Time-Resolved Spectroscopic Study on Photoinduced Electron-Transfer Processes in Zn(II)porphyrin–Zn(II)chlorin–Fullerene Triad

Femtosecond time-resolved transient absorption studies have been performed to investigate the photoinduced energy and electron-transfer processes in Zn(II )porphyrin–Zn(II )chlorin–fullerene triad in which energy and oxidation potential gradients are directed along the donor–acceptor-linked arrays. Fast energy transfer (≈450 fs) from photoexcited Zn(II )porphyrin to Zn(II )chlorin was observed upon selective photoexcitation of Zn(II )porphyrin unit in the triad. In a nonpolar solvent such as toluene, the energy transfer from the excited singlet state of Zn(II )chlorin to fullerene occurs and is followed by the formation of an intermediate state with a time constant of nanoseconds, which was attributed to the intramolecular exciplex between Zn(II )chlorin and fullerene. In benzonitrile, on the other hand, the photoexcitation of the triad results in the fast electron transfer (<1 ps) from photoexcited Zn(II )chlorin to fullerene. The generated charge-separated species recombine with a time constant of ≈12 ps. The relatively fast charge separation and charge recombination rates imply that the strong electronic coupling between Zn(II )chlorin and fullerene moieties is probably induced by the folded conformation between Zn(II )chlorin and fullerene moieties which enhances direct through-space interaction between the proximately contacted π systems.     

Synthesis and Spectral Properties of Ni(II), Pd(II), Pt(II), and Pt(IV) Tetraphenyltetrabenzoporphyrinates

Complexation reactions of 5,10,15,20-tetraphenyltetrabenzoporphyrin and transmetallation of its cadmium complex with nickel(II) acetate, Ni(II), Pd(II), and Pt(II) chlorides in dimethylformamide and phenol have been studied. The corresponding Ni(II), Pd(II), and Pt(II) porphyrinates have been synthesized. Pt^IVBr₂ porphyrinate has been obtained by the treatment of Pt(II) 5,10,15,20-tetraphenyltetrabenzoporphyrinate with bromine in chloroform. The obtained compounds have been characterized by elemental analysis, electronic absorption and ¹H NMR spectroscopy and mass spectrometry.     

Synthesis and spectroscopic studies of biologically active tetraazamacrocyclic complexes of Mn(II), Co(II), Ni(II), Pd(II) and Pt(II)

Complexes of Mn(II), Co(II), Ni(II), Pd(II) and Pt(II) were synthesized with the macrocyclic ligand, i.e., 2,3,9,10-tetraketo-1,4,8,11-tetraazacycoletradecane. The ligand was prepared by the [2 + 2] condensation of diethyloxalate and 1,3-diamino propane and characterized by elemental analysis, mass, IR and ¹H NMR spectral studies. All the complexes were characterized by elemental analysis, molar conductance, magnetic susceptibility measurements, IR, electronic and electron paramagnetic resonance spectral studies. The molar conductance measurements of Mn(II), Co(II) and Ni(II) complexes in DMF correspond to non electrolyte nature, whereas Pd(II) and Pt(II) complexes are 1:2 electrolyte. On the basis of spectral studies an octahedral geometry has been assigned for Mn(II), Co(II) and Ni(II) complexes, whereas square planar geometry assigned for Pd(II) and Pt(II). In vitro the ligand and its metal complexes were evaluated against plant pathogenic fungi (Fusarium odum, Aspergillus niger and Rhizoctonia bataticola) and some compounds found to be more active as commercially available fungicide like Chlorothalonil.     

Preparation and optical properties of polyimide films linked with porphyrinato Pd (II) and Pt (II) complexes through a triazine ring and application toward oxygen sensors

