We present the first successful isolation and crystallographic characterization of a Mackay 55-metal-atom two-shell icosahedron, Pd55L12(μ3-CO)20 (L = PPri3) (1). Its two-shell icosahedron of pseudo-Ih symmetry (without isopropyl substituents) enables a structural/bonding comparison with interior 55-metal-atom two-shell icosahedral geometries observed within the multi-shell capped 145-metal-atom three-shell Pd145(CO)72(PEt3)30 and 165-metal-atom four-shell Pt-centered (μ12-Pt)Pd164-xPtx(CO)72(PPh3)20 (x ≈ 7) nanoclusters, and within the recently reported four-shell Au133(SC6H4-p-But)52 nanocluster. DFT calculations carried out on a Pd55(CO)20(PH3)12 model analogue, with triisopropyl phosphine substituents replaced by H atoms, revealed a positive +0.84 e charge for the entire Pd55 core, with a highly positive second-shell Pd42 surface of +1.93 e.

The monogold [(μ14-Au)Pd22(CO)20(PEt3)8]+ nanocation (2, with a [(CF3CO2)2H]− counterion) is shown to be a versatile precursor for the generation of three different neutral Au–Pd nanoclusters with double gold content in their distinctly dissimilar bimetallic architectures. These carbon monoxide (CO)-induced conversions are based on the reduction of AuI to Au0 that is controlled by the reaction medium. Under basic and acidic conditions, the known Au2Pd21(CO)20(PEt3)10 (3; >90% yield) and Au2Pd28(CO)26(PEt3)10 (4; ∼40% yield), respectively, were obtained, whereas neutral conditions gave rise to the new (μ12-Au)2Pd42(CO)30(PEt3)12 (1; ∼10–20% yield; all yields based on gold). The molecular structure of 1, established from a 100 K CCD X-ray diffraction study, consists of a five-layer hexoganol close-packed (hcp) Au2Pd42 framework of pseudo-D3h symmetry (crystallographic D3 site symmetry) of the Pd6/AuPd9/Pd12/AuPd9/Pd6 layer sequence, with the Au atoms centering two identical hcp (μ12-Au)Pd12 face-fused anti-cuboctahedral fragments. The 12 Et3-attached P atoms are coordinated to the triangular vertex Pd atoms in the four outer layers (except the middle Pd12); all five layers are stapled by interlayer bridging COs. The radial Aucent–Pd mean distance of 2.79 Å within the two symmetry-equivalent (μ12-Au)Pd12 anti-cuboctahedral fragments of 1 is identical with the radial Pdcent–Pd mean distances within hcp (μ12-Pd)Pd12 anti-cuboctahedral fragments of the two geometrically related nondistorted layered structures of Pd52(CO)36(PEt3)14 and [Ni9Pd33(CO)41(PPh3)6]4– ([PPh4]+ counterion), indicating a strain-free structural effect upon the substitution of Au for Pd in their analogous hcp layer-stacked arrangements. It provides prime evidence for an extension to 1 of our previous self-consistent experimental/theoretical-based hypothesis for delocalization of the 6s valence Au electrons in Au2Pd21 (3) and Au2Pd28 (4) toward a formal closed-shell Au+ configuration that is electronically equivalent to that of zerovalent Pd.

