1 Introduction

The chemistry of platinum is particularly important in the arias of catalysis and biochemistry. The high affinity of platinum atom for phosphorus enables to find effectively to organophosphines. There are numerous published structural studies of platinum complexes. In our previous reviews (Holloway and Melnik, 2002, 2003, 2004a, 2004b, 2005) classified and analyzed almost 2000 platinum complexes.

Stereoselectivity in platinum coordination compounds is very often related to important stereosepecifity of biological systems, catalysis and stereochemical effects in technical processes. Isomers can be broadly classis into two major categories, structural and stereoisomers. The former can be divided into coordination, hydrate, ionization, linkage and polymerization sub-categories, and the latter can be divided into geometric (cis- trans, fac- mer), optical, ligand and distortion isomerism. The huge area of platinum coordination chemistry has been surveyed (Holloway and Melnik, 2002, 2003, 2004a, 2004b, 2005) with over 200 isomeric examples noted. In this review we analyzed and classified structural data for Pt(0)P₃, Pt(0)P₄ and Pt(II)P₄ derivatives.

2 Distortion isomerism

The coexistence of two or more species differing only by degree of distortion of M–L bond distances and L–M–L bond angles is typical of the general class of distortion isomers (Melnik, 1982).

Crystallographic and structural data for Pt(0)P₃ and Pt(0)P₄ distortion isomers are gathered in Table 1. The [Pt(PPh₃)₃] complex exists in two isomeric forms, triclinic (Albano et al., 1966) and monoclinic (Chaloner et al., 1989). Structure of monoclinic [Pt(PPh₃)₃] (Chaloner et al., 1989) is shown in Figure 1 as an example. In each of the isomer a bulky unidentate PPh₃ ligands create an approximately trigonal geometry about the Pt(0) atom. The Pt–P bond distances range from 2.25 to 2.28 Å (Albano et al., 1966) and from 2.261(2) to 2.271(2) Å (Chaloner et al., 1989), and the P–Pt–P bond angles range from 115° to 122°, and from 117.20(5)° to 128.80(0)°, respectively.

Figure 1

Structure of [Pt(PPh₃)₃] (Chaloner et al., 1989).

In the two monoclinic [Pt{η²-Ph₂P(CH₂)₃PPh₂}₂] (Asker et al., 1990; Harvey et al., 1988) isomers a pair of bidentate bis(diphenylphosphine)propane ligands create a distorted tetrahedral geometry (PtP₄) about the Pt(O) atom. Structure of monoclinic [Pt{η₂-Ph₂P(CH₂)₃PPh₂}₂] (Asker et al., 1990) is shown in Figure 2. Each bidentate P,P’-donor ligand forms six-membered metallocyclic ring (PC₃P) with the mean values of P–Pt–P bite angles of 97.2° (Asker et al., 1990) and 97.9° (Harvey et al., 1988). The sum of four Pt–P bond distances are growing in the sequence: 9.15 Å (Asker et al., 1990) < 9.17 Å (Harvey et al., 1988).

Figure 2

Structure of monoclinic [Pt{η²-Ph₂P(CH₂)₃PPh₂}₂] (Asker et al., 1990).

Crystallographic and structural data for Pt(II)P₄ distortion isomers are summarized in Table 2. Colorless [Pt{η²-Ph₂PN(η¹-C₁₀H₁₈)P*(=(SiMe₃)₂Cl}₂] exists in three isomeric forms one monoclinic and two triclinic (Lungu et al., 2009). Structure of monoclinic cis-anti- and triclinic trans-anti-[Pt{η²-Ph₂PN(η¹-C₁₀H₁₈)P*(=(SiMe₃)₂Cl}₂] are shown in Figure 3, as an examples. The pair of bidentate P,P’-donor ligands form a distorted square-planar geometry about the Pt(II) atom (PtP₄). The pair of bidentate P,P’-donor ligands create two four-membered metallocyclic rings (PNP) about each Pt(II) atom with the mean value of P–Pt–P bite angles of 70.7°. The sum of all four Pt–P bond distances are 9.27 Å (monoclinic) and 9.24 Å (both triclinic), which indicates that the inner coordination sphere about the Pt(II) atom in monoclinic isomer is somewhat less crowded than in its triclinic isomers.

