Benign synthesis of carboxamide ligands,H2Me2bqb and H2Me2bpb. Preparation,characterization and electrochemistry of Ni(II) complexes: The crystal structure of [Ni(Me2bqb)]

A novel and highly efficient approach for the synthesis of H₂Me₂bqb and H₂Me₂bpb using ionic liquid as an environmentally benign reaction medium has been developed, eliminating the need for the pyridine as a toxic solvent. The Ni(II) complex of the dianionic ligand Me₂bqb²⁻, [Me₂bqb²⁻ = 1,2-bis(quinoline-2-carboxamide)-4,5-dimethyl-benzene dianion], has been synthesized and characterized by elemental analyses and spectroscopic methods, and the crystal and molecular structure of [Ni(Me₂bqb)] (1), has been determined by X-ray crystallography. The complex exhibits distorted square-planar NiN₄ coordination geometry with two short and two long Ni–N bonds (Ni–N ∼1.85 and ∼1.96 Å, respectively). The electrochemical behavior of [Ni(Me₂bqb)] (1), has been studied by cyclic voltammetry and compared with the analogous complex, [Ni(Me₂bpb)] (2).     

Benign synthesis of quinolinecarboxamide ligands,H2bqbenzo and H2bqb and their Pd(II) complexes: X-ray crystal structure,electrochemical and antibacterial studies

Two carboxamide ligands, H₂bqbenzo {3,4-bis(2-quinolinecarboxamido)benzophenone} and H₂bqb {N,N′-bis[(2-quinolinecarboxamide)-1,2-benzene]}, have been prepared using tetrabutylammonium bromide as an environmentally benign reaction medium. Two new Pd(II) complexes, [Pd^II(bqbenzo)] (1) and [Pd^II(bqb)] (2), have been synthesized, characterized, and their structures determined by single crystal X-ray diffraction. The di-anionic ligands, bqbenzo^2? and bqb^2?, are coordinated via two N_amide atoms and the nitrogens of the two quinoline rings, with Pd?N_amide < Pd–N_quinoline bond lengths. The geometry around palladium(II) in both complexes is distorted square planar. The electrochemical behaviors of the ligands and their Pd(II) complexes have been investigated by cyclic voltammetry in DMF. An irreversible Pd^II/I reduction is observed at ?1.06 V for 1 and at ?1.177 V for 2, indicating the influence of the R substituent on the central phenyl ring of carboxamide ligands on the Pd^II/I reduction potential. The ligands and palladium complexes were also screened for in vitro antibacterial activity. The Pd(II) complexes show strong biological activity against S.typhi and E.coli as Gram ?ve and B.cereus and S.aureus as Gram +ve bacteria comparable to the antibiotic penicillin. The antibacterial results also reveal that coordination of Pd(II) significantly improves the activity.     

Complex Formation of PdII Ion with the Tripodal Ligand Tris[2-(dimethylamino)ethyl]amine

The complex formation of Pd^II with tris[2-(dimethylamino)ethyl]amine (N(CH₂CH₂N(CH₃)₂)₃, Me₆tren) was investigated at 25° and ionic strength I = 1, using UV/VIS, potentiometric, and NMR measurements. Chloride, bromide, and thiocyanate were used as auxiliary ligands. The stability constant of [Pd(Me₆tren)]²⁺ in various ionic media was obtained: log β([Pd(Me₆tren)] = 30.5 (I = 1(NaCl)) and 30.8 (I = 1(NaBr)), as well as the formation constants of the mixed complexes [Pd(HMe₆tren)X]²⁺ from [Pd(HMe₆tren)(H₂O)]³⁺:log K = 3.50 = Cl^?) and 3.64 (X^? = Br^?) and [Pd(Me₆tren)X]⁺ from [Pd(Me₆tren)(H₂O)]²⁺: log K = 2.6 (X^? = Cl^?), 2.8(Br^?) and 5.57 (SCN^?) at I = 1 (NaClO₃). The above data, as well as the NMR measurements do not provide any evidence for the penta-coordination of Pd^II, proposed in some papers.     

Suzuki coupling catalyzed by a homoleptic Pd(I)-Pd(I) solvento complex

The Pd^I-Pd^I bonded complex [Pd₂(CH₃CN)₆][SbF₆]₂ is catalytically active towards Suzuki cross-coupling reactions of aryl bromides or chlorides with various arylboronic acids under mild conditions giving good to excellent yields. Its performance is enhanced by the introduction of stoichiometric or limited phosphines. The effects of different ligands, metal oxidation states [Pd(II), Pd(I) Pd(0)], bases and solvents have been examined.     