This first homopalladium carbido cluster, {Pd4(μ4-C)}Pd32(CO)28(PMe3)14 (1), was isolated (3–7% yields) from an ultimately simplified procedure—the reaction of CHCl3 under N2 with either Pd8(CO)8(PMe3)7 or Pd10(CO)12(PMe3)6 at room temperature. Charge-coupled device (CCD) X-ray diffraction data at 100 K for 1·2.5 C6H14 (1a) and 1·3 CHCl3 (1b) produced closely related molecular parameters for 1. This {Pd4C}Pd32 cluster (1) possesses a highly unusual tetracoordinated carbide atom that causes a major distortion of a central regular Pd4 tetrahedron into a new symmetry type of encapsulated Pd4 cage of pseudo-D2 (222) symmetry. Mean Pd–Pd distances for the three pairs of opposite twofold-equivalent Pd–Pd tetrahedral-like edges for 1a are 2.71, 2.96, and 3.59 Å; the mean of the four Pd–C distances [range, 1.87(2)–1.94(2) Å] is 1.91 Å. An astonishing molecular feature is that this {Pd4C}Pd32 cluster (1) is an isostructural and electronically equivalent analogue of the nanosized Au4Pd32(CO)28(PMe3)14 (2). Cluster 2, likewise a pseudo-D2 molecule, contains a geometrically analogous tetrahedrally deformed interior Au4 entity encapsulated within an identical Pd32(CO)28(PMe3)14 shell; mean distances for the three corresponding symmetry-equivalent pairs of slightly smaller opposite tetrahedral-distorted Au–Au edges are 2.64, 2.90, and 3.51 Å. A computational study by both a natural population analysis (NPA) and an atoms-in-molecules (AIM) method performed on model analogues {Pd4C}Pd32(CO)28(PH3)14 (1-mod) and Au4Pd32(CO)28(PH3)14 (2-mod) suggested that the negatively charged Au4 entity in 2-mod may be described as two weakly interacting electron-pair Au2 intradimers. In contrast, an NPA of the {Pd4C} entity in 1-mod revealed that two similarly oriented identical Pd2 intradimers of 2.71 Å are primarily stabilized by Pd–C bonding with a negatively charged carbide atom. The isostructural stabilizations of 1 and 2 are then attributed to the similar sizes, shapes, and overall negative charge distributions of the electronically equivalent interior {Pd4C} and Au4 entities. This resulting remarkable structural/electronic equivalency between 1 and 2 is consistent with the greatly improved performances of commercial palladium catalysts for vinyl acetate synthesis by gold-atom incorporation to suppress carbonization of the Pd atoms, namely, that the extra Au 6s1 valence electron of each added Au atom provides an effective “negative charge protection” against electron-donating carbon atoms forming Pd carbido species such as {Pd4C}.

This article presents the personal saga of one of the authors (LFD) in the determination of the solid-state structure of Fe3(CO)12. We also present the results of our recent determination of its solid-state structure at low temperature (100 K), in which we have used a modern area-detector diffractometer in order to examine more precisely its temperature-dependent structural variations reported by Braga et al. in 1994 from a point-detector diffractometer. These investigations provide a striking illustration of the remarkable advances over the last six decades in both computational hardware and software packages as well as the recent improvements in hardware data-collection instrumentation that have given rise to X-ray crystallography now being the most powerful (and in most cases the only unambiguous) physical method for elucidating the static structures of complex metal clusters. Other experimental measurements and resulting speculations concerning the dynamic/fluxional behavior of Fe3(CO)12 and closely related analogues in the solid state and in solution are briefly mentioned, as are recent theoretical analyses.

The new Tl(I)–Pd(0) cluster Pd9[μ3/3-Tl(acac)](μ2-CO)6(μ3-CO)3(PPh3)6 (1) was prepared in high yields (over 90%), both by reaction of Pd10(CO)12(PPh3)6 (4), PPh3, and TlPF6 in THF in the presence of acetylacetone (Hacac) and base (NEt3) and by direct reaction of Pd10(CO)12(PPh3)6 with PPh3 and Tl(acac). The composition and molecular structure of 1 were unambiguously established from 100 K CCD X-ray diffractometry studies of two solvated crystals, 1·1.5Hacac·0.5THF (1A) and 1·0.3THF (1B), which showed essentially identical geometries for the entire Pd9Tl(CO)9P6 fragment of pseudo-C3v symmetry; its composition is in agreement with X-ray Tl/Pd field-emission microanalysis with a scanning electron microscope for crystals of 1B. This cluster can be viewed as a markedly deformed Pd6 octahedron (oct) with the three Pd(oct) atoms of one of its eight triangular faces connected both by three edge-bridging wingtip (wt) Pd(μ2-CO)2PPh3 fragments and by a symmetrical capping Tl(I). Three triply bridging carbonyl ligands asymmetrically cap the lower alternate 3-fold-related triangular faces of the Pd6 octahedron, and