Abstract

Dicarbonyldi-μ-chlorodichlorodiplatinum, with molecular formula C₂Cl₄O₂Pt₂ and CAS registry number 17522-99-5, represents the first isolated metal-carbonyl complex in chemical history. This organometallic compound exists as both cis and trans isomers, with the trans configuration being more thermodynamically stable. The compound crystallizes as a volatile yellow solid with a dimeric structure featuring two square-planar platinum centers bridged by chloride ligands. Characteristic infrared spectroscopy reveals a carbonyl stretching frequency of 2139 cm^-1 in dichloromethane solution. ¹⁹⁵Pt NMR spectroscopy shows two distinct platinum signals at 1507 and 1511 ppm in thionyl chloride, indicating dynamic structural behavior in solution. The compound demonstrates significant historical importance as the precursor to modern metal carbonyl chemistry and exhibits interesting dissociation behavior under carbon monoxide atmosphere.

Introduction

Dicarbonyldi-μ-chlorodichlorodiplatinum occupies a pivotal position in the development of organometallic chemistry as the first characterized metal-carbonyl complex. Paul Schützenberger initially synthesized this compound in 1868 through the reaction of platinum black with carbon monoxide and chlorine gases. This discovery fundamentally expanded understanding of transition metal coordination chemistry and provided the foundation for subsequent developments in carbonyl complex chemistry. The compound belongs to the class of binuclear organometallic complexes with platinum in the +2 oxidation state. Its structural features include both terminal and bridging chloride ligands along with linearly coordinated carbon monoxide ligands. Although not widely employed in industrial applications, this compound serves as an important reference material for studying platinum(II) coordination chemistry and metal-metal interactions in dimeric systems.

Molecular Structure and Bonding

Molecular Geometry and Electronic Structure

The molecular structure of trans-dicarbonyldi-μ-chlorodichlorodiplatinum exhibits C₂ symmetry with two platinum centers in distorted square-planar coordination environments. X-ray crystallographic analysis reveals that the μ-chloro ligands and carbonyl ligands on each platinum center maintain coplanar arrangement, while terminal chloride ligands deviate from this plane by approximately 0.228 Å. The bridging chloride ligands demonstrate asymmetric coordination with Pt-Cl bond distances ranging from 2.36 to 2.42 Å, indicating unequal sharing between platinum centers. Platinum atoms adopt dsp² hybridization with formal oxidation state +2 corresponding to d⁸ electronic configuration. The molecular orbital configuration involves significant d_z² character contributing to metal-metal interactions across the chloride bridges. Carbon monoxide ligands bind through σ-donation from carbon to platinum with concomitant π-backdonation from filled platinum d orbitals to empty CO π* orbitals.

Chemical Bonding and Intermolecular Forces

The bonding scheme in dicarbonyldi-μ-chlorodichlorodiplatinum involves predominantly covalent interactions with ionic character in platinum-chloride bonds. Terminal Pt-Cl bonds measure approximately 2.28 Å while bridging Pt-Cl bonds extend to 2.39 Å on average. Carbon monoxide ligands exhibit Pt-C bond distances of 1.85 Å with C-O bond lengths of 1.15 Å. The compound exists as discrete molecular units in solid state with intermolecular interactions dominated by van der Waals forces. The molecular dipole moment measures approximately 3.2 Debye due to asymmetric charge distribution across the dimeric structure. Crystal packing demonstrates weak chloride-chloride interactions at distances of 3.5-3.8 Å between adjacent molecules. The compound displays limited solubility in nonpolar solvents but dissolves readily in chlorinated solvents and coordinating media.

Physical Properties

Phase Behavior and Thermodynamic Properties

Dicarbonyldi-μ-chlorodichlorodiplatinum presents as a crystalline yellow solid with needle-like morphology. The compound sublimes at 120 °C under reduced pressure (0.1 mmHg) and decomposes without melting at temperatures above 180 °C. Density measurements yield values of 3.85 g/cm³ at 25 °C, consistent with platinum-containing compounds. The compound demonstrates moderate volatility for an organometallic complex with vapor pressure of 0.5 mmHg at 100 °C. Thermodynamic parameters include enthalpy of formation ΔH_f = -285 kJ/mol and Gibbs free energy of formation ΔG_f = -250 kJ/mol. Crystal structure determination reveals monoclinic space group P2₁/c with unit cell parameters a = 8.92 Å, b = 11.35 Å, c = 12.78 Å, and β = 105.7°. The refractive index measures 1.78 at 589 nm wavelength.

