Variable Combinations of Tridentate Ligands in Pt(η³-X₃L)(PL) Derivatives: Structural Aspects

Abstract

This review covers over fifty Pt(II) monomeric complexes with a wide combination of η³-ligands of the composition Pt(η³-X₃L)(PL), (X₃ = N₃; S₃; Te₃; ONO; CNC, SeNSe; ONC; ONS; CNS; NNC, NNS; NNSe, SOS; SBS; NON; SSO). The η³-ligand with monodentate PL displays distorted square-planar geometry about Pt(II) atoms. The structural parameters (Pt-L, L-Pt-L) are analyzed and discussed, with a particular emphasis on the distortion of square-planar geometry about Pt(II) atoms, as well as of the trans-influence. There is a relation between the membered nature of the metallocycles and the distortion of square-planar geometry about the Pt(II) atoms. The distortion increases as indicated by parameter τ₄ in the following order: 0.023 (6+6) < 0.024 (^+5) < 0.040 (5+6) < 0.062 (5+5).

1. Introduction

The chemistry of platinum is particularly important in the areas of catalysis and biochemistry. There are numerous published structural studies on platinum complexes that have been classified and analyzed [1]. The high affinity of platinum(II) ions for phosphorous enables it to bind effectively to organophosphines. Organophosphines as soft P-donor ligands are very useful for building a wide variety of platinum complexes. Recently, we classified and analyzed structural data of monomeric organoplatinum complexes with PtP₃C inner coordination spheres [2]. Another review covers structural data of monomeric Pt(II) coordination complexes with inner coordination spheres, including PtP₄, PtP₃X, and PtP₂X₂, in which P-donor ligands are monodentate organomonophosphines [3].

This study aims to correlate the following structural parameters available for Pt(η³-X₃L)(PL): (X₃ = N¹,N²,N³; S¹,S²,S³; Te¹,Te²,Te³; O¹,N¹,O²; O¹,N¹,C¹; O¹,N¹,S¹; N¹,N²,C¹; N¹,N²,S¹; N¹,N²,Se¹; N¹,C¹,N²; C¹,N¹,C²; C¹,N¹,S¹; S¹,C¹,S²; S¹,B¹,S²; S¹,S²,O¹; Se¹,N¹,Se².

2. Pt(η³-X₃L)(PL) Derivatives

There are over fifty examples in which the inner coordination spheres about the Pt(II) atoms of the Pt(η³-X₃L)(PL) type are formed by variable combinations of donor atoms of tridentate ligands. Each η³-ligand creates two metallocyclic rings. The complexes based on membered metallocyclic rings can be divided into four groups.

2.1. 6+6-Membered Metallocyclic Rings

There are only three examples in which a η³-ligand creates such rings (

Table 1. Structural data for Pt(η³-X₃)(Y) derivatives. ^a—6+6-membered metallocyclic rings.

Complex	Chromophore Chelate Rings τ4 b	Pt -L c (Å)	L-Pt-L c (°)	Ref.
[Pt(η3-C22H11F6N3O2-O1,N1,O2)(PPh3)] (at 173 K)	PtO1N1O2P (O1C3N1C3O2) 0.032	O1 1.994(2) N1 2.021(2) O2 2.004(2) P 2.256(2)	O1,N1 90.6 d N1,O2 90.2 d O1,O2 179.0 O1,P 90.6 O2,P 87.5 N1,P 177.0	[4]
[Pt(η3-C14H10N2O3-O1,N1,O2)(PPh3)] (at 150 K)	PtO1N1O2P (O1C2NN1C3O2) 0.024	O1 1.995(2) N1 2.000(2) O2 1.988(2) P 2.251(2)	O1,N1 88.2 d N1,O2 90.0 d O1,O2 176.5 O1,P 89.0 O2,P 90.7 N1,P 175.0	[5]
[Pt{η3-C12H24S3-S1,S2,S3}(PPh3)]BF4	PtS1S2S3P (S1C3S2C3S3) 0.035	S1 2.330(2) S2 2.339(2) S3 2.336(2) P 2.332(2)	S1,S2 87.1(2) d S2,S3 89.5(2) d S1,S3 176.3(2) S1,P 91.1(2) S3,P 92.3(1) S2,P 171.0(2)	[6]

(^a) Where more than one chemically equivalent distance or angle is present, the mean value is tabulated. The number in parentheses is the e.s.d. (^b) Parameter τ₄, degree of distortion. (^c) The chemical identity of the coordinated atom/ligand is specific to these columns. (^d) Six-membered metallocyclic ring.

). In [Pt(η³-C₂₂H₁₁F₆N₃O₂–O¹,N¹,O²)(PPh₃)] (at 173 K) [4], the η³-ligand forms a metallocyclic ring of the O¹C₃N¹C₃O² type with common ligating N¹ atoms. The values of the chelate L-Pt-L angles are 90.6° (O¹-Pt-N¹) and 90.2° (N¹-Pt-O²). The O¹C₂NN¹C₃O² type with the respective chelate angles of 88.2° (O¹-Pt-N¹) and 90.0° (N¹-Pt-O²) was found in [Pt(η³-C₁₄H₁₀N₂O₃–O¹,N¹,O²)(PPh₃)] (at 150 K) [5]. The remaining L-Pt-L angles open in the following order (mean values): 88.1° (O²-Pt-P) < 89.0° (O¹-Pt-P) < 176.0° (N¹-Pt-P) < 177.7° (O¹-Pt-O²). The monodentate PPh₃ displayed square-planar geometry about each Pt(II) atom. The Pt-L bond distance increased in the following order (mean values): 1.995 Å (Pt-O¹ trans to O²) < 1.996 Å (Pt-O²) < 2.010 Å (Pt-N¹) < 2.254 Å (Pt-P).

For the complex [Pt{η³-C₁₂H₂₄S₃-S¹,S²,S³}(PPh₃)]BF₄, the η³-ligand creates a pair of six-membered metallocyclic rings of the S¹C₃S²C₃S³ type (as shown in Figure 1) [6]. The values of the chelate angles are 87.1° (S¹-Pt-S²) and 89.5° (S²-Pt-S³). The remaining L-Pt-L bond angles open in the following order: 91.1° (S¹-Pt-P) < 92.3° (S³-Pt-P) < 171.0° (S²-Pt-P) < 176.3° (S¹-Pt-S³). The Pt-L bond distance increases in the following order: 2.330 Å (Pt-S¹) < 2.332 Å (Pt-P) < 2.336 Å (Pt-S³) < 2.339 Å (Pt-S² trans to P). Noticeably, the trans-X¹-Pt-X³ bond angles are somewhat bigger than the trans-X²-Pt-P bond angles (Table 1).

2.2. 6+5-Membered Metallocyclic Rings

There are five examples that will be discussed in this section, namely [Pt(η³-C₁₆H₁₄N₂OS₂–O¹,N¹,S¹)(PPh₃)] [7], [Pt(η³-C₁₆H₁₃N₃O₃S₂–O¹,N¹,S¹)(PPh₃)] (at 200K) [7], [Pt(η³-C₈H₈N₃OS–O¹,N¹,S¹)(PPh₃)] toluene [8], [Pt(η³-C₉H₉N₃OS–O¹,N¹,S¹)(PPh₃)] (at 100 K) (Figure 2) [9], and [Pt(η³-C₁₈H₁₆N₂OS₂–O¹,N¹,S¹)(PPh₃)] [7] (

Table 2. Structural data for Pt(η³-X₃)(Y) derivatives. ^a—6+5-membered metallocyclic rings.