the three other PPh3 ligands are each coordinated to Pd atoms in the geometrically opposite staggered Pd(oct)3 face. The 6s25d10 Tl(I) is also equivalently attached to both chelating O atoms of a bidentate acetylacetonate (acac) monoanion. Although the C2 axis of the pseudo-C2v planar Tl(acac) fragment is approximately parallel to the pseudo-C3 axis of the TlPd9 core, the orientation of the Tl(acac) plane relative to the octahedral-based Pd9 geometry is considerably different for each of the three independent nondisordered molecules of 1 in 1A and 1B; these different planar Tl(acac) orientations may be mainly attributed to anisotropic crystal-packing effects. Coordination of the Tl(I) atom to the three Pd(oct) atoms of the Pd9 core presumably occurs via its so-called “inert” 6s2 electron pair with resulting three short Tl–Pd(oct) connectivities of mean distance 2.83 Å; these connectivities together with three longer Tl–Pd(wt) ones of mean distance 3.15 Å give rise to a (crown-like)Pd6 sextuple (μ3/3-Tl) coordination mode. Of particular stereochemical interest is a comparison of solution behavior of 1 with that for the known structurally related analogue, Pd9[μ3-TlCo(CO)3L](μ2-CO)6(μ3-CO)3L6 (2) (with L = PEt3 instead of PPh3). In 2 the Tl(I) is alternatively attached to a trigonal-bipyramidal Co(CO)3L monoanion and primarily coordinated to the three inner Pd(oct) atoms of a similar PR3/CO-ligated octahedron; corresponding Tl–Pd(oct) and Tl–Pd(wt) mean distances for two independent molecules in 2 are 2.77 and 3.31 Å, respectively. Variable-temperature 31P{1H} NMR solution data of 1 indicate the occurrence of presumed fast wobbling-like motion of the [μ3/3-Tl(acac)] entity about the pseudo-C3 axis of the Pd9(μ2-CO)6(μ3-CO)3P6 fragment without Pd–Tl detachment (i.e., the entire cluster of 1 remains intact). In direct contrast, corresponding temperature-dependent 31P and 13C NMR data of 2 instead are consistent with rapid, reversible dissociation/association of the entire [μ3-TlCo(CO)3L] ligand from the analogous Pd9(μ2-CO)6(μ3-CO)3P6 fragment of 2. This highly dissimilar dynamic solution behavior that points to a stronger Tl(I) attachment to the Pd9 core in 1 than that in 2 may be attributed from the above crystallographic evidence to greater involvement of the outer three wingtip Pd(wt) atoms in bonding connectivities to the Tl(I) in 1 compared to predominant bonding connectivities of only the three inner Pd(oct) atoms to the Tl(I) in 2. 1H NMR solution spectra of 1 also suggest significant covalent character in the bidentate Tl–O(acac) bonding in 1 based upon the observation of H(acac)–Tl coupling; this premise is consistent with its Tl–O distances of 2.35 Å (av) being ca. 0.2 Å shorter than those of 2.52 Å (av) found in crystalline Tl(acac), which with no observed H–Tl NMR coupling in solution implies ionicity of its bidentate Tl–O bonding. Both 1 and 2 conform to an 86 CVE count expected for an octahedral metal polyhedron based upon the Tl(I) and each wingtip Pd(μ2-CO)2L fragment contributing 2 and 4 CVEs, respectively.

Initially isolated from Pd10(CO)12(PEt3)6 (5) and Au(SMe2)Cl precursors in a two-step carbon monoxide (CO)-involved procedure, the nanosized interpenetrating bicuboctahedral gold (Au)–palladium (Pd) Au2Pd28(CO)26(PEt3)10 (1) was then directly obtained in 25–30% yield from the CO-induced reaction of the CO-stable Au-centered cuboctahedral Au2Pd21(CO)20(PEt3)10 (3) with the structurally analogous CO-unstable Pd23(CO)20(PEt3)10 (4). Our hypothesis that this latter synthesis is initiated by the reaction of 3 with coordinatively unsaturated homopalladium species resulting from CO-induced fragmentation of 4 was subsequently substantiated by the alternatively designed synthesis of 1 (∼25% yield) from the CO-induced reaction of 3 with the structurally dissimilar CO-unstable Pd38(CO)28(PEt3)12 (6). The composition of 1, unambiguously established from a 100 K CCD X-ray diffractometry study, is in accordance with single-crystal X-ray Au–Pd field-emission microanalysis. The pseudo-C2h 30-atom Au2Pd28 geometry of 1 may be formally derived via substitution of the interior (μ12-Pd)2 moiety in the interpenetrating bicuboctahedral Pd20 kernel of the known isostructural Pd30(CO)26(PEt3)10 (2) with the corresponding interior (μ12-Au)2 moiety, in which the otherwise entire metal-core geometry and CO/PR3-ligated environment are essentially not altered. Of major significance is that this interior nonisovalent Pd-by-Au replacement in 2 produces CO-stable 1, whereas nanosized CO/PR3-ligated homopalladium Pdn clusters with n > 10 are generally unstable under CO. Because the two adjacent encapsulated Au atoms of 2.811(1) Å separation are not present on the metal surface, isolation of 1 under CO is ascribed to an