Additional Coordination Sites Research Articles

Mechanism of Cycloisomerisation of 1,6‐Heptadienes Catalysed by [(tBuCN)2PdCl2]: Remarkable Influence of Exogenous and Endogenous 1,6‐ and 1,5‐Diene Ligands

The mechanism of the highly regioselective cycloisomerisation of dimethyl hept-1,6-dienyl-4,4-dicarboxylate (1) by a neutral pre-catalyst, [(tBuCN)(2)PdCl(2)] (8), to generate dimethyl 3,4-dimethylcyclopent-2-ene-1,1-dicarboxylate (3) has been investigated by isotopic labelling (reactions involving single and mixed samples of 1,1,2,6,7,7-[(2)H(6)]-1; 3,3,5,5-[(2)H(4)]-1; 1,7-(Z,Z)-[(2)H(2)]-1; [1,3-(13)C(1),5,7-(13)C(1)]-1 and [1,3-(13)C(1),6-(2)H(1)]-1) and by study of the reactions of dimethyl 1-aryl-hept-1,6-dienyl-4,4-dicarboxylates (9 a-e, where aryl is p-C(6)H(4)-X; X=H, OMe, Me, Cl, CF(3)) and dimethyl hept-1,5-dienyl-4,4-dicarboxylate (14), a 1,5-diene isomer of 1. The mechanism proposed involves the generation of a monochloro-bearing palladium hydride which undergoes a simple hydropalladation, carbopalladation, Pd/H dyotropy, beta-H elimination sequence to generate 3. A key point that emerges is that chelation of the 1,6-diene 1 at various stages in the mechanism plays an important role in determining the regioselectivity of the reaction. The selective generation of 3 with pre-catalysts of the form L(2)PdCl(2), as compared to the generation of dimethyl 3-methylene-4-methyl-cyclopentane-1,1-dicarboxylate (2) with pre-catalysts of the form [(MeCN)(2)Pd(allyl)]OTf (5) is ascribed to the absence of chloride ion in the latter, which makes an additional coordination site available throughout turnover. Liberation of the product 3 when [(tBuCN)(2)PdCl(2)] (8) is employed as pre-catalyst, is proposed to proceed via a mono- to bidentate switch in the pi-coordination of diene 1 (eta(2) to bis-eta(2)) displacing pi-coordinated 3 from Pd. When 1-aryl-1,6-dienes 9 are employed as substrates, the electron-donor property of the aryl group is found to influence the regioselectivity of cyclisation. Electron-withdrawing groups favour dimethyl 3-arylmethyl-4-methylcyclopent-2-ene-1,1-dicarboxylates (10), whilst electron-donating aryl groups favour 3-arylidene-4-methyl-cyclopentane-1,1-dicarboxylates (11). The regioselectivity (10/11) correlates with the Hammett sigma(+) values (rho(+)=1.3, r (2)=0.975) indicative of a strong pi-resonance contribution from the aryl ring rather than a simple sigma-inductive effect. Intermolecular modulation of regioselectivity is observed and the net effect proposed to arise through the (pi-->d) donation ability of the vinyl arene in the diene displacing product (10/11) via a mono- to bidentate switch in coordination. The isomerisation process increasingly sequesters Pd as turnover proceeds leading to a powerful inhibition mechanism and ultimately a limitation in turnover number to about 80.

Biomimetic Hydrolysis of Penicillin G Catalyzed by Dinuclear Zinc(II) Complexes: Structure–Activity Correlations in β‐Lactamase Model Systems

A series of highly preorganized pyrazolate-based dinuclear zinc complexes has been studied as functional synthetic analogues of metallo-beta-lactamases, a class of bacterial enzymes that cause serious clinical problems because of their degradation of common beta-lactam antibiotics. We have investigated the hydrolytic cleavage of penicillin G mediated by the different dinuclear zinc complexes, and have deduced structure-activity correlations. While cooperative effects of the adjacent metal ions might be operative, these are found to either enhance or diminish beta-lactamase activity with respect to a single free zinc. Drastic differences in activity are ascribed to a lack of accessible binding sites after incorporation of the substrate within the bimetallic pocket of 2 and 4, whereas partial detachment of hemilabile ligand side arms in 1 and 3 opens up available coordination sites for nucleophile activation and/or for binding and polarisation of the beta-lactam amide oxygen atom. This interpretation has been corroborated by NMR spectroscopic and mass spectrometric evidence as well as by X-ray crystallography of several adducts formed between the pyrazolate-based dinuclear zinc scaffolds and the small substrate analogue oxazetidinylacetate (oaa), 5-7. In all adducts, the carboxylate group of oaa is the primary anchoring site and is nested in a bridging position within the bimetallic pocket. However, zinc binding of the beta-lactam amide oxygen atom has been confirmed crystallographically for the first time in 7, in which additional open-site coordination sites are available.