Spectroscopic Characteristics

Infrared spectroscopy shows characteristic carbonyl stretching vibrations at 2139 cm^-1 in dichloromethane, 2142 cm^-1 in thionyl chloride, 2135 cm^-1 in cyclohexane, and 2132 cm^-1 in benzene. These frequencies indicate minimal backbonding compared to later transition metal carbonyls. Platinum-chloride stretching vibrations appear at 325 cm^-1 and 295 cm^-1 for terminal and bridging modes respectively. ¹⁹⁵Pt NMR spectroscopy in thionyl chloride solution reveals two distinct signals at 1507 ppm and 1511 ppm relative to Na₂PtCl₆, suggesting dynamic equilibrium between different solvated species. ¹³C NMR spectroscopy displays a resonance at 139.9 ppm for carbonyl carbon atoms with ¹⁹⁵Pt-¹³C coupling constant of 1958.8 Hz. Mass spectrometric analysis shows molecular ion peak at m/z = 654 corresponding to C₂Cl₄O₂Pt₂ with characteristic fragmentation pattern including loss of CO ligands and chloride abstraction.

Chemical Properties and Reactivity

Reaction Mechanisms and Kinetics

Dicarbonyldi-μ-chlorodichlorodiplatinum undergoes dissociation under carbon monoxide atmosphere to form monomeric Pt(CO)₂Cl₂ species. This reaction proceeds with second-order kinetics with respect to CO pressure and exhibits rate constant k = 2.3 × 10^-3 M^-1s^-1 at 25 °C in thionyl chloride solution. The reaction initially produces trans-Pt(CO)₂Cl₂, which isomerizes to cis configuration with half-life of 4 hours at room temperature. The dissociation activation energy measures 45 kJ/mol with entropy of activation ΔS^‡ = -35 J/mol·K. Olefin addition reactions cause cleavage of chloride bridges to form monomeric PtCl₂(CO)(olefin) complexes with reaction completion within 2 hours in toluene at 25 °C. The compound serves as precursor for silica-supported platinum catalysts through surface adsorption and subsequent thermal decomposition liberating CO₂ and HCl at 40 °C under water vapor.

Acid-Base and Redox Properties

The compound demonstrates Lewis acidic character at platinum centers with propensity for chloride abstraction by strong Lewis acids. Treatment with aluminum chloride generates Pt(CO)Cl₃^- species through chloride coordination. Electrochemical measurements reveal quasi-reversible oxidation at E_1/2 = +1.25 V versus SCE corresponding to Pt(II)/Pt(III) couple. Reduction occurs at E_1/2 = -0.85 V versus SCE associated with platinum-centered electron transfer. The compound maintains stability in neutral and acidic conditions but undergoes gradual hydrolysis in aqueous media with half-life of 12 hours at pH 7. Carbon monoxide ligand substitution proceeds through dissociative mechanism with rate constant k = 8.7 × 10^-5 s^-1 for acetonitrile exchange at 25 °C. The compound functions as pre-catalyst for hydrochlorination of cyclohexene with turnover frequency of 15 h^-1 at 50 °C.

Synthesis and Preparation Methods

Laboratory Synthesis Routes

The original synthesis developed by Schützenberger employs platinum black reacted with equimolar mixture of carbon monoxide and chlorine gases at elevated temperatures. Optimal formation occurs at 150-180 °C with reaction completion within 4 hours. The volatile yellow product sublimes from the reaction zone and collects on cooler surfaces. Modern laboratory synthesis utilizes trans-PtCl₂(NCEt)(CO) as precursor through thermal elimination of ethyl isocyanide in mesitylene solvent. Dissolution of cis-PtCl₂(NCEt)(CO) in dry mesitylene followed by solvent evaporation at reduced pressure yields dicarbonyldi-μ-chlorodichlorodiplatinum with 85% conversion after two cycles. Alternative preparation starts with PtCl₄^2- anion carbonylated under aluminum chloride catalysis in chlorinated solvents. Treatment of resulting Pt(CO)Cl₃^- with stoichiometric aluminum chloride under nitrogen atmosphere affords the dimeric complex in 70-75% isolated yield after recrystallization from toluene.

Analytical Methods and Characterization

Identification and Quantification

Characteristic identification employs infrared spectroscopy with primary diagnostic feature being carbonyl stretching vibration between 2130-2145 cm^-1 depending on solvent environment. ¹⁹⁵Pt NMR spectroscopy provides unambiguous confirmation through characteristic chemical shifts between 1500-1520 ppm with specific coupling patterns. Quantitative analysis utilizes ultraviolet-visible spectroscopy with maximum absorption at 320 nm (ε = 4500 M^-1cm^-1) in dichloromethane solution. X-ray powder diffraction confirms crystalline phase identity through comparison with calculated pattern from single-crystal data. Elemental analysis requires combustion methods with typical results: C 3.67%, Cl 21.67%, Pt 59.65% (calculated: C 3.66%, Cl 21.66%, Pt 59.64%). Chromatographic purification employs silica gel columns with dichloromethane/hexane eluent systems with R_f = 0.45 in 1:1 mixture.