electronic property. The virtually ideal geometrical site-occupancy fit between 1 and 2 provides definite crystallographic evidence for extensive delocalization in 1 of the two valence Au 6s electrons over the entire cluster (instead of a “localized” covalent Au–Au electron-pair interaction). Gradient-corrected (pseudo-scalar-relativistic) density functional theory (DFT) calculations were performed on the isostructural Au2Pd28(CO)26(PH3)10 (1-H) and Pd30(CO)26(PH3)10 (2-H) model clusters along with hypothetical [Au2Pd28(1-H)]2+ and [Pd30(2-H)]2– analogues (with phosphine ethyl substituents replaced by hydrogen ones). Natural population analysis of these four model clusters revealed similar highly positively charged metal surfaces of 28 Pd atoms relative to the two negatively charged interior metal atoms, which reflect a partially oxidized metal surface due to dominant CO back-bonding. The surprising observation that each less electronegative interior Pd atom in 2-H is more negatively charged by 0.30e than each interior Au atom in 1-H points to a more cationic Au in 1 than interior Pd in 2; this unexpected (opposite) charge difference is consistent with delocalization of each Au 6s valence electron toward a Au+ configuration. This premise is in agreement with the calculated Wiberg bond index (WBI) value of 0.055 for the Au–Au bond order in 1-H versus the WBI single-bond value of 1.01 obtained from analogous DFT calculations for the bare, neutral Au2 dimer, which has a much shorter spectroscopically determined gas-phase distance of 2.472 Å (that corresponds to a “localized” electron-pair interaction). Isolation of 1 under CO is of prime importance in nanoscience/nanotechnology in establishing relative stabilizations toward CO in well-defined CO/PEt3-ligated nonisovalent Pd2-by-Au2-substituted Au2Pdn–2 clusters [namely, n = 30 (1) and 23 (3)]. These important stereochemical implications have a direct relevance to the recent report of the higher tolerance to CO poisoning of highly active Au–Pd nanoparticle catalysts used for the complete conversion of formic acid into high-purity hydrogen (and CO2) for chemical hydrogen storage.

Our synthetic exploratory efforts to obtain new nanosized Au–Pd carbonyl/phosphine clusters by use of the trimethylphosphine precursor Pd10(CO)12(PMe3)6 with smaller-sized PMe3 ligands (versus precursors with relatively larger PEt3 ligands) have previously produced via reaction with Au(SMe2)Cl an unusual Au4Pd32(CO)28(PMe3)14 (2) containing a pseudo-D2 36-atom Au4Pd32 core-geometry with a highly distorted encapsulated Au4 tetrahedron. Herein we report a striking illustration that analogous precursors under different reaction conditions have given rise to another new type of Au–Pd cluster, Au4Pd28(CO)22(PMe3)16 (1), that not only has a completely dissimilar Au4Pd28 core-geometry with a nearly regular encapsulated Au4 tetrahedron but also a totally different postulated multitwinned-composite growth-pattern. This extraordinary cluster, which was obtained from the reaction of Pd10(CO)12(PMe3)6 with Au(PPh3)Cl or Au(SMe2)Cl (estimated yield, ∼20–40%), has a 32-atom Au4Pd28 framework that roughly conforms to cubic T (23) symmetry that is maintained by inclusion of the 16 PMe3 P atoms but is completely reduced to general C1 (1) symmetry by inclusion of the 22 bridging CO ligands. 1 was isolated under different crystallization conditions to give two solvated crystal forms: namely, 1a as diisopropyl-solvated triclinic (\(\hbox{P}\bar 1 \) with Z = 2), and 1b as THF/hexane-solvated monoclinic (P2/c with Z = 4). A comparative analysis of resulting low-temperature CCD X-ray diffractometry determinations revealed an amazing molecular similarity between the actual shapes of the highly deformed Au4Pd28 architectures and ligand connectivities of 1 within the two dissimilar crystal structures. These results clearly indicate that the large observed localized geometrical distortions of 1 are primarily induced by intracluster strain-releasing effects and not by crystal-packing interactions. We propose a multitwinned growth-pattern of its Au4Pd28 core involving the formation of a Au4Pd24 composite-twinned framework formed from four markedly deformed interpenetrating three-layer Au-centered (Pd3)A(Au(n)Pd6)B(Au3)C cuboctahedra (n = 1–4) that are oriented along the four localized threefold axes of the Au4 tetrahedron. The other four outermost (external) Pd atoms that are tetrahedrally disposed about the Au4Pd24 composite presumably provide stabilization by face-condensations (i.e., three tetracapped, one tricapped). This new type of multitwinned bimetallic cluster has direct relevance to both ligated and non-ligated (bare) non-crystalline metal nanoparticles, of which many have been postulated to be multitwinned.