Convenient synthetic routes to MePdII,Me2PdII,and Me3PdIV complexes. The crystal structure of the MePdII complex [MePd(2,2′-bipyridyl)(γ-picoline)]BF4

The pyridazine complex [Me₂Pd(pyd)]_n, stable as a solid on storage at ca. −20°C and obtained on reaction of trans-PdCl₂(SMe₂)₂ pyridazine at low temperature, is an excellent precursor for the synthesis of Me₂Pd^II and Me₃Pd^IV complexes under mild conditions, in p Similarly, [MePd(SMe₂)(μ-I)]₂ is a suitable precursor for the synthesis of neutral and cationic MePd^II complexes, including [MePd(2,2′-     

Tripropylarsane Complexes of Palladium(II) and Platinum(II) — Syntheses,Spectroscopy, and Structures

Several palladium(II) and platinum(II) complexes of tripropylarsanes (AsR₃; R = Pr, iPr) with the formulae, [MCl₂(AsR₃)₂], [M₂Cl₂(μ‐Cl)₂(AsR₃)₂], [Pd₂Me₂(μ‐Cl)₂(AsR₃)₂], [Pd₂X₂(μ‐Pz)₂(AsR₃)₂] (X = Cl or Me, Pz = pyrazolate), [Pd₂Cl₂(μ‐Y)₂(AsR₃)₂] (Y = OAc or SPh), [MCl(S₂CNEt₂)(AsR₃)] and [PdCp(Cl)(AsiPr₃)] (M = Pd or Pt) have been prepared. All the complexes have been characterised by elemental analyses, IR and ¹H NMR spectroscopy. The stereochemistry of the complexes has been deduced from the spectroscopic data. The structures of [Pd₂Me₂(μ‐X)₂(AsiPr₃)₂] (X = Cl or Pz) have been established by single crystal X‐ray diffraction analyses. Both of the complexes have sym‐trans configuration. Strong trans influence of the methyl group is reflected on the Pd—X bond distances.     

31P-NMR and X-Ray Studies of the Complexes [HgX2 (1)]. (1=2, 11-bis (diphenylphosphinomethyl)benzo [c]phenanthrene,X=Cl,I)

The ³¹P{¹H}-NMR characteristics of the complexes [HgX₂( 1 )] and [HgX₂-(PPh₂Bz)₂] (X = NO₃, Cl, Br, I, SCN, CN) and the solid state structures of the complexes [HgCl₂( 1 )] and [HgI₂( 1 )] ( 1 = 2,11-bis (diphenylphosphinomethyl)benzo-[c]phenanthrene) have been determined. The ¹J(¹⁹⁹Hg, ³¹P) values increase in the order CN < I < SCN < Br < Cl < NO₃. The two molecular structures show a distorted tetrahedral geometry about mercury. Pertinent bond lengths and bond angles from the X-ray analysis are as follows: Hg? P = 2.485(7) Å and 2.509 (8) Å, Hg? Cl = 2.525 (8) Å and 2.505 (10) Å, P? Hg? P = 125.6(3)°, Cl? Hg? Cl = 97.0(3)° for [HgCl₂( 1 )] and Hg? P = 2.491 (10) Å and 2.500(11) Å, Hg? I = 2.858(5) Å and 2.832(3) Å, P? Hg? P = 146.0(4)°, I? Hg? I = 116.9(1)° for [HgI₂( 1 )]. The equation, derived previously, relating ¹J(¹⁹⁹Hg, ³¹P) and the angles P? Hg? P and X? Hg? X is shown to be valid for 1 .     

Bimetallic neutral palladium (II) bis(dithiolene) complex: Unusual synthesis,structural and theoretical study

The unusual synthesis of the dimeric dithiolene complex [(PPh₃)Pd(ethylene-1,2-dithiolate)]₂ (1), containing the simplest dithiolene ligand, has been achieved through the reaction between tetrathiafulvalene (TTF) and Pd(PPh₃)₄. The complex shows a folded structure in the solid state, according to single crystal X-ray analysis performed on crystals grown from two different system solvents and conditions, with a central [Pd₂S₂] ring folded about the S···S hinge by 67.9°. The optimized geometry at the DFT level is in excellent agreement with the experimental structure. Moreover, TD-DFT calculations allowed the assignment of the low energy band arising at 576 nm to the HOMO→LUMO transition, between frontier orbitals having mixed metal and dithiolene character.     