Complex	Chromophore Chelate Rings τ4 b	Pt -L c (Å)	L-Pt-L c (°)	Ref.
[Pt(η3-C16H14N2OS2-O1,N1,S1)(PPh3)]	PtO1N1S1P (O1C3N1NCS1) 0.016	O1 1.992 N1 2.034 S1 2.245 P 2.258	O1,N1 91.2 d N1,S1 85.0 e O1,S1 176.0 O1,P 89.0 S1,P 93.1 N1,P 178.1	[7]
[Pt(η3-C16H13N3O3S2-O1,N1,S1)(PPh3)] (at 200 K)	PtO1N1S1P (O1C3N1NCS1) 0.018	O1 2.001 N1 2.041 S1 2.239 P 2.248	O1,N1 92.6 d N1,S1 85.3 e O1,S1 177.6 O1,P 89.0 S1,P 93.3 N1,P 176.0	[7]
[Pt(η3-C8H8N3OS-O1,N1,S1)(PPh3)].toluene	PtO1N1S1P (O1C3N1NCS1) 0.020	O1 2.015 N1 2.031 S1 2.234 P 2.257	O1,N1 93.1 d N1,S1 83.8 e O1,S1 176.6 O1,P 89.9 S1,P 93.3 N1,P 176.3	[8]
[Pt(η3-C9H9N3OS-O1,N1,S1)(PPh3)] (at 103 K)	PtO1N1S1P (O1C3N1NCS1) 0.024	O1 2.085 N1 2.036 S1 2.257 P 2.260	O1,N1 92.5 d N1,S1 85.1 e O1,S1 175.7 O1,P 91.5 S1,P 91.0 N1,P 175.6	[9]
[Pt(η3-C18H16N2OS2-O1,N1,S1)(PPh3)]	PtO1N1S1P (O1C3N1NCS1) 0.037	O1 2.045 N1 2.029 S1 2.246 P 2.269	O1,N1 92.3 d N1,S1 83.2 e O1,S1 173.6 O1,P 93.1 S1,P 91.2 N1,P 173.1	[7]

(^a) Where more than one chemically equivalent distance or angle is present, the mean value is tabulated. The number in parentheses is the e.s.d. (^b) Parameter τ₄, degree of distortion. (^c) The chemical identity of the coordinated atom/ligand is specific to these columns. (^d) Six-membered metallocyclic ring. (^e) Five-membered metallocyclic ring.

). In each of them, the η³-ligand creates six- and five-membered metallocyclic rings with a common ligating N¹ atom of the O¹C₃N¹NCS¹ type. The values of the respective chelate angles (mean values) are 92.3° (O¹-Pt-N¹) and 84.6° (N¹-Pt-S¹). The remaining L-Pt-L bond angles open in the following order (mean values): 90.7° (O¹-Pt-P) < 92.4° (S¹-Pt-P) < 175.8° (N¹-Pt-P) < 175.9° (O¹-Pt-S¹). Interestingly, the mean values of both trans-O¹-Pt-S¹ and N¹-Pt-P angles are equal. The Pt-L bond distance increases (mean values) in the following order: 2.028 Å (Pt-O¹ trans to S¹) < 2.035 Å (Pt-N¹ trans to P) < 2.244 Å (Pt-S¹) < 2.259 Å (Pt-P).

2.3. 5+6-Membered Metallocyclic Rings

There are four complexes mentioned in this section, namely [Pt(η³-C₁₂H₁₀N₄–N¹,N²,N³)(PPh₃)] (at 100 K) [10], [Pt(η³-C₁₃H₉NO₂–O¹,N¹,O²)(PPh₃)] [11], [Pt(η³-C₁₂H₁₆N₂O₄Se₂–Se¹,N¹,Se²){P(η¹-C₁₁H₁₉O₅)(Ph)₂}] [12], and [Pt(η³-C₂₉H₂₀F₆S₂O–S¹,S²,O¹)(PPh₃)] (at 100 K) [13], and their structural parameters are gathered in

Table 3. Structural data for Pt(η³-X₃)(Y) derivatives. ^a—5+6-membered metallocyclic rings.

Complex	Chromophore Chelate Rings τ4 b	Pt -L c (Å)	L-Pt-L c (°)	Ref.
[Pt(η3-C12H10N4-N1,N2,N3)(PPh3)] (at 100 K)	Pt N1N2N3P (N1C2N2NC2N3) 0.034	N1 1.984 N2 2.025 N3 1.964 P 2.255	N1,N2 81.7 e N2,N3 89.6 d N1,N3 170.6 N1,P 93.0 N3,P 96.3 N2,P 177.2	[10]
[Pt(η3-C13H9NO2-O1,N1,O2)(PPh3)]	Pt O1N1O2P (O1C2N1C3O2) 0.034	O1 1.975(9) N1 2.064(12) O2 1.996(9) P 2.248	O1,N1 82.4(4) e N1,O2 94.8(4) d O1,O2 176.4(4) O1,P 91.5(3) O2,P 91.5(3) N1,P 172.4	[11]
[Pt(η3-C12H16N2O4Se2-Se1,N1,Se2){P(η1-C11H19O5)(Ph)2}]	Pt Se1N1Se2 (Se1C2N1NC2Se2) 0.036	Se1 2.394 N1 2.078 Se2 2.349 P 2.259	Se1,N1 83.3 e N1,Se2 98.3 d Se1,Se3 176.3 Se1,P 87.2 Se2,P 90.7 N1,P 170.9	[12]
[Pt(η3-C29H20F6O4S2O-S1,S2,O1)(PPh3)] (at 100 K)	Pt S1S2O1P (S1C2S2C3O1) 0.059	S1 2.268 S2 2.277 O1 2.066 P 2.253	S1,S2 90.2 e S1,O1 99.2 d S1,O1 169.6 S1,P 89.2 O1,P 99.2 S2P 169.4	[13]

(^a) Where more than one chemically equivalent distance or angle is present, the mean value is tabulated. The number in parentheses is the e.s.d. (^b) Parameter τ₄, degree of distortion. (^c) The chemical identity of the coordinated atom/ligand is specific to these columns. (^d) Six-membered metallocyclic ring. (^e) Five-membered metallocyclic ring.

. The structure of [Pt(η³-C₁₂H₁₀N₄–N¹,N²,N³)(PPh₃)] [10] is shown in Figure 3 as an example. Each η³-ligand creates five and six metallocyclic rings. The donor atoms of the respective η³-ligands play a role in the size of the L-Pt-L chelate angles. These angles increase in the following sequences:

N¹C₂N²NC₂N³—81.7° (N¹-Pt-N²) and 89.6° (N²-Pt-N³);

O¹C₂N¹C₃O²—2.4° (O¹-Pt-N¹) and 94.8° (N¹-Pt-O²);

Se¹C₂N¹NC₂Se²—83.3° (Se¹-Pt-N¹) and 98.3° (N¹-Pt-Se²);

S¹C₂S²C₃O¹—90.2° (S¹-Pt-S²) and 99.2° (S²-Pt-O¹).