Chemical Control of the DNA Light Switch: Cycling the Switch ON and OFF

The emission of the DNA light-switch complex [Ru(bpy)2(tpphz)]2+ (bpy = 2,2'-bipyridine, tpphz = tetrapyrido[3,2-a:2',3'-c:3' ',2' '-h:2' '',3' ''-j]phenazine) can be reversibly turned ON and OFF over several cycles. The tpphz and taptp (taptp = 4,5,9,18-tetraazaphenanthreno[9,10-b] triphenylene) ligands in [Ru(bpy)2(tpphz)]2+ and [Ru(bpy)2(taptp)]2+, respectively, intercalate between the DNA bases, and a 50-fold increase in emission intensity of [Ru(bpy)2(tpphz)]2+ is observed upon DNA intercalation. The [Ru(bpy)2(tpphz)]2+ DNA light switch can be turned OFF statically in the presence of Co2+, Ni2+, and Zn2+, and the emission can be fully restored by the addition of EDTA. Cycling of the DNA light switch OFF and ON can be accomplished through the successive introduction of Co2+ and EDTA, respectively, to solutions of DNA-bound [Ru(bpy)2(tpphz)]2+. Owing to the absence of additional coordination sites, the emission of DNA-intercalated [Ru(bpy)2(taptp)]2+ is not quenched by transition metal ions in solution. To our knowledge, this work presents the first example of a reversible DNA light switch.

Comparative study of novel sulfonylcalix[4]arene derivative compounds

Density functional theory (DFT) was applied to investigate the structure and conformational equilibrium of tetraaminosulfonylcalix[4]arene ( 1) and tetramercaptosulfonylcalix[4]arene ( 2) compounds, which are expected to contribute for the design of novel molecular receptors in supramolecular chemistry. All the calculations predict the 1,3-alternate as the most stable conformer for both compounds, which contrasts with an earlier theoretical study on the corresponding thiacalix[4]arene derivatives where cone was shown to be the most stable. The strong intramolecular hydrogen bonding between the substituent groups at the lower rim and the oxygen atoms of sulfone groups seem to play an important role in the stabilization of the conformers. Besides, the influence of the solvent chloroform was taken into account in this study. The results show that chloroform does not modify the stability ordering of the conformers, but leads to a reduction in the energy gaps between them. The presence of additional coordination sites provided by sulfone groups may induce new binding properties of the host molecule towards a variety of metal ions.

2-Oxoquinoline 8-Monooxygenase Oxygenase Component: Active Site Modulation by Rieske-[2Fe-2S] Center Oxidation/Reduction

2-Oxoquinoline 8-monooxygenase is a Rieske non-heme iron oxygenase that catalyzes the NADH-dependent oxidation of the N-heterocyclic aromatic compound 2-oxoquinoline to 8-hydroxy-2-oxoquinoline in the soil bacterium Pseudomonas putida 86. The crystal structure of the oxygenase component of 2-oxoquinoline 8-monooxygenase shows a ring-shaped, C3-symmetric arrangement in which the mononuclear Fe(II) ion active site of one monomer is at a distance of 13 A from the Rieske-[2Fe-2S] center of a second monomer. Structural analyses of oxidized, reduced, and substrate bound states reveal the molecular bases for a new function of Fe-S clusters. Reduction of the Rieske center modulates the mononuclear Fe through a chain of conformational changes across the subunit interface, resulting in the displacement of Fe and its histidine ligand away from the substrate binding site. This creates an additional coordination site at the mononuclear Fe(II) ion and can open a pathway for dioxygen to bind in the substrate-containing active site.