5‐(3‐Aminophenyl)‐10,15,20‐tri(4‐methylphenyl) porphyrinato Pd (II) and Pt (II) complexes ( 2a ‐ Pd ) and ( 2a ‐ Pt ), respectively, were prepared from 5‐(3‐nitrophenyl)‐10,15,20‐tri(4‐methyl‐phenyl)porphyrin via two‐step reactions, and reacted with cyanuric chloride to produce corresponding porphyrin derivatives ( 3a ‐ Pd ) and ( 3a ‐ Pt ) with a dichlorotriazine ring. Aromatic polyimides were prepared using diamine ( 4 ); triazine dichlorides having porphyrin units ( 3a ‐ Pd ), ( 3a ‐ Pt ), ( 3c ‐ Pd ), and ( 3c ‐ Pt) ; fluoro‐functionality 6‐(p‐perfluorononenyl oxyanilino)triazine‐2,4‐dichloride ( 6 ); and tetracarboxylic dianhydride ( 5 ) in N‐methyl‐2‐pyrrolidone (NMP) at an elevated temperature up to 300 °C. The resulting viscous polymeric solution was cast on a glass plate, affording well‐proportioned reddish transparent films with number‐average molecular weights of 25,000–38,000. Glass transition temperatures of the polymers were ～230 °C; the films were stable up to 400 °C in air. The film emission spectra showed a broad peak ～670 nm, similar to those of porphyrins ( 2a ‐ Pd ) and ( 2a ‐ Pt ) dispersed in a polystyrene matrix. While the luminescence of these polymer films was quenched with oxygen, it rapidly recovered under a deoxygenated atmosphere. The polyimide film sensitivity to oxygen was higher under low oxygen concentrations than those of porphyrins ( 2a ‐ Pd ) and ( 2a ‐ Pt ) dispersed in polystyrene. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017 , 55, 1086–1094     

New Optical Sensors for Oxygen Based on Phosphorescent Cationic Water-Soluble Pd(II), Pt(II), and Rh(III) Porphyrins

A new material is proposed for optical sensors for molecular oxygen; the material is obtained by the introduction of Pd(II), Pt(II), and Rh(II) complexes of cationic water-soluble porphyrins into an MF-4SK ion-exchange polymer membrane. The phosphorescence of immobilized metal porphyrins is efficiently quenched by molecular oxygen both in the gas phase and in aqueous solutions. The Stern–Volmer relationships for phosphorescence quenching are linear over the entire range of oxygen concentrations. The new material exhibits good response times and high photochemical stability.     

Core-modified porphyrin incorporating a phenolate donor. Characterization of Pd(II), Ni(II), Zn(II), Cd(II), and Fe(III) complexes

Coordinating properties of acetoxybenziporphyrin, (TPBPOAc)H, have been investigated for a number of metal ions. Insertion of Ni, Pd, and Fe results in the cleavage of the acetoxy group leading to complexes (TPBPO)Ni(II), (TPBPO)Pd(II), and (TPBPO)Fe(III)X containing a M-O bond. No cleavage is observed with Zn(II) and Cd(II), which form complexes (TPBPOAc)M(II)Cl, where M = Zn, Cd. (TPBPO)Ni(II) can also be obtained from the dication of hydroxybenziporphyrin, [(TPBPOH)H(3)]Cl(2), which is prepared by acid hydrolysis of the acetoxy compound. The diamagnetic (TPBPO)Ni(II) can be transformed into the paramagnetic (TPBPOAc)Ni(II)Cl in a reaction with acetyl chloride. X-ray structures have been determined for (TPBPO)Pd(II) and (TPBPOAc)Zn(II)Cl. In the palladium species, the phenolate moiety forms a strong bond to the Pd ion and an unusual interaction geometry is observed, enforced by the macrocyclic environment. Association of a TFA molecule to the phenolic oxygen does not cause significant structural changes in the (TPBPO)Pd(II) molecule. In (TPBPOAc)Zn(II)Cl, the metal ion weakly interacts with the phenolic fragment. The paramagnetic Fe(III) complexes, (TPBPO)Fe(III)X, have been investigated with (1)H NMR spectroscopy. The observed spectral patterns are consistent with the presence of a high-spin Fe(III) center and pi delocalization of spin density onto the phenoxide fragment. Each of the compounds (TPBPO)Fe(III)X exists in solution as a mixture of two isomers, which for X = I are shown to remain in a temperature-dependent equilibrium. The observed isomerism results from two nonequivalent orientations of the axial halide with respect to the puckered macrocyclic ring.     