The original synthesis and stereochemical characterization of the [Pd12(μ3-I)2(μ4-I)3(μ2-CO)6L6]+ monocation (1) (L = PEt3; [PF6]− salt) was an outgrowth of our investigation of the chemical behavior of two unusual thallium–palladium clusters: (μ6-Tl)[Pd3(CO)3L3]2+ (2) which possesses a Pd3TlPd3 sandwich framework, and [Tl2Pd12(CO)9L9]2+ (3) which may be viewed as edge-fusions of three Pd5 trigonal bipyramids to a central Tl2Pd3 trigonal bipyramid. Room-temperature reactions of 2 and 3 with I2 in THF gave rise in each case to small yields (<10%) of 1 along with two square-planar palladium(II) co-products, trans-Pd2(μ2-I)2I2L2 (7) and trans-PdI2L2 (8) (L = PEt3). The geometries and compositions of 1, 7, and 8 were unequivocally established from low-temperature CCD X-ray crystallographic determinations. 1 was characterized by solid-state/solution IR and multinuclear (31P, 13C, 1H) NMR spectra. The 12-atom metal-core architecture of this geometrically unprecedented Pd6(μ3-Pd)6(μ3-I)2(μ4-I)3 kernel of 1 of crystallographic D3 (32) site symmetry may be envisioned as a distorted hexacapped octahedral Pd(oc)6Pd(cap)6 core with its two metal-uncappedtrans octahedral Pd(oc)3 faces additionally capped by iodide μ3-I atoms. The three tetracapping μ4-I atoms are each coordinated to two Pd(oc) and two adjacent Pd(cap) atoms. A comparative geometrical/qualitative bonding analysis of the hexacapped octahedral Pd(oc)6Pd(cap)6 core in 1 with the structurally analogous cores in the recently reported Pd12 clusters, Pd12(μ2-CO)6(μ3-CO)6(PR3)6 (R = n-Bu (4), Ph (5)), revealed significantly different architectural features but yet emphasized the importance of the Pd(cap) atoms in stabilizing the Pd(oc) octahedra in 1, 4, and 5. In fact, strong bonding interactions of the μ3-I and μ4-I atoms to the Pd12 polyhedron in 1 are evidenced by its black-violet crystals being air-stable for at least one month and by 1 dissolved in THF, acetone, or acetonitrile not undergoing decomposition to AgI upon addition of Ag(OAc). An exploration via the structure-to-synthesis approach of possible preparative pathways involving 14 different chemical reactions was carried out in order to isolate 1 in much higher yields. This systematic investigation demonstrated the importance of conproportionation reactions (i.e., Pd(0) + Pd(II) → Pd(1/2) in 1) utilizing the co-products trans-Pd2(μ2-I)2I2L2 (7) and trans-PdI2L2 (8), in different chemical reactions as palladium(II) precursors; high yields of 1 (ca. 50% based upon 6) were obtained from conproportionation reactions in THF of palladium(0) Pd10(CO)12L6 (6) with 7 in the presence of Pd(OAc)2 and (NBu4n)(PF6). The square-planar palladium(II) geometries of the iodide-bridged dimeric 7 and monomeric 8 are compared with each other and with those of the previous crystallographically determined 8 (at room temperature) and several crystallographically known analogues (with different phosphine ligands); corresponding molecular parameters were found to be in remarkably close agreement with distinct bond-length variations in bridging Pd–I(b) and terminal Pd–P bonds being readily attributed to the well-documented trans influence.This geometrically unprecedented and unusually stable Pd(+1/2) monocation (1) of crystallographic D3 (32) site symmetry was initially isolated in small yields (<10%), together with two structurally determined square-planar Pd(II) co-products, trans-Pd2 (μ2-I)2I2L2 (7) and trans-PdI2L2 (8) (L = PEt3), and characterized by low-temperature CCD X-ray diffractometry, solid-state/solution IR, and multinuclear (31P, 13C, 1H) NMR measurements. A systematic synthetic exploration involving 14 different chemical reactions demonstrated the importance of conproportionation reactions in increasing the yields of 1 to ca. 50%. Comparative structural/bonding analyses of 1, 7, and 8 with geometrically known analogues are given.

