CHAPTER ONE
Introduction
The chemistry of platinum and sulfur has received tremendous attention since they were discovered owing to their interesting chemical activities. Platinum earned its name only in the 18th century, although it was known since ancient times. The first known reference to platinum is contained in the writings of the Italian physician, scholar, and poet Julius Caesar Scaliger (1484-1558). Scaliger apparently saw platinum while visiting Central America in 1557. Platinum was later found in large quantities as an impurity in 1750 by Spaniards mining for silver in Rio Pinto, Colombia. It was then named “little silver” (sometimes called “white gold”). Its first complete description was given by the Spanish military leader Don Antonio de Ulloa (1716-95). While serving in South America from 1735 to 1746, de Ulloa collected samples of platinum and later wrote a report describing the metal. De Ulloa is often given credit for discovering platinum (in 1735) on the basis of the report. The name ʺplatinumʺ was derived from the Spanish platina, meaning “little silver”. Since its discovery, platinum and many of its compounds have been used in catalysis, synthetic precursors and therapeutic medicine. They have continued to gain important prominence in many areas.These include synthetic chemistry where they are employed in synthesizing novel molecular systems, for which examples include (2,2‘–bipyridine)dichloroplatinum(II) and cis–Bis-(acetonitrile)dichloroplatinum(II) used in Proton Enhanced Membrane (PEM) fuel cells. They are also used in self-assembly of supramolecular structures (Roxanne et al., 2008) e.g., {Pt(en)(4,4‘–dipyridyl)}4, as synthetic precursors e.g. cis-PtCl2(PPh3)2 (Chatt and Mingos, 1970; Ugo et al.,1971). In therapeutic medicine they are used as antitumor drugs (Ovejero et al., 2007) e.g., cis-platin, PtCl2(NH3)2 {cis-diaminedichloridoplatinum(II), used for treatment of testicular and ovarian cancers (Cotton et al.,1972) }.The glass industry uses platinum for optical fibers and liquid crystal display (or LCD) glasses, especially for laptops (B. V. Lenntech, personal writing on platinum).
Platinum has six isotopes, with 194, 195 and 196 as the most abundant ones. It exists in variable oxidation states ranging from 0 to VI, although several barium palatinates have been synthesized in which the platinum exhibits negative oxidation states ranging from –1 to –2, e.g., BaPt, Ba3Pt2 and Ba2Pt (Karpov, Konuma and Jansen, 2006). Jansen (2005) had previously shown that cesium platinate contains Pt2– anions. Complexes of certain carbonyl anions with the general formula [Pt3(CO)3(µ-CO)3]n2- possess formal negative oxidation states. Jansen (2005) reiterated that the negative oxidation states exhibited by platinum are however unusual for metallic elements, although they are attributed to the relativistic stabilization of the 6s orbitals. Oxidation states of II and IV are however most common. As expected, tetracoordinated Pt(II) complexes tend to adopt 16-electron square planar geometries. The I and III oxidation states are less common and are often stabilized by metal bonding in bimetallic (or polymetallic) species. Zerovalent platinum complexes usually possess a tetrahedral structure and are stabilized by phosphine ligands e.g.,{Pt(PPh3)3} and {Pt[P(OR)3]4}. The tetracarbonyl complex, {Pt(CO)4} does not exist unlike the nickel analog although {Pt(PF3)4} and {Pt[PF(CF3)2]4} have been synthesized and characterized (Cotton et al.,1972). Complexes of Pt0 have been employed in catalysis, especially in C–C coupling reactions because of the ease with which they undergo oxidative addition reactions. Low-valence platinum forms numerous clusters, usually based on the Pt3 triangle, as in the carbonyl phosphine complexes (Figure 1.0). The cluster, [Pt15(CO)30]2– catalyzes the hydrogenation of MeCN, PhCHO and other organic substances. The higher states V and VI are seen only in a few fluoro compounds (Cotton et al., 1972).
Leave a Reply
You must be logged in to post a comment.