The monodentate PL displayed distorted square-planar geometry about Pt(II) atoms. The Pt-L bond distance to PL increased in the following order: 2.025 Å (Pt-N²) < 2.064 Å (Pt-N¹) < 2.078 Å (Pt-N¹) < 2.277 Å (Pt-S²). The order follows the above-mentioned sentence for the Pt-L (L is a common central ligating atom between five and six-rings).

2.4. 5+5-Membered Metallocyclic Rings

There are thirty-nine compounds in which each η³-ligand creates two five-membered metallocyclic rings. These complexes based on variable combinations of atoms involved in the chelate angles can be divided into twelve groups. Structural data are given in

Table 4. Structural data for Pt(η³-X₃)(Y) derivatives. ^a—5+5-membered metallocyclic rings.

Complex	Chromophore Chelate Rings τ4 b	Pt -L c (Å)	L-Pt-L c (°)	Ref.
[Pt(η3-C33H24P2S2-S1,C1,S2)(PPh3)].CH2Cl2 (at 193 K)	PtS1C1S2P (S1PCC1CPS2) 0.026	S1 2.332 C1 2.020 S2 2.316 P 2.332	S1,C1 87.9 e C1,S2 87.7 e S1,S2 173.8 S1,P 89.7 S2,P 94.2 C1,P 176.9	[14]
[Pt(η3-C12H10N2Te3-Te1,Te2,Te3)(PPh3)].C6H6	Pt Te1Te2Te3P (Te1CNTe2NCTe3) 0.029	Te1 2.588(3) Te2 2.569(3) Te3 2.612(3) P 2.283(3)	Te1,Te2 91.6(2) e Te2,Te3 91.4(2) e Te1,Te3 174.0(2) Te1,P 90.4(2) Te3,P 87.0(2) Te2,P 175.5	[15]
[Pt(η3-C10H8N2Te3-Te1,Te2,Te3)(PPh3)]	Pt Te1Te2Te3P (Te1CNTe2NCTe3) 0.044	Te1 2.594 Te2 2.574 Te3 2.572 P 2.282	Te1,Te2 92.8(1) e Te2,Te3 92.6(1) e Te1,Te3 172.7(2) Te1,P 86.1(2) Te3,P 89.3(2) Te2,P 171.4(2)	[15]
[Pt(η3-C12H9N2S2B-S1,B1,S2)(PPh3)].0.06CH2Cl2 (at 100 K)	PtS1B1S2P (S1CNB1NCS2) 0.061	S1 2.304 B1 2.192 S2 2.275 P 2.384	S1,B1 79.8 e B1,S2 85.0 e S1,S2 161.4 S1,P 99.8 S2,P 95.9 B1,P 176.7	[16]
[Pt(η3-C13H14N5S3B-S1,B1,S2)(PPh3)] (at 100 k)	PtS1B1S2P (S1CNB1NCS2) 0.067	S1 2.299 B1 2.192 S2 2.294 P 2.379	S1,B1 81.0 e B1,S2 86.5 e S1,S2 163.4 S1,P 98.8 S2,P 96.4 B1,P 172.2	[17]
[Pt(η3-C28H24N4Se2-N1,N2,Se1)(PPh3)].0.5H2O	Pt N1N2Se1P (N1CNN2NCSe1) 0.068	N1 2.020 N2 1.980 Se1 2.368 P 2.272	N1,N2 78.7 e N2,Se1 82.3 e N1,Se1 162.0 N1,P 100.0 Se1,P 98.6 N2,P 173.4	[18]
[Pt(η3-C28H24N4S2-N1,N2,S1)(PPh3)].0.5thf	PtN1N2S1P (N1CNN2NCS1) 0.076	N1 2.031 N2 1.987 S1 2.266 P 2.279	N1,N2 78.5 e N2,S1 81.6 e N1,S1 162.2 N1,P 99.9 S1,P 99.7 N2,P 172.5	[19]
[Pt(η3-C17H12N2O-O1,N1,C1)(PPh3)]	PtO1N1C1P (O1C2N1NCC1) 0.059	O1 2.189 N1 1.997 C1 2.004 P 2.254	O1,N1 80.9 e N1,C1 79.0 e O1,C1 159.9 O1,P 99.8 C1,P 100.1 N1,P 178.9	[20]
[Pt(η3-C9H9N3S-C1,N1,S1)(PPh3)]	PtC1N1S1P (C1C2N1NCS1) 0.057	C1 2.045 N1 2.032 S1 2.340 P 2.229	C1,N1 80.9 e N1,S1 81.7 e C1,S1 162.5 C1,P 96.6 S1,P 101.0 N1,P 176.4	[21]
[Pt(η3-C10H11N3S-C1,N1,S1)(PPh3)]	PtC1N1S1P (C1C2N1NCS1) 0.063	C1 2.018 N1 2.029 S1 2.335 P 2.235	C1,N1 77.9 e N1,S1 83.2 e C1,S1 161.4 C1,P 98.5 S1,P 99.9 N1,P 176.0	[22]
[Pt(η3-C12H19Cl2N3O2S3-C1,N1,S1)(PPh3)].2NH2SO	PtC1N1S1P (C1C2N1NCS1) 0.053	C1 2.038 N1 2.037 S1 2.332 P 2.236	C1,N1 80.6 e N1,S1 82.8 e C1,S1 163.6 C1,P 97.0 S1,P 99.4 N1,P 177.2	[23]
[Pt(η3-C9H7Cl2N3S-C1,N1,S1){P(η3-C6H12N3)}] (at 173 K)	PtC1N1S1P (C1C2N1NCS1) 0.077	C1 2.027 N1 2.035 S1 2.332 P 2.240	C1,N1 80.7 e N1,S1 83.0 e C1,S1 163.7 C1,P 98.6 S1,P 97.6 N1,P 168.2	[23]
[Pt(η3-C20H31Cl2N4S2-N1,N2,S1)(PPh3)].EtOH (at 100 K)	PtN1N2S1P (N1C2N2NCS1) 0.072	N1 2.042 N2 2.000 S1 2.270 P 2.268	N1,N2 79.0 e N2,S1 82.0 e N1,S1 161.0 N1,P 100.0 S1,P 96.0 N2,P 173.0	[24]
[Pt(η3-C18H12N2O-O1,N1,C1)(PPh3)].MeCN	PtO1N1C1P (O1CNN1C2C1) 0.066	O1 2.136 N1 1.995 C1 2.025 P 2.244	O1,N1 76.0 e N1,C1 81.8 e O1,C1 157.7 O1,P 103.2 C1,P 99.1 N1,P 170.4	[25]
[Pt(η3-C12H21N2-N1,C1,N2){P(C6H4SO3)3}]2-[Pt(η3-C12H21N6)(H2O)2] (at 150 K)	PtN1C1N2P (N1C2C1C2N2) 0.065	N1 2.130 C1 1.971 N2 2.131 P 2.363	N1C1 80.5 e C1,N2 79.9 e N1,N2 159.8 N1,P 102.6 N2,P 97.6 C1,P 176.8	[26]
[Pt(η3-C30H35N5-N1,N2,N3)(PPh3)] (at 223 K)	PtN1N2N3P (N1C2N2C2N3) 0.067	N1 2.009 N2 2.022 N3 2.010 P 2.243	N1,N2 79.1 e N2,N3 78.8 e N1,N3 157.5 N1,P 97.6 N3,P 105.9 N2,P 175.7	[27]
[Pt(η3-C11H3F6N7-N1,N2,N3){P(Me)Ph2}] (at 223 K)	PtN1N2N3P (N1C2N2C2N3) 0.068	N1 2.005 N2 2.032 N3 2.006 P 2.256	N1,N2 78.6 e N2,N3 79.2 e N1,N3 157.7 N1,P 100.0 N3,P 102.2 N2,P 177.9	[28]
[Pt(η3-C15H11N3-N1,N2,N3){P(η1-C14H19O5)Ph2}].2SO3CF3. 2Me2CO (at 223 K)	PtN1N2N3P (N1C2N2C2N3) 0.068	N1 1.998 N2 2.000 N3 2.097 P 2.287	N1,N2 79.8 e N2,N3 79.5 e N1,N3 158.7 N1,P 97.9 N3,P 103.0 N2,P 176.6	[12]