Mössbauer study of (Bi,Pb)-2223 superconductors irradiated with86Kr ions

57Fe Mossbauer spectroscopy was used to study the effects of heavy ion irradiation on (Bi0.93Pb0.17)Sr1.9Ca2.05(Cu1.02Fe0.01)3Oy superconductors. The Mossbauer spectra of the irradiated superconductor showed significant changes in the subspectra belonging to different Cu microenvironments into which Fe ions were substituted. The relative occurrence of Fe occupying the square pyramidal fivefold oxygen coordinated Cu sites decreased while Fe3+ substituting the square planar fourfold oxygen coordinated Cu sites increased upon bombardment by 246 MeV 86Kr8+ ions. This observation was interpreted on the basis of partial replacement of the apical oxygen atoms in the square pyramids surrounding one of the Cu sites. Such oxygen vacancies create additional square planar coordination sites for Cu. These results also support the covalent character of Cu-O bonds in the superconducting layer as well as the ionic character of bond of apical oxygen to Cu.

Synthesis of Novel Pd−NCN Pincer Complexes Having Additional Nitrogen Coordination Sites and Their Application as Catalysts for the Heck Reaction

A novel Pd−NCN pincer complex, 7, bearing two additional nitrogen atoms has been synthesized and characterized. The dynamic NMR study of 7 shows the two-site exchange reaction in the NCN pincer ligand with a relatively low ΔG⧧ value, suggesting easy interconversion in the pincer complex. The use of 7 as a catalyst in the Heck reaction was studied. Catalyst 7 is quite effective for all aryl iodides. A turnover number (TON) value of 4.3 × 106 was observed with 4-tolyl iodide. However, 7 is not an effective catalyst for nonactivated bromobenzene and chlorobenzene substrates. According to the study of the Heck reaction, the facile interconversion of the pincer ligand positively affects the catalytic activity.

Synaptotagmins form a hierarchy of exocytotic Ca(2+) sensors with distinct Ca(2+) affinities.

Synaptotagmins constitute a large family of membrane proteins implicated in Ca(2+)-triggered exocytosis. Structurally similar synaptotagmins are differentially localized either to secretory vesicles or to plasma membranes, suggesting distinct functions. Using measurements of the Ca(2+) affinities of synaptotagmin C2-domains in a complex with phospholipids, we now show that different synaptotagmins exhibit distinct Ca(2+) affinities, with plasma membrane synaptotagmins binding Ca(2+) with a 5- to 10-fold higher affinity than vesicular synaptotagmins. To test whether these differences in Ca(2+) affinities are functionally important, we examined the effects of synaptotagmin C2-domains on Ca(2+)-triggered exocytosis in permeabilized PC12 cells. A precise correlation was observed between the apparent Ca(2+) affinities of synaptotagmins in the presence of phospholipids and their action in PC12 cell exocytosis. This was extended to PC12 cell exocytosis triggered by Sr(2+), which was also selectively affected by high-affinity C2-domains of synaptotagmins. Together, our results suggest that Ca(2+) triggering of exocytosis involves tandem Ca(2+) sensors provided by distinct plasma membrane and vesicular synaptotagmins. According to this hypothesis, plasma membrane synaptotagmins represent high-affinity Ca(2+) sensors involved in slow Ca(2+)-dependent exocytosis, whereas vesicular synaptotagmins function as low-affinity Ca(2+) sensors specialized for fast Ca(2+)-dependent exocytosis.

Hydrothermal syntheses and structural characterizations of organic-inorganic hybrid materials of the M(II)-ligand/vanadium oxide system (M(II) = Cu(II) and Zn(II); ligand = 2,2'-bipyridine and 2,2':6',2"-terpyridine).

Investigation into the incorporation of complex transition metal-organic units into vanadium oxide structures has resulted in the preparation of several novel composite materials. Hydrothermal reactions of V(2)O(5), 2,2'-bipyridine, an appropriate Zn or Cu starting material, and H(2)O under a variety of conditions yielded the organic-inorganic hybrid materials [[Zn(2,2'-bpy)](2)V(4)O(12)] (1) and [Cu(2,2'-bpy)V(4)O(10.5)] (2). Blocking an additional coordination site on the secondary metal center by using a tridentate organonitrogen ligand, 2,2':6',2' '-terpyridine in place of 2,2'-bipyridine, allowed the isolation of [Cu(terpy)V(2)O(6)] (3) and [[Zn(terpy)](2)V(6)O(17)] (4). The structure of 1 is a two-dimensional zinc vanadate layer, composed of rings containing four corner-sharing [VO(4)] vanadium(V) tetrahedra linked through six zinc square pyramids, with the 2,2'-bipyridine groups attached to the zinc centers and directed above and below the plane of the layer. In contrast to 1, the layer of 2 is based on a two-dimensional vanadium oxide substructure composed of vanadium(IV) square pyramids and vanadium(V) tetrahedra with copper square pyramids attached through corner-sharing interactions with vanadium tetrahedra such that the bipyridine ligands attached to the copper sites form staggered stacks above and below the plane of the layer. Compound 3 consists of one-dimensional vanadium oxide chains of corner-sharing tetrahedra linked through copper-terpyridine units into a two-dimensional bimetallic oxide of composition [CuV(2)O(6)], while the layer structure of 4 contains more complex one-dimensional vanadium oxide chains composed of fused rings of six corner-sharing vanadium oxide tetrahedra which are linked into a layer through [Zn(terpy)](2+) units.