Synthesis and photoluminescent properties of a Schiff-base ligand and its mononuclear Zn(II), Cd(II), Cu(II), Ni(II) and Pd(II) metal complexes

Mononuclear Zn(II), Cd(II), Cu(II), Ni(II) and Pd(II) metal complexes of Schiff-base ligand(HL₁) derived from 8-acetyl-7-hydroxycoumarin and P-phenylenediamine were prepared and characterized by microanalytical, mass, UV–Vis, IR, ¹H NMR, ¹³C NMR, ESR, conductance and fluorescence studies. The measured low molar conductance values in DMSO indicate that the complexes are non-electrolytes. The structures of the solid complexes under study are established by using IR, electronic and ESR spectroscopy suggesting that Zn(II) and Ni(II) complexes are octahedral, Cd(II) complex is tetrahedral, Cu(II) and Pd(II) complexes are square planar. The ESR spectrum of the Cu(II) complex in DMSO at 298 and 150 K was recorded and its salient features are reported, it supports the mononuclear structure. The Schiff base exhibited photoluminescence originating from intraligand (π–π*) transitions. Metal-mediated enhancement is observed on complexation of HL with Zn(II) and Cd(II), whereas metal-mediated fluorescence quenching occurs in Cu(II), Ni(II) and Pd(II).     

Catalytic effects of peroxidase-like metalloporphyrins on the fluorescence reaction of homovanillic acid with hydrogen peroxide

Summary The catalytic effects of peroxidase-like metalloporphyrins (Me-P) on the fluorescence reaction of homovanillic acid with hydrogen peroxide have been studied. These metalloporphyrins are the complexes of Mn with tetrakis(carboxyphenyl)porphyrin (TPPC) and trikis(sulfophenyl)porphyrin(TPPS₃), Fe, Co, Ni, Cu, Zn, Ag and Sn with tetrakis(sulfophenyl)porphyrin(TPPS₄), and Rh, Pt and Pd with tetrakis(N-methylpyridiniumyl)porphyrin-(TMPyP) and hemin. The complexes of Mn, Fe, Co, Rh and Pt with porphyrins catalyzed the formation of the fluorescence product, while the complexes of Ni, Cu, Zn, Ag, Sn and Pd did not. Traces of hydrogen peroxide and glucose can be determined using the metalloporphyrins. The characteristics of peroxidase-like metalloporphyrins have been compared with those of horseradish peroxidase (HRP).     

TRANSFER OF EXCITATION ENERGY BETWEEN PORPHYRIN CENTERS OF A COVALENTLY-LINKED DIMER*

Abstract— Three covalently-linked porphyrin hybrid dimers were synthesized, each containing a metallotetraarylporphyrin [Zn(II), Cu(II), or Ni(II)], and a free base tetraarylporphyrin. Transfer of singlet excitation energy from the metalloporphyrin center to the free base porphyrin center was determined by measuring fluorescence properties. The Zn hybrid dimer displayed excellent intramolecular transfer of energy ( 85%) from the excited singlet state of the Zn(II) chromophore to the free base chromophore. No evidence for such transfer of the excited singlet state energy was found in the Ni(II) or Cu(II) analogues. From our experimental data, the fluorescence quantum yield of the Zn hybrid dimer was the same as for the free base monomer porphyrin (0.11; Seybold and Gouterman, 1969). Thus, the covalent attachment of another fluorescent porphyrin center effectively doubled the antenna size without decreasing the quantum yield even though the fluorescence quantum yield of the Zn(II) containing monomer was substantially less (0.03, according to Seybold and Gouterman, 1969) than that of the free base porphyrin. The donor-acceptor distance and the rate constant for energy transfer were calculated using the Forster equation. Assuming random orientation, a donor-acceptor distance of 15 Å was calculated with an associated rate constant (k_ci) for energy transfer of 1.9 ± 10₉ s_–1.     

Synthesis, Structure, and Antimicrobial Activity of Complexes of Pt(II), Pd(II), and Ni(II) with the Condensation Product of 2-(Diphenylphosphino)benzaldehyde and Semioxamazide

Complexes of Pt(II), Pd(II), and Ni(II) with the condensation derivative of 2-(diphenylphosphino)benzaldehyde and semioxamazide were synthesized, characterized, and their antimicrobial activity was evaluated. The ligand and the complexes were characterized by spectroscopic methods with the particular accent on NMR spectral analysis. For the palladium(II) complex, the crystal structure was determined by X-ray analysis. In all the complexes the ligand is coordinated as a tridentate via a P, N, O donor set. The Pd(II) and Pt(II) complexes have a square planar geometry, whereas the geometry of the Ni(II) complex is tetrahedral. The ligand showed antibacterial and antifungal activity, which was enhanced upon complexation.     