The preparation, isolation, and structural/bonding characterization of four high-nuclearity neutral homopalladium clusters, Pd16(CO)13(PMe3)9  1, Pd35(CO)23(PMe3)15  2, Pd39(CO)23(PMe3)16  3 and Pd59(CO)32(PMe3)21  4, and a minor bimetallic product, Pd29Ni3(CO)22(PMe3)13  5, are given. The four homopalladium clusters were characterized by CCD X-ray crystallographic determinations, elemental analyses, IR, multinuclear NMR, and cyclic voltammetry; because 5 was obtained in very low yields, both its molecular geometry and composition were established from the X-ray crystallographic analysis. These five clusters, obtained from reactions of a ccp Pd–Ni carbonyl cluster precursor and PMe3 (with or without acetic acid), exhibit five different types (four unprecedented) of centered icosahedral-based transition-metal frameworks: (1) the Pd16 core in 1 possesses a centered Pd13 icosahedron. (2) The Pd35 and Pd39 cores in 2 and 3 each have a face-fused centered Pd23 biicosahedron with linear (pseudo-D3h) and bent (pseudo-C2v) geometries, respectively; the pseudo-D3h central Pd29 polyhedron of the Pd35 core in 2 approximately conforms to five interpenetrating centered icosahedra. (3) The crystallographic-D3 (32) Pd59 core in 4 has two centered Pd13 icosahedra that are indirectly connected viatrans double face-sharing with an inner face-fused Pd9 bioctahedron; the entire nanosized face-condensed Pd59 core has 11 interior Pd(i) atoms. (4) The Pd29Ni3 core in 5 contains a pseudo-Td central Pd26 polyhedron comprised of four interpenetrating centered icosahedra. The existence of these highly condensed icosahedral-based metal carbonyl clusters, found only for Pd but not for the other eight Group 8–10 transition metals, may be ascribed to Pd metal having the weakest metal–metal bonding (i.e., smallest cohesive energy). Electronic closed-shell stabilization of each of these clusters is indicated by electron-counting condensation rules giving calculated values in exact agreement with observed electron counts for the metal cores in 1, 2, 4, and 5 (i.e., irregular condensations prevent a reliable electron count in 3). Proposed growth sequences provide logical pathways in the formation of the central palladium fragments in 2, 3, 4 and 5 from the centered Pd13 icosahedral fragment in 1.

Structural/bonding considerations of two new Pt–Au clusters, [Pt3(AuPPh3)5(μ2-CO)2(CO)2PPh3]+ (1) and [(μ6-Au){Pt3(μ2-CO)3(PMe3)4}2]+ (2) isolated (as chloride salts), revealed: (i) that the heretofore unknown 20-electron Pt-centered Pt2Au5 icosahedral cage fragment (five missing vertices) of 1 is best viewed as a 44-electron triangular Pt3 adduct of a nearly planar 39-electron [Pt3(μ2-CO)2L3]+ (L3 = (CO)2PPh3) and five one-electron donating AuPPh3 ligands; and (ii) that the geometrically distorted trimethylphosphine “full” Pt3AuPt3 sandwich of 2 is the first example of two nucleophilic 44-electron triangular Pt3(μ2-CO)3L4 (3 : 3 : 4) units (L = PMe3) which asymmetrically encapsulate a central electrophilic Au(I).