Palladium(II) and platinum(II) complexes of Schiff bases derived from 2,6-diacetylpyridine and S-methyldithiocarbazate and the X-ray crystal structure of the [Pd(mdapsme)Cl] complex

Condensation of 2,6-diacetylpyridine (dap) with S-methyldithiocarbazate (smdtc) in a 1:2 molar ratio yields a bicondensed pentadentate Schiff base (H₂dapsme) which reacts with K₂MCl₄ (M = Pd^II, Pt^II) giving stable complexes of empirical formula, [M(dapsme)] · 0.5Me₂CO. These complexes have been characterized by a variety of physico-chemical techniques. Condensation of dap with smdtc in a 1:1 molar ratio also yields the bicondensed Schiff base (H₂dapsme) as the major product, but a mono-condensed one-armed Schiff base (Hmdapsme) is also obtained as a minor product. The latter reacts with K₂PdCl₄ in an EtOH–H₂O mixture yielding a crystalline complex of empirical formula, [Pd(mdapsme)Cl], the crystal structure of which has been determined by X-ray diffraction. The complex has a distorted square-planar structure in which the ligand is coordinated to the palladium(II) ion as a uninegatively charged tridentate chelating agent via the pyridine nitrogen atom, the azomethine nitrogen atom and the thiolate sulfur atom; the oxygen atom of the acetyl group does not participate in coordination.     

The Quest for PdII–PdII Interactions: Structural and Spectroscopic Studies and Ab Initio Calculations on Dinuclear [Pd2(CN)4(μ‐diphosphane)2] Complexes

Structural and spectroscopic properties of and theoretical investigations on dinuclear [Pd₂(CN)₄(P–P)₂] (P–P=bis(dicyclohexylphosphanyl)methane ( 1 ), bis(dimethylphosphanyl)methane ( 2 )) and mononuclear trans‐[Pd(CN)₂(PCy₃)₂] ( 3 ) complexes are described. Xray structural analyses reveal Pd???Pd distances of 3.0432(7) and 3.307(4) Å in 1 and 2 , respectively. The absorption bands at λ>270 nm in 1 and 2 have 4d →5p_σ electronic‐transition character. Calculations at the CIS level indicate that the two low‐lying dipole‐allowed electronic transition bands in model complex [Pd₂(CN)₄(μ‐H₂PCH₂PH₂)₂] at 303 and 289 nm are due to combinations of many orbital transitions. The calculated interaction‐energy curve for the skewed dimer [{trans‐[Pd(CN)₂(PH₃)₂]}₂] is attractive at the MP2 level and implies the existence of a weak Pd^II–Pd^II interaction.     

Unusual crystal and molecular structure of palladium iminoquinonoid complex Pd2Cl2(PPh3)2L (L = N,N′-di(4-methyl-4-trichloromethylcyclohexa-2,5-dien-1-ylidene)hydrazine)

The structure of the first iminoquinonoid complex Pd₂Cl₂(PPh₃)₂L·0.4(Me₂CO), where L = N,N′-di(4-methyl-4-trichloromethylcyclohexa-2,5-dien-1-ylidene)hydrazine, was established by X-ray crystallography. The centrosymmetric and non-centrosymmetric molecules of meso-form (RS) and the molecule of optically active (SS) threo-form were found to be simultaneously present (cocrystallize) in the crystal. The molecule in the crystal has a general formula [Pd(meso-L)]₂[Pd(threo-L)].     

Palladium(II) Acetylide Complexes with Pincer‐Type Ligands: Photophysical Properties,Intermolecular Interactions,and Photo‐cytotoxicity

Two classes of cationic palladium(II) acetylide complexes containing pincer‐type ligands, 2,2′:6′,2′′‐terpyridine (terpy) and 2,6‐bis(1‐butylimidazol‐2‐ylidenyl)pyridine (C^N^C), were prepared and structurally characterized. Replacing terpy with the strongly σ‐donating C^N^C ligand with two N‐heterocyclic carbene (NHC) units results in the Pd^II acetylide complexes displaying phosphorescence at room temperature and stronger intermolecular interactions in the solid state. X‐ray crystal structures of [Pd(terpy)(C≡CPh)]PF₆ ( 1 ) and [Pd(C^N^C)(C≡CPh)]PF₆ ( 7 ) reveal that the complex cations are arranged in a one‐dimensional stacking structure with pair‐like Pd^II⋅⋅⋅Pd^II contacts of 3.349 Å for 1 and 3.292 Å for 7 . Density functional theory (DFT) and time‐dependent density functional theory (TD‐DFT) calculations were used to examine the electronic properties. Comparative studies of the [Pt(L)(C≡CPh)]⁺ analogs by ¹H NMR spectroscopy shed insight on the intermolecular interactions of these Pd^II acetylide complexes. The strong Pd−C_carbene bonds render 7 and its derivative sufficiently stable for investigation of photo‐cytotoxicity under cellular conditions.     