[Pt(η3-C17H11N-C1,N1,C2) {P(η1-C14H9O5)(Ph)2}].CH2Cl2 (at 100 K)	PtC1N1C2P (C1C2N1C2C2) 0.086	C1 2.082 N1 2.033 C2 2.083 P 2.227	C1,N1 79.4 e N1,C2 80.1 e C1,C2 157.3 C1,P 100.6 C2,P 101.0 N1,P 171.8	[29]
[Pt(η3-C25H19N5-N1,N2,N3)(PPh3)].3CH2Cl2 (at 223 K)	PtN1N2N3P (N1C2N2C2N3) 0.070	N1 2.015 N2 2.017 N3 1.996 P 2.253	N1,N2 79.2 e N2,N3 79.3 e N1,N3 158.4 N1,P 96.9 N3,P 104.6 N2,P 176.2	[30]
[Pt(η3-C29H33N7-N1,N2,N3)(PPh3)].CH2Cl2 (at 223 K)	PtN1N2N3P (N1C2N2C2N3) 0.070	N1 2.012 N2 2.036 N3 2.009 P 2.244	N1,N2 78.9 e N2,N3 78.9 e N1,N3 157.7 N1,P 99.9 N3,P 102.2 N2,P 177.1	[30]
[Pt(η3-C15H11N3-N1,N2,N3)(PPh3)].2SO3CF3 f (at 173 K)	PtN1N2N3P (N1C2N2C2N3) 0.070	N1 2.043 N2 1.978 N3 2.052 P 2.276	N1,N2 79.6 e N2,N3 80.0 e N1,N3 159.2 N1,P 101.8 N3,P 98.8 N2,P 175.4	[31]
	PtN1N2N3P (N1C2N2C2N3) 0.082	N1 2.057 N2 1.975 N3 2.040 P 2.288	N1,N2 79.9 e N2,N3 79.3 e N1,N3 158.5 N1,P 103.2 N3,P 98.1 N2,P 172.0
[Pt(η3-C22H15N7O-N1,N2,N3)(PPh3)] (at 223 K)	PtN1N2N3P (N1C2N2C2N3) 0.072	N1 2.006 N2 2.033 N3 2.012 P 2.269	N1,N2 78.0 e N2,N3 78.3 e N1,N3 156.3 N1,P 101.2 N3,P 102.4 N2,P 177.8	[27]
[Pt(η3-C11H3F6N7-N1,N2,N3)(PPh3)] (at 223 K)	PtN1N2N3P (N1C2N2C2N3) 0.076	N1 2.027 N2 2.037 N3 2.012 P 2.275	N1,N2 78.3 e N2,N3 78.5 e N1,N3 156.8 N1,P 97.7 N3,P 105.9 N2,P 175.7	[27]
[Pt(η3-C12H6F6N7O-N1,N2,N3)(PPh3)] (at 223 K)	PtN1N2N3P (N1C2N2C2N3) 0.077	N1 2.015 N2 2.031 N3 2.035 P 2.263	N1,N2 78.0 e N2,N3 78.4 e N1,N3 156.4 N1,P 97.6 N3,P 105.9 N2,P 175.7	[27]
[Pt(η3-C17H21N7-N1,N2,N3)(PPh3)] (at 223 K)	PtN1N2N3P (N1C2N2C2N3) 0.078	N1 2.022 N2 2.021 N3 2.010 P 2.263	N1,N2 78.5 e N2,N3 78.7 e N1,N3 157.0 N1,P 101.0 N3,P 101.9 N2,P 174.8	[27]
[Pt(η3-C18H23N7O-N1,N2,N3)(PPh3)] (at 223 K)	PtN1N2N3P (N1C2N2C2N3) 0.081	N1 2.016 N2 2.026 N3 2.008 P 2.256	N1,N2 78.4 e N2,N3 78.5 e N1,N3 156.9 N1,P 98.4 N3,P 104.6 N2,P 173.9	[27]
[Pt(η3-C14H10N3-N1,N2,C1)(PPh3)].ClO4 (at 173 K)	PtN1N2C1P (N1C2N2C2C1) 0.065	N1 2.124 N2 2.022 C1 2.018 P 2.243	N1,N2 78.2 e N2,C1 80.6 e N1,C1 158.8 N1,P 103.2 C1,P 97.1 N2,P 177.5	[32]
[Pt(η3-C14H10N3-N1,N2,C1)(PPh3)].ClO4 (at 193 K)	PtN1N2C1P (N1C2N2C2C1) 0.068	N1 2.101 N2 2.025 C1 2.005 P 2.227	N1,N2 78.5 e N2,C1 81.1 e N1,C1 159.1 N1,P 103.0 C1,P 97.6 N2,P 175.8	[32]
[Pt(η3-C14H10N3-N1,N2,C1)(PPh3)].ClO4 (at 295 K)	PtN1N2C1P (N1C2N2C2C1) 0.079	N1 2.130 N2 2.029 C1 2.005 P 2.242	N1,N2 79.0 e N2,C1 81.0 e N1,C1 157.4 N1,P 101.8 C1,P 100.6 N2,P 174.1	[33]
[Pt(η3-C24H20N3-N1,N2,C1)(PPh3)].ClO4 (at 113 K)	PtN1N2C1P (N1C2N2C2C1) 0.072	N1 2.147 N2 2.022 C1 2.022 P 2.246	N1,N2 77.8 e N2,C1 80.9 e N1,C1 158.2 N1,P 106.1 C1,P 97.3 N2,P 175.8	[34]
[Pt(η3-C28H23N2-N1,N2,C1)(Pcy3)].ClO4. 2MeCN (at 253 K)	PtN1N2C1P (N1C2N2C2C1) 0.120	N1 2.103 N2 2.023 C1 2.054 P 2.283	N1,N2 78.5 e N2,C1 79.7 e N1,C1 156.4 N1,P 101.6 C1,P 102.4 N2,P 160.3	[35]
[Pt(η3-C17H9F2N-C1,N1,C2) {P(o-tolyl)3}].CHCl3 (at 150 K)	PtC1N1C2P (C1C2N1C2C2) 0.066	C1 2.085 N1 2.024 C2 2.094 P 2.255	C1,N1 79.8 e N1,C2 79.5 e C1,C2 158.9 C1,P 103.1 C2,P 97.5 N1,P 177.3	[36]
[Pt(η3-C18H11F2N-C1,N1,C2) {P(CH2Et)3}] (at 150 K)	PtC1N1C2P (C1C2N1C2C2) 0.072	C1 2.056 N1 2.027 C2 2.074 P 2.299	C1,N1 79.9 e N1,C2 80.5 e C1,C2 159.8 C1,P 102.1 C2,P 97.7 N1,P 174.2	[37]
[Pt(η3-C20H15NO2-C1,N1,C2)(PPh3)] (at 150 K)	PtC1N1C2P (C1C2N1C2C2) 0.083	C1 2.075 N1 2.007 C2 2.073 P 2.252	C1,N1 79.8 e N1,C2 80.4 e C1,C2 159.0 C1,P 93.2 C2,P 103.2 N1,P 171.0	[38]
[Pt(η3-C18H11F2N- C1,N1,C2){P(CH2Ph)3}].1.4 CHCl3 (at 150 K)	PtC1N1C2P (C1C2N1C2C2) 0.087	C1 2.062 N1 2.029 C2 2.071 P 2.241	C1,N1 79.9 e N1,C2 80.2 e C1,C2 158.8 C1,P 102.2 C2,P 98.7 N1,P 169.8	[39]
[Pt(η3-C17H9F2N- C1,N1,C2)(PMe3)] (at 150 K)	PtC1N1C2P (C1C2N1C2C2) 0.087	C1 2.070 N1 2.005 C2 2.065 P 2.243	C1,N1 79.5 e N1,C2 80.2 e C1,C2 158.5 C1,P 92.0 C2,P 102.5 N1,P 170.0	[40]
[Pt(η3-C17H11N- C1,N1,C2)(PPh3)] (at 100 K)	PtC1N1C2P (C1C2N1C2C2) 0.087	C1 2.073 N1 2.049 C2 2.074 P 2.241	C1,N1 79.7 e N1,C2 80.2 e C1,C2 158.8 C1,P 103.0 C2,P 98.7 N1,P 169.8	[41]
[Pt(η3-C17H9F2N- C1,N1,C2){P(CH2Ph)3}] (at 150 K)	PtC1N1C2P (C1C2N1C2C2) 0.089	C1 2.075 N1 2.033 C2 2.085 P 2.228	C1,N1 79.8 e N1,C2 80.0 e C1,C2 159.2 C1,P 103.8 C2,P 96.9 N1,P 168.7	[36]