Phosphorus-Based Ambidentate Chelating Ligands: Pyridyl-N− and Imido-N−Metal Coordination in the Py2P(NSiMe3)2Anion

Monoanionic heteroallylic ligand systems [R−N−E−N−R]- (E = Si(R2), S(R2) or S(R), C(R), and P(R2)) are versatile chelating substituents both in main group and transition metal chemistry as they provide sufficient steric demand and solubility to the products. Their application is only limited by the rigid bite of the ligands as the N···N distance cannot be tuned to the various radii of different metals. In this paper we present the new concept of opening the ligand periphery to additional coordination. The NP(R2)N- chelate in classical aminoiminophosphoranates is extended by additional coordination sites in the organic substituents (e.g., 2-pyridyl (Py) instead of phenyl (Ph)). Py2P{N(H)SiMe3}(NSiMe3) (1) is the starting material for a new class of complexes as deprotonated 1 contains along with the NPN- chelate the pyridyl ring nitrogen atoms to generate a side-selective Janus face ligand. In [(THF)Sr{Py2P(NSiMe3)2}2] (2) and [(4,4‘-bipy)Ba{Py2P(NSiMe3)2}2]n (3) both pyridyl rings are involved in metal co...

A novel trinuclear palladium cluster compound: di-μ3-chloro-tris[chloro(triphenylphosphine-P)palladium] acetone solvate

The title triangular tripalladium cluster, [Pd3­Cl5­(C18­H15P)3]·­C3H6O, (I), has a trigonal-bipyramidal framework of Pd3(μ3-Cl)2, with the two Cl atoms in apical positions. Each Pd atom in the framework has two additional coordination sites to establish square-planar cis-PdL2(μ3-Cl)2 geometry. Three P atoms are located on the same side of the plane defined by the Pd3 triangle, which leads to a pseudo-C3v symmetry for the core framework of Pd3(μ3-Cl)2P3Cl3. The average Pd—Cl distance trans to PPh3 is 2.473 (8) Å, which is significantly longer than the average Pd—Cl distance of 2.294 (4) Å for those trans to terminal Cl, due to the strong trans effect of a P atom compared with a Cl atom. Compound (I) has 49 valence electrons and shows a rhombic electron-spin resonance signal, indicating an S = ½ ground state.

A novel trinuclear palladium cluster compound: di-mu 3-chloro-tris

The title triangular tripalladium cluster, [Pd3­Cl5­(C18­H15P)3]·­C3H6O, (I), has a trigonal-bipyramidal framework of Pd3(μ3-Cl)2, with the two Cl atoms in apical positions. Each Pd atom in the framework has two additional coordination sites to establish square-planar cis-PdL2(μ3-Cl)2 geometry. Three P atoms are located on the same side of the plane defined by the Pd3 triangle, which leads to a pseudo-C3v symmetry for the core framework of Pd3(μ3-Cl)2P3Cl3. The average Pd—Cl distance trans to PPh3 is 2.473 (8) Å, which is significantly longer than the average Pd—Cl distance of 2.294 (4) Å for those trans to terminal Cl, due to the strong trans effect of a P atom compared with a Cl atom. Compound (I) has 49 valence electrons and shows a rhombic electron-spin resonance signal, indicating an S = ½ ground state.