Mono and dinuclear Pd(II) complexes with pyrazole and imidazole-type ligands Synthesis, characterization and thermal behaviour

The cyanate-bridged cyclopalladated compound [Pd(C²,N-dmba)(μ-NCO)]₂ (dmba=N,N-dimethylbenzylamine) reacts in acetone with pyrazole (pz), 3,5-dimethylpyrazole (dmpz), imidazole (imz) and 2-methylimidazole (mimz) to give [Pd₂(C²,N-dmba)₂(μ-NCO)(μ-pz)] (1), [Pd₂(C²,N-dmba)₂(μ-NCO)(μ-dmpz)] (2), [Pd(C²,N-dmba)(NCO)(imz)] (3) and [Pd(C²,N-dmba)(NCO)(mimz)] (4), respectively. The compounds were characterized by elemental analysis, IR spectroscopy and TG. The thermal decomposition of the compounds occurs in three consecutive steps and the final decomposition products were identified as Pd(0) by X-ray powder diffraction. The thermal stability order of the complexes is 2>3>1>4.     

Acetylene Dithiolate Linking up the [Tp′W(CO)(CN)] Moiety with RuII or PdII

A series of heterodinuclear complexes with acetylene dithiolate (acdt^2?) as the bridging moiety were synthesised by a facile one‐pot procedure that avoided use of the highly elusive acetylene dithiol. Generation of the W–Ru complex [Tp′W(CN)(CO)(C₂S₂)Ru(η⁵‐C₅H₅)(PPh₃)] (Tp’=hydrotris(3,5‐dimethylpyrazolyl)borate) and the W–Pd complexes [Tp′W(CN)(CO)(C₂S₂)Pd(dppe)] and [Tp′W(CO)₂(C₂S₂)Pd(dppe)][PF₆] (dppe=1,2‐bis(diphenylphoshino)ethane), which exhibit a [W(η²‐κ²‐C₂S₂)M] core (M=Ru, Pd), was accomplished by using a transition‐metal‐assisted solvolytical removal of the Me₃Si‐ethyl thiol protecting groups. All intermediate species of the reaction have been fully characterised. The highly coloured W–Ru complex [Tp′W(CN)(CO)(C₂S₂)Ru(η⁵‐C₅H₅)(PPh₃)] shows reversible redox chemistry, as does the prototype complex [Tp′W(CO)₂(C₂S₂)Ru(η⁵‐C₅H₅)(PPh₃)][PF₆]. Single crystal X‐ray diffraction and IR, EPR and UV/Vis spectroscopic studies in conjunction with DFT calculations prove the high electronic delocalisation of states over the acdt^2? linker. Comparative studies revealed a higher donor strength and more pronounced dithiolate character of acdt^2? in [Tp′W(CN)(CO)(C₂S₂)Ru(η⁵‐C₅H₅)(PPh₃)] relative to [Tp′W(CO)₂(C₂S₂)Ru(η⁵‐C₅H₅)(PPh₃)]⁺. In addition, the influence of the overall complex charge on the metric parameters was investigated by single‐crystal X‐ray diffraction studies with the W–Pd complexes [Tp′WL₂(C₂S₂)Pd(dppe)] (L=(CN^?)(CO) or (CO)₂). The central [W(C₂S₂)Pd] units exhibit high structural similarity, which indicates the extensive delocalisation of charge over both metals.     

Synthesis and structural characterization of mono- and dinuclear NiII and PdII complexes derived from tetradentate 1,7-bis-(pyridin-2-yl)-2,6-diaza-1,6-heptadiene

The reaction of Schiff base 1,7-bis-(pyridin-2-yl)-2,6-diaza-1,6-heptadiene (L) with either NiCl₂·6H₂O or [Pd^IICl₂(CH₃CN)₂]/Na[BF₄] in 1?:?1 stoichiometry yielded mononuclear ionic complexes, trans-[Ni^II(L)(H₂O)₂]Cl₂·3H₂O (1·3H₂O) and [Pd^II(L)][BF₄]₂ (2), respectively; the reaction of L with [Pd^IICl₂(CH₃CN)₂] in 1?:?2 ratio yielded dinuclear cis-[Pd^II ₂(μ-L)Cl₄] (3). Complexes 1–3 were characterized by vibrational spectroscopy and X-ray diffraction; diamagnetic 2 and 3 were also characterized by NMR in solution. The molecular structures of 1 and 2 displayed tetradentate coordination of L with formation of two five-membered and one six-membered chelate rings for both complexes. In 3, L showed bidentate coordination mode for each pyridylimine toward Pd^II. Complex 1 has distorted octahedral geometry around Ni^II and an extended hydrogen-bond network; distorted square planar geometry around Pd^II in 2 and 3 was observed.     

