What is Magnus's green salt? Describe its Synthesis, Structure and Bonding.
Magnus's green salt is an inorganic compound having the formula [Pt(NH3)4][PtCl4]. It is a dark green, quasi-one-dimensional linear chain compound, consisting of a chain of platinum atoms. This compound is discovered by Heinrich Gustav Magnus in the early 1830s.
Synthesis of Magnus's Green Salt
Magnus's green salt is synthesized by mixing the aqueous solutions of [Pt(NH3)4]+2 and [PtCl4]−2 which produce a deep green solid precipitate.
[Pt(NH3)4]Cl2 + K2[PtCl4] → [Pt(NH3)4][PtCl4]↓ + 2KCl
Under certain conditions of concentration, temperature, or solvent variations, a pink polymorph (Magnus's Pink Salt) may be formed which have different structure.
Structure of Magnus's Green Salt
Magnus's green salt is a linear chain polymer. Its structure consists of alternating [Pt(NH3)4]+2 cations and [PtCl4]−2 anions joined together and forms a quasi-one-dimensional structure. The platinum atoms are separated by 3.25 Å along the chain, and the structure is characterized by square planar (tetragonal) geometry around each platinum centre. This arrangement leads to the formation of infinite chains of platinum atoms, which is unusual among platinum compounds. This arrangement is responsible for the dark green color of this compound. The bond length of Pt–Cl is 2.34 Å in [PtCl4]−2 and of Pt–N is 2.06 Å in [Pt(NH3)4]+2
Bonding of Magnus's Green Salt
The bonding in Magnus's Green Salt involves a combination of intramolecular and intermolecular interactions, contributing to its unique properties.
Platinum(II) is coordinated by four ammonia ligands in [Pt(NH3)4]+2 and arranged as square planar geometry. Similarly, Platinum(II) is coordinated by four chloride ligands in [PtCl4]−2 and arranged as square planar geometry.
The square-planar geometry arises from the strong ligand field splitting of the d8 Pt(II) ion, with the dx2 – y2 orbital at high energy and the dz2 orbital in lower energy, facilitating potential intermolecular interactions.
The Pt---Pt (distance of 3.25 Å) interactions are due to the overlap of the 5dz2 orbitals of adjacent Pt atoms, forming a quasi-one-dimensional band structure. However, semiempirical molecular orbital studies indicate there is no significant covalent bonding between Pt atoms. Due to weak Pt–Pt interactions, Magnus's green salt shows semiconducting properties and characteristic green color, as they influence electronic transitions in the visible spectrum.
Infrared spectroscopy and structural analyses show there is no significant >N-H...Cl hydrogen bonding between the ammonia ligands and chloride ligands