Synthesis, Structure, and Antimicrobial Activity of Complexes of Pt(II), Pd(II), and Ni(II) with the Condensation Product of 2-(Diphenylphosphino)benzaldehyde and Semioxamazide

Summary. Complexes of Pt(II), Pd(II), and Ni(II) with the condensation derivative of 2-(diphenylphosphino)benzaldehyde and semioxamazide were synthesized, characterized, and their antimicrobial activity was evaluated. The ligand and the complexes were characterized by spectroscopic methods with the particular accent on NMR spectral analysis. For the palladium(II) complex, the crystal structure was determined by X-ray analysis. In all the complexes the ligand is coordinated as a tridentate via a P, N, O donor set. The Pd(II) and Pt(II) complexes have a square planar geometry, whereas the geometry of the Ni(II) complex is tetrahedral. The ligand showed antibacterial and antifungal activity, which was enhanced upon complexation.     

Efficient excitation energy transfer in long meso-meso linked Zn(II) porphyrin arrays bearing a 5,15-bisphenylethynylated Zn(II) porphyrin acceptor

Electronically coupled porphyrin arrays are suitable for artificial light harvesting antenna in light of a large absorption cross-section and fast excitation energy transfer (EET). Along this line, an artificial energy transfer model system has been synthesized, comprising of an energy donating meso-meso linked Zn(II) porphyrin array and an energy accepting 5,15-bisphenylethynylated Zn(II) porphyrin linked via a 1,4-phenylene spacer. This includes an increasing number of porphyrins in the meso-meso linked Zn(II) porphyrin array, 1, 2, 3, 6, 12, and 24 (Z1A, Z2A, Z3A, Z6A, Z12A, and Z24A). The intramolecular singlet-singlet EET processes have been examined by means of the steady-state and time-resolved spectroscopic techniques. The steady-state fluorescence comes only from the acceptor moiety in Z1A-Z12A, indicating nearly the quantitative EET. In Z24A that has a molecular length of ca. 217 A, the fluorescence comes largely from the acceptor moiety but partly from the long donor array, indicating that the intramolecular EET is not quantitative. The transient absorption spectroscopy has provided the EET rates in real time scale: (2.5 ps)(-1) for Z1A, (3.3 ps)(-1) for Z2A, (5.5 ps)(-1) for Z3A, (21 ps)(-1) for Z6A, (63 ps)(-1) for Z12A, and (108 ps)(-1) for Z24A. These results have been well explained by a revised F?rster equation (Sumi formula), which takes into account an exciton extending coherently over several porphyrin pigments in the donor array, whose length is not much shorter than the average donor-acceptor distance. Advantages of such strongly coupled porphyrin arrays in light harvesting and transmission are emphasized in terms of fast EET and a large absorption cross-section for incident light.     

Spectrometric Studies on the Complexation of Pd(II), Fe(III) and Pt(IV) with mandelazo I

Abstract

A spectrometric study of the reaction between Pd(II), Fe(III) and Pt(IV) ions, and Mandelazo I was carried out. The optimum conditions favouring the formation of the complexes are extensively investigated. The stoichiometry of the complexes formed in solution (1:2, 1:1, 1:1), their apparent stability constants (5.45 × 10⁹, 2.39 × 10⁶, 4.12 × 10⁵) and the ranges for obedience to beer's law (0.2 – 6.4, 0.25 – 7.0, 1.5 – 42.0 μg/mL) are reported for Pd(II), Fe(III) and Pt(IV), respectively. The effect of some metal ions including Cu(II), Zn(II), Mn(II), Cd(II), Hg(II), Co(II), Ni(II), Be(II), Al(III), Th(IV) and U(VI), on the maximum absorbance of the formed complexes was also investigated.     

Experimental evidence for the noninnocence of o-aminothiophenolates: coordination chemistry of o-iminothionebenzosemiquinonate(1-) pi-radicals with Ni(II), Pd(II), Pt(II)