Binuclear Orthometalated N,N-Dimethylbenzylamine Complexes of Palladium(II): Synthesis,Structures and Thermal Behavior

Abstract

Organochalcogenolate-bridged cyclometalated palladium(II) complexes of the formulae, [Pd₂(μ-Epy)₂(Me₂NCH₂C₆H₄-C,N)₂] (2) (E = S (2a), Se (2b)), [Pd₂(μ-SAr)(μ-Cl)(Me₂NCH₂C₆H₄-C,N)₂] (3) (Ar = Ph (3a), Mes (Mes = 2,4,6-Me₃C₆H₂) (3b)) and [Pd₂(μ-SeAr)₂(Me₂NCH₂C₆H₄-C,N)₂] (4) (Ar = Ph (4a), Mes (4b)), have been synthesized by the reactions of [Pd₂(μ-Cl)₂(Me₂NCH₂C₆H₄-C,N)₂] with lead or sodium salts of the chalcogenolate ligand. These complexes have been characterized by elemental analysis, mass spectral data, and NMR (¹H and ⁷⁷Se{¹H}) spectroscopy. The molecular structure of 2, determined by single crystal X-ray diffraction analysis, revealed a Epy-bridged head-to-tail arrangement in which the eight-membered “(PdECN)₂” ring adopts a distorted twist boat conformation. The Pd····Pd separation in 2a is within the van-der-Waals interaction but in 2b it is too large to support the presence of any metal–metal interaction. The thermal behavior of these complexes has been studied by thermogravimetric analysis.     

Hydrogenolysis of Dinuclear PCNR Ligated PdII μ-Hydroxides and Their Mononuclear PdII Hydroxide Analogues

The hydrogenolysis of mono- and dinuclear Pd^II hydroxides was investigated both experimentally and computationally. It was found that the dinuclear μ-hydroxide complexes {[(PCN^R)Pd]₂(μ-OH)}(OTf) (PCN^H=1-[3-[(di-tert-butylphosphino)methyl]phenyl]-1H-pyrazole; PCN^Me=1-[3-[(di-tert-butylphosphino)methyl]phenyl]-5-methyl-1H-pyrazole) react with H₂ to form the analogous dinuclear hydride species {[(PCN^R)Pd]₂(μ-H)}(OTf). The dinuclear μ-hydride complexes were fully characterized, and are rare examples of structurally characterized unsupported singly bridged μ-H Pd^II dimers. The {[(PCN^Me)Pd]₂(μ-OH)}(OTf) hydrogenolysis mechanism was investigated through experiments and computations. The hydrogenolysis of the mononuclear complex (PCN^H)Pd-OH resulted in a mixed ligand dinuclear species [(PCN^H)Pd](μ-H)[(PCC)Pd] (PCC=a dianionic version of PCN^H bound through phosphorus P, aryl C, and pyrazole C atoms) generated from initial ligand “rollover” C−H activation. Further exposure to H₂ yields the bisphosphine Pd⁰ complex Pd[(H)PCN^H]₂. When the ligand was protected at the pyrazole 5-position in the (PCN^Me)Pd−OH complex, no hydride formed under the same conditions; the reaction proceeded directly to the bisphosphine Pd⁰ complex Pd[(H)PCN^Me]₂. Reaction mechanisms for the hydrogenolysis of the monomeric and dimeric hydroxides are proposed.     