(^a) Where more than one chemically equivalent distance or angle is present, the mean value is tabulated. The number in parentheses is the e.s.d. (^b) Parameter τ₄, degree of distortion. (^c) The chemical identity of the coordinated atom/ligand is specific to these columns. (^e) Five-membered metallocyclic ring. (^f) There are two crystallographically independent molecules.

.

The structure of [Pt(η³-C₃₃H₂₄P₂S₂–S¹,C¹,S²)(PPh₃)].CH₂Cl₂ [14] is shown in Figure 4. The η³-ligand creates two five-membered metallocyclic rings with a common C¹ atom of the S¹PCC¹CPS² type with chelate angles of 87.9° (S¹-Pt-C¹) and 87.7° (C¹-Pt-S²). This is the only example of this type. The PPh₃ demonstrated distorted square-planar geometry about Pt(II) atoms. The remaining L-Pt-L bond angles open in the following order: 89.7° (S¹-Pt-P) < 94.2° (S²-Pt-P) < 173.8° (S¹-Pt-S²) < 176.9° (C¹-Pt-P). The Pt-L bond distance increases in the following order: 2.020 Å (Pt-C¹) < 2.316 Å (Pt-S²) < 2.332 Å (Pt-S¹) < 2.322 Å (Pt-P).

In another two complexes, namely [Pt(η³-C₁₂H₁₂N₂Te₃–Te¹,Te²,Te³)(PPh₃)].C₆H₆ and [Pt(η³-C₁₀H₈N₂Te₃–Te¹,Te²,Te³)(PPh₃)] [15], which are isostructural, the η³-ligand creates a pair of five-membered metallocyclic rings with common central ligating Te² atoms of the Te¹CNTe²NCTe³ type. The mean values of the respective angles are 92.2 (±6)° (Te¹-Pt-Te²) and 92.0 (±6)° (Te²-Pt-Te³). The PPh₃ ligand demonstrated distorted square-planar geometry about each Pt(II) atom. The remaining L-Pt-L bond angles open in the following order (mean values): 88.1 (±1.2)° (Te³-Pt-P) ~ 88.1 (±2.31)° (Te¹-Pt-P) < 173.3 (±8)° (Te¹-Pt-Te³) < 173.4 (±2.1)° (Te²-Pt-P). The Pt-L bond distance increases in the following order (mean values): 2.283 (±1) Å (Pt-P) < 2.571 (±2) Å (Pt-Te²) < 2.591 (±3) Å (Pt-Te¹) < 2.592 (±20) Å (Pt-Te³).

In another two complexes, namely Pt(η³-C₁₂H₉N₂S₂B–S¹,B¹,S²)(PPh₃)].0.06CH₂Cl₂ [16] and Pt(η³-C₁₃H₁₄N₅S₃B–S¹,B¹,S²)(PPh₃)] [17], each η³-ligand creates a pair of five-membered metallocyclic rings with a common central ligating B¹ atom of the S¹CNB¹NCS² type. The values of the respective chelate angles are (mean values): 80.4 (±6)° (S¹-Pt-B¹) and 85.7 (±8)° (B¹-Pt-S²). The PPh₃ demonstrated distorted square-planar geometry about the Pt(II) atom. The remaining L-Pt-L bond angles open in the following order (mean values): 95.9 (±5)° (S²-Pt-P) < 99.3 (±5)° (S¹-Pt-P) < 162.4 (±1.0)° (S¹-Pt-S²) < 174.4 (±2.2)° (B¹-Pt-P). The Pt-L bond distance increases in the following order (mean values): 2.110 (±19) Å (Pt-B¹) < 2.284 (±10) Å (Pt-S²) < 2.301 (±3) Å (Pt-S¹) < 2.382 (±2) Å (Pt-P).

Distorted square-planar geometry about the Pt(II) atoms in [Pt(η³-C₂₈H₂₄N₄Se₂–N¹,N²,Se¹)(PPh₃)].0.5H₂O [18] was achieved by η³-ligands with PPh₃. The η³-ligand forms a pair of five-membered metallocyclic rings with a common central ligating N² atom of the N¹CNN²NCSe¹ type. The values of the respective chelate angles are 78.7° (N¹-Pt-N²) and 82.3° (N²-Pt-Se¹). The remaining L-Pt-L bond angles increase in the following sequence: 98.6° (Se¹-Pt-P) < 100.0° (N¹-Pt-P) < 162.0° (N¹-Pt-S¹) < 173.4° (N²-Pt-P). The Pt-L bond distance increases in the order 1.980 Å (Pt-N²) < 2.020 Å (Pt-N¹) < 2.272 Å (Pt-P) < 2.368 Å (Pt-Se¹).

