Dicarbonyldi-μ-chlorodichlorodiplatinum. trans-Dicarbonyldi-μ-chlorodichlorodiplatinum is the first ever isolated metal-carbonyl complex. It was first synthesized by Paul Schützenberger in 1868. This discovery of  opened the door for the exploration of metal-carbonyl complexes and represents a significant breakthrough in the field of organometallic chemistry. Though  itself is not used in industry, metal carbonyls are an important class of catalysts for a wide range of industrial processes including hydroformylation amongst others. As such, the discovery of  was an important milestone both in the synthesis of organometallic compounds and in the development of many important catalysts widely used today.

Structure and characterization 
Dicarbonyldi-μ-chlorodichlorodiplatinum has both cis and trans isomers. Unless noted otherwise,  in this article specifically refers to the trans isomer.

Crystal structure 
The crystal structure of  has been obtained.  can be roughly thought of as two square planar  centers, but the complex does not perfectly align with that model; the two platinum centers deviate from the square planar geometry of  monomers. The μ-chloro ligands and the  ligand on each  center are coplanar, but the terminal  on both  centers is 0.228 Å out of plane, giving the complex overall C2 symmetry. Additionally, the bridging chlorines are not equally shared between the two  centers.

Spectroscopic characterization 
IR spectroscopy and NMR have also both been used to characterize . The CO stretching frequency for  is 2139 cm−1 in DCM (the C-O stretching frequency has also been measured in thionyl chloride, cyclohexane and benzene, toluene and n-heptane).  NMR reveals two  peaks for  (at 1507 and 1511 ppm) in thionyl chloride. This is believed to be due to the formation of two distinct  dimers in solution (where solvation effects cause slight changes in  chemical shift) or stacking of dimers in solution giving rise to distinct  shifts.  NMR reveals a 139.9 shift for the carbonyl carbons with a  coupling constant of 1958.8 Hz.

Synthesis

Schützenberger's discovery of  

Schützenberger first synthesized  in 1868. The complex was sourced from  black and   and  gases, but there are conflicting reports on whether he flowed a mixture of the gasses over platinum black or first formed  and subsequently exposed the compound to . However, Schützenberger's isolated product is consistently reported to be a volatile yellow solid. Schützenberger also observed that performing the reaction at different temperatures yields compounds with different melting points. Based on this observation, Schützenberger identified that the composition of the platinum complex formed under these conditionsis determined by the temperature at which the reaction proceeds.  is formed at a narrow temperature range with this procedure, but the diplatinum complex (and liberation of two  equivalents) is favored at higher temperatures.

More recent syntheses 
More recently,  has been accessed from other platinum complexes. Dell'Amico et al. reported the formation of  from the dimerization of trans- with the production of 2 equivalents of  as a byproduct. To obtain ,  cis- is dissolved in dry mesitylene. The solution is then heated, allowing the solvent to evaporate.  NMR analysis of the resulting solid reveals a mixture of trans- and . Repeating this dissolution/evaporation procedure yields an orange residue showing complete conversion to .

Alternatively, has been prepared from

.  can be generated in chlorinated solvents by treating  with  under a  atmosphere.  can also be accessed by allowing  to equilibrate under a  atmosphere at room temperature, but this equilibration occurs over the course of approximately 40 hours.   can then be generated from  by treatment with  under a nitrogen atmosphere. Interestingly, treating  with  under a  atmosphere, trans- is formed instead.

Reactivity 

 can dissociate into mono-platinum complexes. By dissolving  in thionyl chloride and exposing the solution to atmospheric pressure of  at room temperature,  is formed. Interestingly, trans- is observed shortly after exposure to , but after allowing the reaction to proceed for 24 hours, the cis isomer is exclusively observed. Trans- can also be isolated from  by performing the reaction at -80 °C. This reaction is observed to be complete after 10 hours. Trans- is stable at -80 °C, but rapidly converts to the cis isomer at room temperature.  Alternatively, olefin substitution can cause the dissociation of the diplatinum complex.  and an olefin (cyclohexene, ethylene or propylene) can be combined in toluene, causing the dissociation of the diplatinum complex to form two equivalents of cis-.

 has also been explored as a molecular precursor for loading platinum onto silica (which has been subsequently used to hydrogenate cyclohexene on a laboratory scale). To do so, dry silica is treated with  in toluene at room temperature under a nitrogen atmosphere. The  containing fragments appear to migrate to the surface of the silica based on the orange color of the material after treatment. Authors attribute this color change to adsorption of a  center onto the  surface followed by cleavage of the μ-chloro linkages to form surface bound  species. The treated  particles were then recovered by filtration and exposed to water vapor at 40 °C to liberate  and , leaving metallic  loaded on silica.

 has also been used as a starting material for the formation of more complicated platinum containing complexes. Panuzi et al. report the formation of a diplatinum complex formed by reacting a functionalized xanthene with  (shown in the figure below). To synthesize the bidentate xanthene ligand, 4,5-dibromo-2,7-di-tert-butyl-9,9-dimethylxanthene and excess  are refluxed in dry quinoline for 24 hours. The resulting solid is then washed with diethyl ether and extracted into chloroform. The  solid is then purified on a silica column with a 40:1 DCM:methanol eluent (40% yield). To form the platinum complex, the synthesized ligand is combined with excess  in toluene. The mixture was allowed to stand overnight at 5 °C, where yellow crystals formed. These crystals were then isolated and washed with dry toluene, yielding the pure diplatinum xanthene complex.

 has also been reported as a pre-catalyst for the  catalyzed hydrochlorination of cyclohexene. Alper et al. report the reaction of  and  to form  and liberate .  can then be reacted with two equivalents of cyclohexene to regenerate . The overall reaction from these two steps is the catalyzed chlorination of cyclohexene.

References 

Carbonyl complexes
Platinum complexes
Chloro complexes