The ligand 2-mercapto-3,5-di-tert-butylaniline, H[L(AP)], an o-aminothiophenol, reacts with metal(II) salts of Ni and Pd in CH3CN or C2H5OH in the presence of NEt3 under strictly anaerobic conditions with formation of beige to yellow cis-[M(II)(L(AP))2] (M = Ni (1), Pd (2)) where (L(AP))1- represents the o-aminothiophenolate(1-) form. The crystal structure of cis-[Pd(II)(L(AP))2][HN(C2H5)3][CH3CO2] has been determined by X-ray crystallography. In the presence of air the same reaction produces dark blue solutions from which mixtures of the neutral complexes trans/cis-[M(II)(L(ISQ))2] (M = Ni (1a/1b), Pd (2a/2b), and Pt (3a/3b)) have been isolated as dark blue-black solid materials. By using HPLC the mixture of 3a/3b has been separated into pure samples of 3a and 3b, respectively; (L(ISQ))1- represents the o-iminothionebenzosemiquinonate(1-) pi-radical. The structures of 1a.dmf and 3a.CH2Cl2 have also been determined. All compounds are square-planar and diamagnetic. 1H NMR spectroscopy established the cis <==> trans equilibrium of 1a/1b, 2a/2b, and 3a/3b in CH2Cl2 solution where the isomerization rate is very fast for the Ni, intermediate for the Pd, and very slow for the Pt species. It is shown that the electronic structures of 1a/1b, 2a/2b, 3a, and 3b are best described as diradicals with a singlet ground state. The spectro- and electrochemistries of all complexes display the usual full electron transfer series where the monocation, the neutral species, the mono- and dianions have been spectroscopically characterized. X-band EPR spectra of the monocations [1a/1b]+ and [3a]+ support the assignment of an oxidation-state distribution as predominantly [M(II)(L(ISQ))(L(IBQ))]+ where (L(IBQ))0 represents the o-iminothionequinone level. In contrast, the EPR spectra of the monoanions [1a/1b]- and [3a]- indicate an [M(II)(L(ISQ))(L(AP)-H)]- distribution but with a significant contribution of the [M(I)(L(ISQ))(2)]- resonance hybrid; (L(AP)-H)2- represents the o-imidothiophenolato(2-) oxidation level. Analysis of the geometric features of 120 published structures of complexes containing ligands of the o-aminothiophenolate type show that high precision X-ray crystallography allows to discern the differing protonation and oxidation levels of these ligands. o-Aminothiophenolates are unequivocally shown to be noninnocent ligands; the (L(ISQ))1- radical form is quite prevalent in coordination compounds and the electronic structure of a number of published complexes must be reconsidered.     

Mass spectral behavior of some homoleptic and mixed aryldichalcogenide bis(diphenylphosphino)ferrocenenickel(II), palladium(II), and platinum(II), and bis(diisopropylphosphino)ferrocenepalladium(II) complexes

The mass spectral behavior of a number of organometallic complexes containing the Group 10 metals Ni, Pd, and Pt, together with various thiolate ligands were studied. For Pd, two main types of complexes, differing by the substituents on the phosphorus atom were studied. Types I and II were substituted with bis(diphenylphosphino)ferrocene and bis(diisopropylphosphino)ferrocene ligands, respectively. The Ni complexes, except for one, and the Pd Type I complexes had no molecular radical cations (M(+.)) in their EI spectra. On the other hand, all the Pt complexes showed intense M(+.) ions in their EI spectra indicating that these complexes were more stable as radical cations than those of Ni and Pd. The FAB and MALDI spectra of all the complexes displayed intense quasi-molecular ions (MH(+)) and the fragmentations in both modes were similar. The MALDI spectra of several complexes displayed only M(+.) ions while one gave evidence of both MH(+) and M(+.) ions. Several Pd Type II complexes yielded intense M(+.) in their EI spectra.     

Thermal behaviour of Co(II), Ni(II), Cu(II), Zn(II), Hg(II) and Pd(II) complexes with isatin-β-thiosemicarbazone

The isatin-β-thiosemicarbazone (ITC) complexes of Co(II), Ni(II), Cu(II), Zn(II), Hg(II) and Pd(II) were prepared and characterized by elemental analysis, as well as molar conductivity, magnetic susceptibility, FTIR, UV-Vis and ¹H NMR spectroscopic methods. The complexes were also studied for its thermal stability. They all behaviour as anhydrous complexes and its thermolysis passes through the stages of deamination (517–547 K) and complete thermal decomposition (619–735 K).     