Figure 3

Structure of cis-anti- and triclinic trans-anti-[Pt{η²-Ph₂PN(η₁-C₁₀H₁₈)P(=(SiMe₃)₂Cl}₂] (Lungu et al., 2009).

Complex [Pt{η²-(Ph)(O)P(CH₂)₂P(OH)(Ph)}₂] exists in two isomeric forms triclinic syn-, and monoclinic meso- (Powell et al., 1995), and in addition the triclinic isomer contains two crystallographically independent molecules within the same crystal. The pairs of the bidentate P,P’- donor ligands create five-membered metallocyclic rings (PC₂P) about each Pt(II) atom with the mean value of P–Pt–P bite angles of 86.1°. The sum of four Pt–P bond distances growing in the order: 9.17 Å (molecule 1, triclinic) < 9.18 Å (molecule 2, triclinic) < 9.22 Å (monoclinic). The size of the inner coordination sphere about the Pt atoms increases in the given order.

In two monoclinic, meso- and rac-[Pt{η²-(Ph)(OH) P(CH₂)₂P(OH)(Ph)}₂]Cl₂ (Powell et al., 1995) a pair of bidentate ligands, both PC₂P, giving five-membered metallocyclic rings with the values of P–Pt–P angles of 84.1° and 84.5°, respectively. The mean values of Pt–P bond distances are 2.305 Å (meso-) and 2.301 Å (rac-).

In [Pt{η²-Ph₂P(CH₂)₂PPh₂}₂]X₂ (X = Cl, monoclinic) (Engelhardt et al., 1984), (X = I, triclinic) (Ferguson et al., 1993) a pair of bidentate ligands form five-membered metallocyclic rings (PC₂P) with the mean values of P–Pt–P angles of 82,0°. The mean values of Pt–P bond distances are 2.334 and 2.328 Å, respectively.

Two dissimilar bidentate ligands in tetragonal, meso-, and triclinic rac-[Pt{η²-(Me)(C₁₅H₂₃)P*(CH₂)P*(C₁₅H₂₃) (Me)}{η²-Ph₂P(CH₂)₂PPh₂}]⋅2(CF₃SO₃) (Chapp et al., 2012) build up a distorted square-planar geometry about Pt(II) atoms. One of the bidentate ligand forms four-(P*CP*) and another one forms five-(PC₂P) membered metallocyclic rings with the values of P–Pt–P angles of 73.2° and 83.6° in tetragonal isomer and 72.1° and 83.7° in triclinic.

There are two monoclinic isomers [Pt{η⁴-C₄₂H₄₂P₄)] (BPh₄)₂ (Brüggeller and Hübner, 1990; Brüggeller et al., 1992) in which tetradentate ligand forms three five-membered rings (Figure 4). The mean Pt–P bond distances of 2.258(6) Å for intra P atoms are shorter by 0.069 Å than those for inter P atoms 2.327(0) Å (Brüggeller and Hübner, 1990) as expected. The values in Brüggeller et al. (1992) are 2.275(5) Å, 0.052 Å and 2.327(5) Å.

Figure 4

Structure of [Pt{η⁴-C₄₂H₄₂P₄)] (Brüggeller and Hübner, 1990; Brüggeller et al., 1992).

3 Conclusions

The distortion isomers can be divided into several sub-groups. One example, [Pt{η²-Ph₂P(CH₂)₃PPh₂}₂] exists in two monoclinic isomeric forms (Asker et al., 1990; Harvey et al., 1988). Another one [Pt{η²-Ph₂PN(η¹-C₁₀H₁₈) P(=(SiMe₃)₂Cl}₂] also exists in three isomeric forms, two triclinic and one monoclinic (Lungu et al., 2009). The remaining examples exist in two isomeric forms with homo- as well as hetero-crystal classes. In two derivatives both isomers classes belong to the homo-monoclinic (Brüggeller and Hübner., 1990; Brüggeller et al., 1992; Powell et al., 1995). The remaining examples differ from each other not only by degree of distortion but also by crystal class. In three of these one isomer is triclinic and one monoclinic (Albano et al., 1966; Chaloner et al., 1989; Engelhardt et al., 1984; Ferguson et al., 1993; Powell et al., 1995). In one example one is tetragonal and the other monoclinic (Chapp et al., 2012).