Purity Assessment and Quality Control

Purity determination relies on complementary techniques including differential scanning calorimetry showing sharp sublimation endotherm at 120 °C. Thermogravimetric analysis demonstrates single-step mass loss corresponding to complete sublimation. High-performance liquid chromatography with UV detection at 320 nm provides purity assessment with detection limit of 0.1% for common impurities including Pt(CO)₂Cl₂ and PtCl₂ derivatives. ¹⁹⁵Pt NMR spectroscopy allows quantitative determination with detection limit of 2% for platinum-containing impurities. The compound exhibits stability under nitrogen atmosphere for extended periods with less than 5% decomposition after 6 months storage at -20 °C. Handling requires anhydrous conditions due to sensitivity toward hydrolysis with formation of platinum oxides and hydrochloric acid.

Applications and Uses

Industrial and Commercial Applications

Dicarbonyldi-μ-chlorodichlorodiplatinum serves primarily as a laboratory reagent rather than industrial catalyst due to its historical significance and fundamental chemical properties. The compound finds application as molecular precursor for supported platinum catalysts through vapor deposition onto silica surfaces. This application exploits the compound's volatility and clean decomposition pathway to generate metallic platinum nanoparticles on various oxide supports. Surface science studies utilize the compound as model system for understanding platinum carbonyl chemistry on solid surfaces. The compound functions as synthetic intermediate for preparation of more elaborate platinum complexes through bridge-cleavage reactions with donor ligands. Educational applications include demonstration of early metal carbonyl chemistry and illustration of trans-effect in square-planar platinum complexes.

Research Applications and Emerging Uses

Current research applications focus on the compound's utility as precursor for platinum nanomaterials with controlled morphology. Vapor-phase deposition using this compound produces platinum films with unusual crystallographic orientations due to specific decomposition pathways. Materials science investigations employ the compound as molecular building block for supramolecular assemblies through coordination of the chloride bridges to other metal centers. Catalysis research explores its potential in hydrocarbon transformation reactions, particularly hydrochlorination and hydroformylation processes. The compound serves as reference material for theoretical studies of metal-metal interactions and bonding in binuclear platinum systems. Emerging applications include use in chemical vapor deposition processes for microelectronic applications where low-temperature decomposition provides advantages over traditional platinum precursors.

Historical Development and Discovery

The discovery of dicarbonyldi-μ-chlorodichlorodiplatinum by Paul Schützenberger in 1868 marked the inception of metal carbonyl chemistry as a distinct field within organometallic chemistry. Schützenberger's original publication described the reaction of platinum sponge with carbon monoxide and chlorine gases, producing a volatile yellow compound initially formulated as PtCOCl₂. Subsequent investigations throughout the late 19th and early 20th centuries established the dimeric nature of the compound and its structural relationship to other platinum halide complexes. The development of vibrational spectroscopy in the 1930s allowed definitive characterization of the carbonyl bonding through infrared measurements. X-ray crystallographic determination in the 1960s provided unambiguous structural assignment confirming the bridged dimeric formulation. The compound's historical significance lies in its role as prototype for countless metal carbonyl complexes discovered subsequently, including those of iron, nickel, and cobalt that became industrially important catalysts. Modern reinvestigations continue to reveal new aspects of its chemistry and reactivity.

Conclusion

Dicarbonyldi-μ-chlorodichlorodiplatinum represents a landmark compound in the development of organometallic chemistry with continuing scientific relevance. Its historical significance as the first isolated metal-carbonyl complex provides context for understanding the evolution of coordination chemistry. The compound exhibits distinctive structural features including asymmetric chloride bridging and distorted square-planar geometry around platinum centers. Spectroscopic characterization reveals dynamic behavior in solution with equilibrium between different dimeric forms. Synthetic accessibility through multiple routes ensures continued availability for research purposes. The compound's reactivity patterns, particularly its dissociation under carbon monoxide atmosphere and bridge-cleavage reactions with olefins, provide fundamental insights into platinum(II) chemistry. Future research directions likely include expanded applications in materials science as precursor for well-defined platinum nanomaterials and continued fundamental studies of metal-metal interactions in bimetallic systems.