Syntheses, Structures, and Magnetic Properties of Di-μ3-Chloride-Bridged Tripalladium Compounds, [Pd3(μ3-Cl)2(HqnS)6]Cl2 (HqnS = Quinoline–2(1H)–thione) and [Pd3(μ3-Cl)2(Et2dtc)2(PPh3)2]·C6H6(Et2dtc = N, N′–Diethyldithiocarbamate)

Abstract Two triangular tripalladium compounds, [Pd3(μ3-Cl)2(HqnS)6]Cl2 (1) (HqnS = quinoline–2(1H)–thione), and [Pd3(μ3-Cl)2(Et2dtc)2(PPh3)2]·C6H6 (2) (Et2dtc = N, N′-diethyldithiocarbamate anion), were prepared. Their single crystal X-ray structure analyses revealed they have a similar trigonal bipyramidal framework of Pd3(μ3-Cl)2 with two chlorine atoms in apical positions. Each Pd atom in all clusters has two additional coordination sites to establish a square planar cis-PdL2(μ3-Cl)2 geometry. In compound 1, six sulfur atoms of six HqnS ligands with thion form are coordinated to the vacant sites of the Pd3(μ3-Cl)2 core. Two coordination sites of one of the three Pd atoms in 2 are occupied by two triphenylphosphines and those of the other two are coordinated by two S atoms of Et2dtc. Although both compounds have 50 valence electrons, only 1 shows paramagnetic behavior. Crystallographic data are as follows. 1: Pd3Cl4S6N6C54H42, triclinic, P1- a = 14.988(5), b = 17.328(3), c = 12.642(1) Å, α = 93.61(1), β = 96.06(2), γ = 71.79(2)°, V = 3099(1) Å3, and Z = 2. 2: Pd3Cl2P2S4N2C52H56, monoclinic, P21, a = 12.278(5), b = 17.426(6), c = 12.630(3) Å, β = 94.65(3)°, V = 2693(2) Å3, and Z = 2.

Reaction of titanocene dihalides with Na2H2 EDTA. Crystal structure of [Ti(EDTA)(H2O)

Titanocene complexes ([Ti(η5-C5H4R)2X2];R = H, SiMe3;X=Cl, Br) react with Na2H2EDTA in aqueous methanol to give an identical product ([Ti(EDTA)(H2O)] by cleavage of the halogen and cyclopentadienyl ligands. The structure of [Ti(EDTA)(H2O)] has been determined by X-ray diffraction; crystal data: monoclinica=13.923(6),b=7.048(3),c=13.252(5) A, β=90.81(1)°, space groupP21/c,Z=4. In this complex, Ti has a sevenfold coordination with a hexadentateEDTA4− ligand and a water molecule occupying an additional coordination site.

Potentiometric investigation of Schiff base equilibria involving salicylaldehyde and tridentateα-aminoacids with Cu(II), Ni(II) and Zn(II) in aqueous medium

Potentiometric determination of equilibrium constants and species distribution of Schiff base complexes, in aqueous solution, involving salicylaldehyde (A) and tridentateα-amino acids (B)(L-threonine, L-glutamine and L-glutamic acid) with metal ions Cu(II), Ni(II) and Zn(II)(M) indicates preference for the formation of Schiff base complexes rather than the ligands binding as such to the metal ions. Binary stability constants follow the Irving-Williams order Cu(II)>Ni(II)>Zn(II). The influence of additional coordination sites such as -OH, -CONH2 and -COOH in the formation of ternary complexes is also discussed.

Bond length–bond order relations and calculated geometries for some benzenoid aromatics, including phenanthridine. Structures of 5,6-dimethylphenanthridinium triflate, [N-(6-phenanthridinylmethyl)-aza-18-crown-6-κ5 O,O',O'',O''',O''''](picrate-κ2-O,O')potassium, and [N,N'-bis(6-phenanthridinyl-κN-methyl)-7,16-diaza-18-crown-6-κ4 O,O',O'',O''']sodium iodide dichloromethane solvate