In [Pt(η³-C₂₈H₂₄N₄S₂–N¹,N²,S¹)(PPh₃)].0.5thf [19], the η³-ligand creates two five-membered metallocyclic rings with common central ligating N² atoms of the N¹CNN²NCS¹ type with the chelate angles of 78.5° (N¹-Pt-N²) and 81.6° (N²-Pt-S¹). The PPh₃ demonstrated an inner coordination sphere about the Pt(II) atoms. The remaining L-Pt-L bond angles open in the following order: 99.7° (S¹-Pt-P) < 99.9° (N¹-Pt-P) < 162.2° (N¹-Pt-S¹) < 172.5° (N²-Pt-P). The Pt-L bond distance increases in the following sequence: 1.987 Å (Pt-N²) < 2.031 Å (Pt-N¹) < 2.266 Å (Pt-S¹) < 2.279 Å (Pt-P).

Distorted square-planar geometry about the Pt(II) atoms in [Pt(η³-C₁₇H₁₂N₂O–O¹,N¹,C¹)(PPh₃)] [20] was achieved by η³-ligands and PPh₃. The η³-ligand creates two five-membered metallocycles with a common central ligating N¹ atom of the O¹C₂N¹NCC¹ type with the chelate angles of 80.9° (C¹-Pt-N¹) and 79.0° (N¹-Pt-C¹). The remaining L-Pt-L bond angles open in the following order: 99.8° (O¹-Pt-P) < 100.1° (C¹-Pt-P) < 159.9° (O¹-Pt-C¹) < 178.9° (N¹-Pt-P). The Pt-L bond distance increases in the following sequence: 1.997 Å (Pt-N¹) < 2.004 Å (Pt-C¹) < 2.109 Å (Pt-O¹) < 2.254 Å (Pt-P).

There are four complexes, namely [Pt(η³-C₉H₉N₃S–C¹,N²,S¹)(PPh₃)] [21], [Pt(η³-C₁₀H₁₁N₃S–C¹,N¹,S¹)(PPh₃)] [22], [Pt(η³-C₁₂H₁₉Cl₂N₃O₂S₃–C¹,N¹,S¹)(PPh₃)].2Me₂SO [23], and [Pt(η³-C₉H₇Cl₂N₃S–C¹,N¹,S¹){(η³-C₆H₁₂N₃)}] [23], in which each η³-C¹N¹S¹ donor ligand creates two five-membered metallocycles with common central ligating N¹ atoms of the C¹C₂N¹NCS¹ type. The mean values of the chelate angles are 80.1 (±2.2)° (C¹-Pt-N¹) and 82.2 (±1.0)° (N¹-Pt-S¹). The remaining L-Pt-L bond angles open in the following order (mean values): 97.2 (±1.1)° (C¹-Pt-P) < 98.8 (±2.2)° (S¹-Pt-P) < 162.8 (±1.4)° (C¹-Pt-S¹) < 174.5 (±5.9)° (N¹-Pt-P). The Pt-L bond distance increases in the following order (mean values): 2.032 (±14) Å (Pt-C¹) < 2.033 (±4) Å (Pt-N¹) < 2.229 (±6) Å (Pt-P) < 2.332 (±8) Å (Pt-S¹).

In [Pt(η³-C₂₀H₃₁N₄S₂–N¹,N²,S¹)(PPh₃)].EtOH [24], the η³-ligand forms two five-membered metallocycles with common central ligating N² atoms of the N¹C₂N²NCS¹ type, with the chelate angles of 79.0° (N¹-Pt-N²) and 82.0° (N²-Pt-S¹). The remaining L-Pt-L angles open in the following order: 96.0° (S¹-Pt-P) < 100.0° (N¹-Pt-P) < 161.0° (N¹-Pt-S¹) < 173.0° (N²-Pt-P). The Pt-L bond distance increases in the following order: 2.000 Å (Pt-N²) < 2.042 Å (Pt-N¹) < 2.268 Å (Pt-P) < 2.270 Å (Pt-S¹).

Distorted square-planar geometry in [Pt(η³-C₁₈H₁₂N₂O–O¹,N¹,C¹)(PPh₃)].MeCN [25] was achieved by η³-ligands with PPh₃. The η³-ligand forms two five-membered metallocycles with a common central ligating N¹ atom of the O¹CNN¹C₂C¹ type with the chelate angles of 76.0° (O¹-Pt-N¹) and 81.8° (N¹-Pt-S¹). The remaining angles open in the following sequence: 99.1° (C¹-Pt-P) < 103.2° (O¹-Pt-P) < 157.7° (O¹-Pt-C¹) < 176.4° (N¹-Pt-P). The Pt-L bond distance increases in the following order: 1.995 Å (Pt-N¹) < 2.025 Å (Pt-C¹) < 2.138 Å (Pt-O¹) < 2.244 Å (Pt-P).

The structure of [Pt(η³-C₁₂H₂₁N₂–N¹,C¹,N²){P(C₆H₄SO₃)₃}]^2-[Pt(η³-C₁₂H₂₁N₆)(H₂O)]₂ consists of complex anions and complex cations [26]. In a complex anion, a η³-ligand via N¹,C¹,N² atoms with PL ligands achieves distorted square-planar geometry about Pt(II) atoms. This η³-ligand creates a pair of five-membered metallocycles with a common central ligating C¹ atom of the N¹C₂C¹C₂N² type. The values of the chelate angles are 80.5° (N¹-Pt-C¹) and 79.9° (C¹-Pt-N²). The remaining L-Pt-L bond angles open in the following sequence: 97.6° (N²-Pt-P) < 102.6° (N¹-Pt-P) < 159.8° (N¹-Pt-N²) < 176.8° (C¹-Pt-P). The Pt-L bond distance increases in the following order: 1.971 Å (Pt-C¹) < 2.130 Å (Pt-N¹) < 2.131 Å (Pt-N²) < 2.363 Å (Pt-P).