The synthesis, isolation, and stereochemical characterization of Au2Pd41(CO)27(PEt3)15 (1) are described. This nanosized Au2Pd41 cluster (maximum metal-core diameter, 1.04 nm) was originally obtained with Au2Pd21(CO)20(PEt3)10 as low-yield by-products together with Pd145(CO)x(PEt3)30 (x ∼ 60) from the reaction of Pd(PEt3)2Cl2 and Au(PPh3)Cl in DMF with NaOH under CO atmosphere. The subsequent preparation of Au2Pd21(CO)20(PEt3)10 in greatly improved yields (preceding article) thereby provided the starting material that led to the isolation of 1 in reasonable yields (54%) from an overnight refluxing of the preformed Au2Pd21 cluster in THF under N2. Both the composition (subsequently ascertained from elemental analysis) and molecular geometry of 1 were unequivocally established from a low-temperature CCD X-ray diffraction study, which revealed a cubic unit cell of P213 symmetry with four molecules of 1 and four co-crystallized triphenylphosphine oxide molecules each lying on a crystallographic three-fold axis. The entire Au2Pd41 core of pseudo-C3h symmetry may be viewed as a central Au2Pd29 fragment of pseudo-D3h symmetry composed of two heretofore geometrically unknown 13-coordinated Au-centered (μ13-Au)Pd13 polyhedra that share a common internal Pd(i)3 triangular face perpendicular to the C3 principal axis and of three three-fold-related interpenetrating 12-coordinated Pd-centered (μ12-Pd)Au2Pd10 icosahedra. A comparative analysis of this central Au2Pd29 fragment in 1 with an internal Au(i)2Pd(i)3 trigonal bipyramid vs. the corresponding central Pd29 fragment in the known homopalladium Pd35(CO)23(PMe3)15 (2) with an internal Pd(i)5 trigonal bipyramid resulting from five interpenetrating 12-coordinated Pd-centered [(μ12-Pd)Pd12] icosahedra is particularly illuminating; it provides a striking illustration of the remarkable observed difference between Pd- vs. Au-centered polyhedra which is attributed to a large electronegativity-mismatch in radial bonding interactions that occurs upon replacement of the Pd-centered atom with a highly electronegative Au-centered atom. The entire Au2Pd41 core-geometry is obtained by additional face-condensations of 12 tetracapping Pd(cap) atoms. This cluster is stabilized by 15 PEt3 ligands and 27 doubly- and triply-bridging CO ligands. A close geometrical resemblance between the three three-fold-related Au2Pd14 moities within the Au2Pd41 core in 1 and the entire Au2Pd14 core in the known [Au2Pd14(CO)9(PMe3)11]2+ dication (3) is observed; resulting stereochemical implications are given.

Reactions of Pd(PEt3)2Cl2 and Au(PPh3)Cl in DMF with NaOH under CO atmosphere gave rise to the unique capped three-shell homopalladium Pd145(CO)x(PEt3)30 (x ∼ 60) and two neutral Au–Pd clusters: Au2Pd21(CO)20(PEt3)10 (1) and Au2Pd41(CO)27(PEt3)15 (following article). Similar reactions with Pd(PMe3)2Cl2 being used in place of Pd(PEt3)2Cl2 afforded Au2Pd21(CO)20(PMe3)10 (2), the trimethylphosphine analogue of 1, and the electronically equivalent [AuPd22(CO)20(PPh3)4(PMe3)6]− monoanion (3) as the [PPh4]+ salt. Each of these three air-sensitive 23-atom heterometallic Au–Pd clusters was obtained in low yields (7–25%); however, their geometrical similarities with the known cuboctahedral-based homopalladium Pd23(CO)20(PEt3)10 (4), recently obtained in good yields from Pd10(CO)12(PEt3)6, suggested an alternative preparative route for obtaining 1. This “structure-to-synthesis” approach afforded 1 in 60–70% yields from reactions of Pd10(CO)12(PEt3)6 and Au(PPh3)Cl in DMF with NaOH under N2 atmosphere. Both the compositions and atomic arrangements for 1, 2 and 3 were unambiguously established from low-temperature single-crystal CCD X-ray crystallographic determinations in accordance with their nearly identical IR carbonyl frequencies. Cluster 1 was also characterized by 31P{1H} NMR, cyclic voltammetry (CV) and elemental analysis. The virtually identical Au2Pd21 core-architectures of 1 and 2 closely resemble that of 4, which consists of a centered hexa(square capped)-cuboctahedral Pd19 fragment of pseudo-Oh symmetry that alternatively may be viewed as a centered Pd19 ν2-octahedron (where νn designates (n + 1) equally spaced atoms along each edge). [AuPd22(CO)20(PPh3)4(PMe3)6]− (3) in the crystalline state ([PPh4]+ salt) consists of two crystallographically independent monoanions 3A and 3B; a superposition analysis ascertained that their geometries are essentially equivalent. A CV indicates that 1 reversibly undergoes two one-electron reductions and two one-electron oxidations; these reversible redox processes form the basis for an integrated structural/electronic picture that is compatible with the existence of the electronically-equivalent 1–3 along with the electronically-nonequivalent 4 (with two fewer CVEs) and other closely related species.