Synthesis, thermal, spectral and magnetic studies of complexes of Co(II), Ni(II), Cu(II), Ru(II), Pd(II) and Pt(II) with 2, 3-disubstituted quinazolin-(3H)-4-ones

A number of complexes of Co(II), Ni(II), Cu(II), Ru(II), Pd(II) and Pt(II) with 2-methyl-3-(carboxy methyl) quinazolin (3H)-4-one (MCMQ) and 2-phenyl-3-(carboxy methyl) quinazolin (3H)-4-one (PCMQ) have been synthesized and characterized by analytical, conductivity, thermal, magnetic, infrared, electronic, proton magnetic resonance and electron spin resonance spectral data. Based on analytical data, the stoichiometry and the association with other molecules of the complexes have been determined. Conductivity data show that all these complexes are nonelectrolytes. Infrared and PMR spectral data indicate that both the ligands are uninegative bidentate with all the metal ions. Based on electronic spectral data, the geometries of the complexes have been indicated. Electronic spectral parameters for Co(II) and Ni(II) and ESR parameters for Cu(II) complexes have been calculated and relevant conclusions have been drawn with respect to the nature of bonds present in them.     

Synthesis,physico-chemical characterization and bioevaluation of Ni(II), Pd(II), and Pt(II) complexes with 1-(o-tolyl)biguanide: Antimicrobial and antitumor studies

New complexes of type [M(tbg)₂]Cl₂ [tbg = 1-(o-tolyl)biguanide; M = Ni(II), Pd(II), and Pt(II)] were synthesized and characterized to develop new biologically active compounds. The features of the complexes were assigned from microanalytical and thermal data. The NMR, FT-IR, and UV-Vis spectra were established by comparison with HtbgCl. All complexes exhibit a square-planar geometry resulting from the chelating behavior of tbg. The HtbgCl and [Ni(tbg)₂]Cl₂ complexes were fully characterized by single-crystal X-ray diffraction. The HtbgCl species crystallize in the monoclinic C2/c spatial group, while the Ni(II) complex adopts an orthorhombic Pna2₁ spatial group. The structure is stabilized by a complex hydrogen bonds network. The in vitro antimicrobial assays revealed improved antimicrobial activity for complexes in comparison with the ligand against both planktonic and biofilm embedded microbial cells. The most efficient compound, showing the largest spectrum of antimicrobial activity, including Gram-positive and Gram-negative bacteria, as well as fungal strains, in both planktonic and biofilm growth states was the Pd(II) complex, followed by the Pt(II) complex. The Pt(II) compound exhibited the most significant antiproliferative activity on the human cervical cancer SiHa cell line, inducing a cell cycle arrest in the G2/M phase.     

Cation-Induced dimerization of nickel(II), platinum(II), and palladium(II) meso-tetra(benzo-15-crown-5)porphyrinates

Cation-induced dimerization of nickel(II), platinum(II), and palladium(II) meso-tetra(benzo-15-crown-5)porphyrinates (Ni(II)TCP, Pd(II)TCP, and Pt(II)TCP) on treatment with potassium thiocyanate in a chloroform-methanol solution has been studied by electronic absorption spectroscopy. The formation of [{MTCP}₂(K⁺)₄](SCN^?)₄ in solution induces a hypsochromic shift of the Soret band and a bathochromic shift of the β-band with respect to their positions in the spectrum of MTCP. The equilibrium constants (K) for the 2MTCP + 4K⁺ = [{MTCP}₂(K⁺)₄] processes at 20°C are determined to be as follows: log K _Ni(II)TCP = 27.31 ± 1.67, logK _Pd(II)TCP = 27.16 ± 1.43, and logK _Pt(II)TCP = 26.15 ± 1.56.     

The coordination chemistry of simple phospholes: Complexes of nickel(II), palladium(II) and platinum(II)

The reactions between four very simply substituted phospholes and the chlorides of Ni(II), Pd(II) and Pt(II) are described. The phospholes 1-phenylphosphole, 3-methyl-1-phenyl-phosphole and 3,4dimethyl-1-phenylphosphole all readily form bis-complexes of formula L₂MCl₂ [L = phosphole ligand and M = Ni(II), Pd(II) or Pt(II)] or tris-complexes of formula L₃MCI₂. 1-n-Butyl-3,4-dimethylphosphole appears to form stable complexes only with Ni(II). Evidence is put forward which indicates that the L₂MCl₂ complexes exist in a four-coordinate, square-planar monomeric/five coordinate equilibrium while the L₃MCl₂ complexes are primarily the ionic species [L₃MCl]⁺ Cl^? in solution. Comparisons are made with the behaviour of other simple phospholes which do not form Ni(II) complexes and the results are discussed briefly in terms of both aromatic and non-aromatic phosphole models.