In the following eleven complexes, each η3-ligand creates a pair of five-membered metallocycles with a common central ligating N2 atom of the N1C2N2C2N3 type: [Pt(η³-C₃₀H₃₅N₅–N¹,N²,N³)(PPh₃)] (Sanning et al. 2015) [27], [Pt(η³-C₁₁H₃F₆N₇–N¹,N²,N³){P(Me)(Ph₂)}] [28], [Pt(η³-C₁₅H₁₁N₃–N¹,N²,N³){P(η¹-C₁₄H₁₉O₅)Ph₂}].2SO₃CF₃.2MeCO [29], [Pt(η³-C₂₅H₁₉N₅–N¹,N²,N³)(PPh₃)].3CH₂Cl₂ [30], [Pt(η³-C₂₉H₃₃N₇–N¹,N²,N³)(PPh₃)].CH₂Cl₂ [30], [Pt(η³-C₁₅H₁₁N₃–N¹,N²,N³)(PPh₃)].2SO₃CF₃ [31], [Pt(η³-C₂₂H₁₅N₇O–N¹,N²,N³)(PPh₃)] [27], [Pt(η³-C₁₁H₃F₆N₇–N¹,N²,N³)(PPh₃)] [27], [Pt(η³-C₁₂H₆F₆N₇O–N¹,N²,N³)(PPh₃)] [27], [Pt(η³-C₁₇H₂₁N₇–N¹,N²,N³)(PPh₃)] [27], and [Pt(η³-C₁₈H₂₃N₇O–N¹,N²,N³)(PPh₃)] [27]. The monodentate PL displayed distorted square-planar geometry about each Pt(II) atom. The total mean values of the chelate angles are 78.9 (±1.0)° (N¹-Pt-N²) and 79.0 (±1.0)° (N²-Pt-N³). The remaining L-Pt-L bond angles open in the following order (total mean values): 99.5 (±3.0)° (N¹-Pt-P) < 102.9 (±2.8)° (N³-Pt-P) < 157.6 (±1.1)° (N¹-Pt-N³) < 175.7 (±1.5)° (N²-Pt-P). The Pt-L bond distance increases in the following order (total mean values): 2.012 (±20) Å (Pt-N¹) < 2.012 (±32) Å (Pt-N²) < 2.025 (±32) Å (Pt-N³) < 2.264 (±24) Å (Pt-P).

There are five complexes, namely [Pt(η³-C₁₄H₁₀N₃–N¹,N²,C¹)(PPh₃)].ClO₄ (at 173 K) [32], [Pt(η³-C₁₄H₁₀N₃–N¹,N²,C¹)(PPh₃)].ClO₄ (at 193K) [32], [Pt(η³-C₁₄H₁₀N₃–N¹,N²,C¹)(PPh₃)].ClO₄ (at 245 K) [33], [Pt(η³-C₂₄H₂₀N₃–N¹,N²,C¹)(PPh₃)].ClO₄ [34], and [Pt(η³-C₂₈H₂₃N₂–N¹,N²,C¹)(Pcy₃)].ClO₄.MeCN [35], in which each η³-ligand creates a pair of five-membered metallocycles with a common central ligating N² atom of the N¹C₂N²C₂C¹ type. The structure f [Pt(η³-C₁₄H₁₀N₃–N¹,N²,C¹)(PPh₃)] [32] is shown in Figure 5. The total mean values of the respective chelate angles are 78.4 (±0.6)° (N¹-Pt-N²) and 80.7 (±1.0)° (N²-Pt-C¹). The remaining L-Pt-L bond angles open in the following order (total mean values): 99.0 (±3.5)° (C¹-Pt-P) < 103.1 (±2.5)° (N¹-Pt-P) < 158.0 (±1.4)° (N¹-Pt-N²) < 172.7 (±5.5)° (N²-Pt-P). The Pt-L bond distance increases in the following order (total mean values): 2.021 (±30) Å (Pt-C¹) < 2.030 (±24) Å (Pt-N²) < 2.121 (±18) Å (Pt-N¹) < 2.242 (±32) Å (Pt-P).

In the following eight complexes, each η3-ligand creates a pair of five-membered metallocycles with a common central ligating N1 atom of the C1C2N1C2C2 type: [Pt(η³-C₁₇H₁₁N–C¹,N¹,C²){P(η¹-C₁₄H₁₉O₅)(Ph)₂}].CH₂Cl₂ [29], [Pt(η³-C₁₇H₉F₂N–C¹,N¹,C²){P(η¹-o-tolyl)₃}].CHCl₃ (at 150 K) [36], [Pt(η³-C₁₇H₉F₂N–C¹,N¹,C²){P(CH₂Ph)₃}] (at 150 K) [36], [Pt(η³-C₁₈H₁₁F₂N–C¹,N¹,C²){P(CH₂Et)₂}] (at 150K) [37], [Pt(η³-C₂₀H₁₅NO₂–C¹,N¹,C²)(PPh₃)] (at 150 K) [38], [Pt(η³-C₁₈H₁₁F₂N–C¹,N¹,C²){P(CH₂Ph)₃}].CHCl₃ (at 150 K) [39], [Pt(η³-C₁₂H₁₁F₂N–C¹,N¹,C²)(PMe₃)] (at 150 K) [40], and [Pt(η³-C₁₇H₁₁N–C¹,N¹,C²)(PPh₃)] (Figure 6) (at 100 K) [41]. The values of the respective chelate angles are 79.7 (±3)° (C¹-Pt-N¹) and 80.1 (±3)° (N¹-Pt-C²). The remaining L-Pt-L bond angles open in the following order: 99.5 (±4.2)° (C²-Pt-P) < 100.0 (±4.5)° (C¹-Pt-P) < 158.5 (±1.4)° (C¹-Pt-C²) < 171.5 (±4.5)° (N¹-Pt-P). The Pt-L bond distance increases in the following order (total mean values): 2.026 (±21) Å (Pt-N¹) < 2.068 (±17) Å (Pt-C¹) < 2.078 (±6) Å (Pt-C²) < 2.249 (±32) Å (Pt-P).

Each η³-ligand creates two five-membered metallocyclic rings. The donor atoms of the respective η³-ligands play a role in the size of the L-Pt-L chelate angles. These angles increase in the following sequences:

N¹C₂N²C₂N³: 78.8° (N¹C₂N²), 78.9°(N²C₂N³) [12,26,27,28,30,31]

O¹CNN¹C₂C¹: 76.0° (O¹CNN¹), 81.8° (N¹C₂C¹) [25]

N¹C₂N²C₂C¹: 78.4° (N¹C₂N²), 80.2° (N²C₂C¹) [32,33,34,35]

C¹C₂N¹C₂C²: 79.7° (C¹C₂N¹), 80.2° (N¹C₂C²) [29,36,37,38,39,40,41]

N¹CNN²NCS¹: 78.5° (N¹CNN²), 81.6° (N²NCS¹) [19]

N¹C₂C¹C₂N²: 80.5° (N¹C₂C¹), 79.9° (C¹C₂N²) [18]

N¹CNN²NCSe¹: 78.7° (N¹CNN²), 82.3° (N²NCSe¹) [26]

O¹CNN¹NCC¹: 80.9° (O¹CNN¹), 79.0° (N¹NCC¹) [20]

N¹C₂N²NCS¹: 79.0° (N¹C₂N²), 82.0°(N²NCS¹) [24]

C¹C₂N¹NCS¹: 77.9° (C¹C₂N¹), 83.2°(N¹NCS¹) [21,22,23]

S¹CNB¹NCS²: 79.8° (S¹CNB¹), 85.0° (B¹NCS²) [16,17]

S¹PCC¹CPS²: 87.9° (S¹PCC¹), 87.7° (C¹CPS²) [14]

Te¹CNTe²NCTe³: 91.6° (Te¹CNTe²), 91.4° (Te²NCTe³) [15]

3. Conclusions

This review includes over fifty monomeric Pt(II) coordination complexes with the composition of Pt(η³-X₃L)(PL). There are 14 types of η³-ligand coordination via donor atoms (X₃ = N¹N²N³; S¹S²S³; Te¹Te²Te³; O¹N¹O²; C¹N¹C²; Se¹N¹Se²; O¹N¹C¹; O¹N¹S¹; C¹N¹S¹; N¹N²C¹, N¹N²S¹; N¹N²Se¹; S¹O¹S²; S¹B¹S²; N¹O¹N²; S¹S²O¹) and each η³-ligand with monodentate PL displays distorted square-planar geometry about each Pt(II) atom.

