the word "stable". So maybe they can legitimately claim the first synthesis of stable gold(II) compounds. $\endgroup$ – matt_black Jan 12 at 18:14

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