The crystal structures of the title compounds are studied in order to investigate the role of novel fluoroionophores in complexation of sodium and potassium. In the potassium complex seven coordination, including the picrate ligand, is encountered. An additional coordination site is via the phenanthridine nitrogen at 3.252 (2) Å (second coordination). The complex is of C 1 symmetry and the aza-18-crown-6 macrocylic ring exhibits a crown-type conformation. The 7,16-diaza-18-crown-6 macrocycle accommodates a six-coordinate sodium with two additional ligands, via nitrogen from phenanthridine units. The complex cation shows a crystallographic twofold symmetry. The macrocycle is not of the crown-type conformation. In both complexes the alkali metals are shifted out of the cavity centres towards a picrate ligand in [N-(6-phenanthridinylmethyl)-aza-18-crown-6-κ5 O,O′,O′′,O′′′,O′′′′](picrate-κ2 O,O′)potassium and the phenanthridine units in [N,N′-bis-(6-phenanthridinyl-κN-methyl)-7,16-diaza-18-crown-6-κ4 O,O′,O′′,O′′′]sodium iodide dichloromethane solvate. Semi-empirical and molecular mechanics calculations based on various force fields were used for the optimization of phenanthridine geometry. The values obtained are compared with experimental data. Valence bond calculations of bond lengths in some benzenoid aromatic systems (C—C bonds in benzenoid hydrocarbons, azabenzenoid hydrocarbons and picrate-like systems; C—N bonds in the azabenzenoids; C—O bonds in the picrate-like systems), as well as some analogous Hückel molecular orbital calculations (C—C bonds in the benzenoid hydrocarbons and the azabenzenoids), were found to agree with the observed values (average differences up to 0.015 Å). These approaches can be used by means of bond length-bond order relations for prediction of bond lengths in the phenanthridine units as well as in the picrate.

Fluoride-assisted stabilisation of amino acid complexes of vanadium Synthesis and characterisation

Fluoride amino acid complexes of vanadium(IV) of the type A[VOF 2L(H 2O)] (A = Naor NH 4, L = cysteinate) and A[VOF 3L(H 2O)] (A = NH 4 for L = alanine and A = Na, NH 4 or K for L = serine) have been synthesised. The compounds have been characterised by chemical analyses, chemical determination of the oxidation state of vanadium, solution electrical conductance and magnetic susceptibility measurements, ESR, IR and electronic spectral studies. The vibrational spectra of the complexes indicate coordination of the amino acids through their carboxylate group to the metal centre. Also, the sulphydryl functionality provides an additional coordination site in cysteine-containing complexes. Fluoride appears to act as a stabilising ligand which helps in the solid-state isolation of the complexes.

ALKALINE HYDROLYSIS OF β-AND γ-FUNCTIONAL PHOSPHONIUM SALTS

Abstract A model study of alkaline hydrolysis of β-and γ-heterosubstituted phosphonium salts has been made and used as a synthetic method to obtain diphosphine dioxides affording additional coordination sites in an alkyl chain linked to phosphorus. After synthesis of the corresponding salts, anomalous fragmentations, such as inversion in cleavage selectivity or participation of the reaction solvent, have been pointed out in alkaline hydrolysis of β-functional phosphonium salts. A mechanism is suggested to account for the presence of all compounds detected. In the case of γ-functional phosphonium salts, the alkaline hydrolysis leads only to predicted compounds corresponding to the usual selective cleavage of one phenyl group. The results observed have been used to synthesize symmetric and asymmetric diphosphine dioxides with ether substituted side chains.

Thermal properties of barium complexes used for depositing thin films of YBa2Cu3O7 superconductors

The thermal properties of several barium containing complexes are compared by using thermogravimetric analysis and solids probe mass spectrometry. The properties of complexes containing flourinated and non-fluorinated β-diketonates with and without the presence of a polyether to occupy additional barium coordination sites are compared. Barium complex thermal properties are heavily dependent on the structure of the β-diketones and polyether comprising the complex. Barium complexes containing fluorinated ligands are better suited for chemical vapour deposition of barium than complexes containing non-fluorinated ligands because they efficiently evolve vapours containing barium at lower temperatures.

Bifunctional Cyclopentadienyl Ligands in Organotransition Metal Chemistry

Abstract The coordination chemistry of cyclopentadienyl ligands that contain an additional coordination site appended to the ring-periphery by a covalent bridge Z, C5R4−Z-X or C5R4−Z-L, is outlined. When transition metals are coordinated both to the cyclopentadienyl group C5R4 and the pendent donor site X or L (X = CH2 NR′, O; L = HC[dbnd]CH2 NR2′, PR2′, OR′, SR′), the resulting complexes possess distinct properties due to the chelate effect. In such complexes the coordination sphere is defined by the nature of the bridge Z and the coordinating groups X and L and a series of metallacyclic complexes becomes available of which the chemistry is different from that of their non-bridged analogs.