Halogenid‐Ionen als Katalysator: Metallzentrierte C–C‐Bindungsknüpfung ausgehend von Acetontril

Halide Ions as Catalyst: Metalcentered C–C Bond Formation Proceeded from Acetonitril AlMe₃ reacts at 20 ?C in acetonitrile to the complex [Me₃Al(NCMe)] ( 1 ). By addition of cesium halides (X^– = F^–, Cl^–, Br^–) a trimerisation to the heterocycle [Me₂Al{HNC(Me)}₂C(CN)] ( 2 ) has been observed. The reaction might be carried out under catalytic conditions (1–2 mol% CsX). The gallium complex [Me₂Ga{HNC(Me)}₂ · C(CN)] ( 3 ), generated under similar reaction conditions, can be converted to the silylated compound [Me₂Ga{Me₃SiNC(Me)}₂C(CN)] ( 4 ) by successive treatment with two equivalents n‐butyllithium and Me₃SiCl. 3 reacts under hydrolysis conditions (1 M hydrochloric acid) to the iminium salt [{H₂NC(Me)}₂C(CN)]Cl ( 5 ). A mixture of H₂O, Ph₂PCl and 3 in THF/toluene leads in a unusual conversion to the diphospane derivative [Ph₂P–P(O)(Me₂GaCl)] ( 6 ). 1 , 2 , 4 , 5 and 6 have been characterized by NMR, IR and MS techniques. X‐ray structure analyses were performed with 1 , 2 , 4 and 6 · 0.5 toluene. According this 1 possesses an almost linear axis AlNCC [Al1–N1–C3: 179,5(2)?; N1–C3–C4: 179,7(4)?]. 2 is an AlN₂C₃ six‐membered heterocycle with two iminium fuctions. One N–H group is responsible for a intermolecular chain‐formation through hydrogen bridges to an adjacent nitrile group along the direction [010]. The basic structural motif of the heterocycle 3 has been maintained after silylation to 4 . In 6 · 0.5 toluene an unit Me₂GaCl, originated from 3 , is coordinated to the oxygen atom of the diphosphane oxide Ph₂P–P(O)Ph₂.     

Ag(I) and Pd(II) complexes of a 1,3-dibenzhydryl substituted benzannulated N-heterocyclic carbene: Unexpected rearrangement, structures and catalytic studies

Reaction of the sterically bulky 1,3-dibenzhydrylbenzimidazolium bromide (Bh₂-bimyH⁺Br⁻) (A) with Pd(OAc)₂ in DMSO yielded a mono(carbene) Pd(II) complex 1 with a N-bound benzimidazole derivative, which resulted from an unusual NHC rearrangement reaction. Reaction of A with Ag₂O, on the other hand, cleanly gave the Ag(I) carbene complex [AgBr(Bh₂-bimy)] (2), which has been used as a carbene-transfer agent to prepare the acetonitrile complex trans-[PdBr₂(CH₃CN)(Bh₂-bimy)] (3). Dissociation of acetonitrile from complex 3 and subsequent dimerization afforded the dinuclear Pd(II) complex [PdBr₂(Bh₂-bimy)]₂ (4) in quantitative yield. All complexes were fully characterized by multinuclear NMR spectroscopies, ESI mass spectrometry and X-ray diffraction analysis. Furthermore, the catalytic activity of complex 4 in aqueous Suzuki-Miyaura cross-coupling reactions was studied and compared with that of its previously reported less bulky analogue [PdBr₂(ⁱPr₂-bimy)]₂.     

Characterization of a New Tetranuclear Cluster of Palladium, [Pd4(dppm)4(H)]—

The recently discovered and characterized [Pd₄(dppm)₄(H)]²⁺ cluster catalyst ( 1 ; dppm = Ph₂PCH₂PPh₂), slowly evolves in the presence of the reducing tetraphenylborate anion, to generate a new diamagnetic cluster [Pd₄(dppm)₄(H)]⁺ ( 2 ). The evolution of this starting material 1 , has been monitored using NMR (¹H and ³¹P), UV‐vis and ESR spectroscopy. This new 56‐electron Pd cluster has been characterized from X‐ray crystallography, and consists of a cyclic species exhibiting an approximate puckered square structure. The Pd₂ bond distances are 2.7367(10) and 2.7495(11)Å and indicate the presence of weak bonding. The diagonal Pd···Pd separations are 3.646(10) and 3.590(10)Å indicating that the square is relatively symmetric. Such a structure is unprecedented for “Pd_x(dppm)_x” species. Although not formally observed from the X‐ray data, the hydride is assumed to be fluxional as found in 1 . The cyclic voltammogram for 2 exhibits an irreversible reduction wave at —1.65V vs SCE which is greater than that found for 1 , and corroborates the lower oxidation state for Pd (+1/2). The Pd‐H bonding scheme and MO symmetry for a model cluster where the hydride has been placed at the center of the Pd₄ frame, have been addressed qualitatively using the EHMO model. These calculations demonstrate clearly that the Pd‐H bonding is strong.