In the isomers with Pt(0)P₃ chromophore the mean values of Pt–P bond distances as well as deviation of P–Pt–P bond angles form the ideal value of 120° (triclinic (Albano et al., 1966) vs monoclinic (Chaloner et al., 1989)) are 2.270 Å and 5° vs 2.266 Å and 2.8°, indicated that triclinic isomer is somewhat more distorted than its monoclinic partner.

The two monoclinic isomers with Pt(0)P₄ chromophore are differ not only by degree of distortion tetrahedral geometry (Table 1) but also of space groups: C2/c (Asker et al., 1990) and C2/m (Harvey et al., 1988), respectively.

In [Pt{η²-Ph₂PN(η¹-C₁₀H₁₈)P*(=(SiMe₃)₂Cl}₂] which exists in three isomeric forms (Lungu et al., 2009) the bidentate P,P*-donor ligands coordinated unsymmetrically with the mean Pt–P(Ph₂) and Pt–P*(=(SiMe₃)₂Cl) bond distances of 2.285 and 2.340 Å, respectively. Each bidentate donor ligand forms a four-membered metallocyclic ring (PNP) with the mean values of P–Pt–P bite angles of 70.7°. These rings are responsible for high-degree of distortion of square-planar geometry (Pt(II)P₄).

In another three pairs of the isomers (Engelhardt et al., 1984; Ferguson et al., 1993; Powell et al., 1995) the bidentate P,P- donor ligands create five-membered

metallocyclic rings (PC₂P) (Table 2). There are cooperative effects between the Pt–P bond distance and the value of P–Pt–P bite angle. The mean Pt–P bond distance elongate and the respective angle closes: 2.297 Å and 86.3° (Powell et al., 1995), 2.303 Å and 84.3° (Powell et al., 1995), 2.331 Å and 82.0° (Engelhardt et al., 1984; Ferguson et al., 1993).

In one pair of isomers a square-planar geometry (Pt(II)P₄) is created by dissimilar bidentate P,P donor ligands, one which creates four-(P*CP*) and another one five-(PC₂P) membered metallocycles (Chapp et al., 2012). The mean Pt–P* and Pt–P bond distances are 2.350 and 2.332 Å and the values of the respective angles are 72.6° (P*CP*) and 83.6° (PC₂P).

In remaining two monoclinic isomers (Brüggeller and Hübner, 1990; Brüggeller et al., 1992) a tetradentate ligand created three five-membered metallocycles (PC₂P) with mean P–Pt–P bond angles of 84.3°. The tetradentate ligand coordinated unsymmetrically with the mean Pt–P bond distances of 2.266 Å for intra P atoms and 2.327 Å for inter P atoms.

In transition metal complexes, the oxidation state plays a leading role in the geometry formed and platinum is not an exception. In four-coordinate Pt(0) prefer tetrahedral geometries, while Pt(II) a square planar geometries. The utility of a single metric to as seas molecular shape and degree of distortion as well as exemplified best by equations:

for square planar introduced by Yang et al. (2007).

The values of τ₄ range from 1.00 for a perfect tetrahedral geometry, since 360-2(109.5) = 41; to zero for a perfect square planar geometry, since 360-2(180) = 0. The values of τ₄ for Pt(0) are given in Table 1 and for Pt(II) in Table 2. Noticeable, the total values of the sums of four Pt–P bond distances are 9.15 Å for Pt(I0)P₄ complexes and 9.25 Å for Pt(II)P₄ complexes, indicate the inner coordination sphere is the tetrahedral Pt(0)P₄ complexes are somewhat more crowded than in Pt(II)P₄ square planar complexes.