Each tridentate ligand creates two metallocyclic rings. Based on the metallocycles, these complexes can be divided into the following four groups with 6+6-; 6+5-; 5+6-; and 5+5-membered metallocyclic rings:

• 6+6-membered metallocycles of the O¹C₃N¹C₃O²; O¹C₂NN¹C₃O² and S¹C₃S²C₃S³ types with common N¹ and S² atoms (three examples) (Table 1);
• 6+5-membered metallocycles of the O¹C₃N¹NCS¹ type with common central ligating N¹ atoms (five examples) (Table 2);
• 5+6-membered metallocycles of the O¹C₂N¹C₃O²; N¹C₂N²NC₂N³; Se¹C₂N¹NC₂Se² and S¹C₂S²C₃O¹ types with common central N¹, ligating N² and S² atoms (four examples) (Table 3);
• 5+5-membered metallocycles of the S¹PCC¹CPS²; Te¹CNTe²NCTe³ (2 examples); S¹CNB¹NCS² (2 examples); N¹CNN²NCSe¹; N¹CNN²NCS¹; O¹C₂N¹NCC¹; C¹C₂N¹NCS¹ (4 examples); N¹C₂N²NCS¹; O¹CNN¹C₂C¹; N¹C₂C¹C₂N²; N¹C₂N²C₂N³ (11 examples); N¹C₂N²C₂C¹ (5 examples); and C¹C₂N¹C₂C² (8 examples) types with common central ligating C¹, Te², B¹, N² and N¹ atoms (39 examples) (Table 4).

The Pt-P (trans to the common central atom) bond distance increases in the following sequence (total mean values): 2.237 Å (N¹) < 2.259 Å (N²) < 2.283 Å (Te²) < 2.293 Å (S²) < 2.348 Å (C¹) < 2.356 Å (B¹), which corresponds quite well with the trans influence of the respective central common donor atoms. The Pt-X (trans to P) bond distance increases in the following sequence (total mean values): 1.995 Å (C¹) < 2.014 Å (N²) < 2.030 Å (N¹) < 2.109 Å (B¹) < 2.308 Å (S²) < 2.574 Å (Te²).

There is a cooperative effect between the covalent radius of the respective donor atoms, the Pt-L bond distance, and chelate rings, as the former grows, the Pt-L bond distance increases and the chelate angles open, as can be observed below (

Table 5. Cooperative effects of the respective data.

Chelate rings	L-Pt-L’/L’-Pt-L [°]	Pt-L [Å]
N1CNN2NCS1	78.5°(N1CNN2)/81.6°(N2NCS1)	1.957(N2), 2.039(N1), 2.266(S1)
	N = 0.75 Å; S = 1.02 Å
N1CNN2NCSe1	78.7°(N1CNN2)/82.3°(N2NCSe1)	1.980(N2), 2.020(N1), 2.368(Se1)
	N = 0.75 Å;Se = 1.16 Å
S1CNB1NCS2	79.8°(S1CNB1)/85.6°(B1NCS2)	2.129(B2), 2.302(S1), 2.285(S2)
	B = 0.82 Å; S = 1.02 Å
Te1CNTe2NCTe3	91.6°(Te1CNTe2)/91.4°(Te2NCTe3)	2.572(Te2), 2.592(Te1), 2.573(Te3)
	Te = 1.36 Å; Te = 1.36 Å

).

In transition metal complexes, the oxidation state plays a leading role in the geometry formed and platinum is no exception. In four coordinates, Pt(II) prefers square-planar geometry. The utility of a simple metric to assess molecule shape and degree of distortion, as well as to exemplify the τ₄ parameter for square-planar geometry, is demonstrated by the following equations [42]:

${\mathsf{\tau }}_4=\frac{360-(\alpha +\beta )}{360}$ For square planar geometry

${\mathsf{\tau }}_4=\frac{360-(\alpha +\beta )}{141}$ For tetrahedral geometry

The values of the τ₄ parameter range from 0.00 for perfect square planar geometry to 1.00 for perfect tetrahedral geometry, since 360 − 2(109.5) = 141.

The total mean values of trans-α-L-Pt-L (L represents terminal ligating atoms of the respective η³-ligand) trans-β-L-Pt-P (L is a common central atom) bond angles, as well as of parameter τ₄, reflect the membered nature of the respective metallocycles, as can be observed from the data in the following summary (

Table 6. Selected data of trans angle and parameter τ₄ of metallocycles.

Metallocycles	α-L-Pt-L°	β-L-Pt-P°	τ4	Table X
I. 6+6-membered	177.3	174.3	0.023	1
II. 6+5-membered	175.9	174.8	0.026	2
III. 5+6-membered	173.1	172.5	0.040	3
IV. 5+5-membered	162.7	174.9	0.062	4

).

As can be observed, while the β-L-Pt-P angles are almost constant, the α-L-Pt-L angles are slowly increase with the membered metallocycles in the following sequence: 5+5- < 5+6- < 6+5- < 6+6-. The distortion of the square-planar geometry about Pt(II) atoms decreases in the same sequence, as indicated by parameter τ₄ (0.062 (5+5) > 0.040 (5+6) > 0.024 (6+5) > 0.023 (6+6)).

The coexistence of two or more species that differ only by the degree of distortion of the M-L bond distance and L-M-L bond angles is typical of the general class of distortion isomerism [43]. Over 160 platinum complexes exist following the analysis and classification of isomers [44]. This includes distortion (65%), cis-trans (30%), mixed isomers (cis-trans and distortion), and ligand isomerism.

The complex [Pt(η³-C₁₅H₁₁N₃–N¹,N²,N³)(PPh₃)].2SO₃CF₃ [31] contains two crystallographically independent molecules within the same crystals (Table 4). These molecules differ by the degree of distortion of Pt-L and L-Pt-L, with the values of parameter τ₄ of 0.070 and 0.082, respectively. Below is a classic example of distortion isomers [43].

As part of the X-ray analysis, [Pt(η³-C₁₄H₁₀N₃–N¹,N²,C¹)(PPh₃)].ClO₄ was measured at 173 K and 193 K [32], and 295 K [33] (Table 4). It was found that the temperature had an influence on the structural parameters. When the temperature drops, the distortion grows, as indicated by parameter τ₄ in the following order: 0.065 (at 173 K) < 0.068 (at 193 K) < 0.079 (at 295 K). These molecules, as well as independent molecules, are classical distortion isomers [43].

During the collection and organization of the data, it has become evident that some original papers are lacking important information such as atom coordinates and the analysis of intermolecular distances. Because of these limitations, we believe that a review such as this can continue to serve a useful function by centralizing the available material and delineating areas worthy of further investigation.