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Defini&on: "a finite group of metal atoms that are held together mainly, or at least to a significant extent, by bonds directly between metal atoms, even though some non-‐metal atoms may also be in&mately associated with the cluster" – F.A. Co)on
M-‐M bonds and Cluster chemistry
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Two extreme types of metal clusters: • Molecular clusters are composed of 3 or more metal atoms stabilized by small molecules acAng as ligands, and characterized by metal-‐metal bonds, either localized or delocalized over the metal framework.
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•"Naked clusters" -‐ have no ligands, electronically unsaturated, naked metal parAcles formed in atomic beams; are extremely reacAve unless stabilized by a support or an inert matrix, e.g. [Pb5]2-‐. In this part of the course, we focus on transiAon metal clusters with 2 or more M-‐M bonds, and, parAcularly, late TM clusters with CO ligands.
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Importance: (a) EvoluAon of properAes as cluster size increases (e.g. structure, magneAc behaviour, ionizaAon potenAal): Transi&on from molecular → bulk metal domain. • metal-‐ligand charge transfer interacAons are limited to the metal atoms which are at the surface of the metal cluster; inner-‐core metal atoms behave as a metal parAcle. • properAes of a solid gradually evolve as atoms are brought together to form increasingly larger units.
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(b) InterpretaAon of data obtained from metal surfaces which are acAve heterogeneous catalysis sites (cluster-‐surface analogy). • models for chemi-‐ and physi-‐sorpAon of small molecules onto metallic surfaces -‐ e.g. fluxionality of the coordinated fragments, mechanism of hydrogen diffusion inside a metallic network, the distorAons in metal close-‐packing induced by intersAAal heteroatoms, etc.
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• chemical reac&vity of coordinated ligands in molecular metal clusters has been used for studying the reacAon mechanisms at the metal centres, and for mimicking elementary processes relevant to homogeneous and heterogeneous catalysts.
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c) FascinaAng structures (!). • first reports of organometallic transiAon metal clusters: Fe2(CO)9 (1905), Fe3(CO)12 (1907) and Co2(CO)8 (1910) • computer analysis of X-‐ray crystallographic data → novel structural features
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• variety of metal frame shapes, bonding modes for any given ligand and stereochemistry observed was only possible in the presence of several metal atoms • solid state chemists and physicists also became interested – clusters as par&culate metals enclosed in a ligand sphere.
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• 1000s of examples of ligated metal clusters have been reported. Most common clusters are formed by: -‐ early transiAon metals stabilized by halide or oxygen atoms (aggregates)
-‐ metals of group 6-‐10 stabilized by CO molecules
-‐ transiAon or post-‐transiAon metals having halogen atoms, phosphine groups, and main group elements as ligands • primary analyAcal tool is X-‐ray diffrac&on, bond lengths which are shorter than the sum of the covalent radii being a reliable indicaAon of M-‐M bonding.
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• one of the first reported X-‐ray structural determina&ons of TMCCs was of Fe2(CO)9 (1939), revealed CO ligands bridging the M-‐M bond.
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• historically, first unsupported M–M bonds were found in the analogous complexes Re2(CO)10 and Mn2(CO)10 (1957). Confirmed the existence of unsupported M-‐M bonds.
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• also very early on, the structure of the pentacoordinate Fe5(μ5-‐C)(CO)15 (1962) and [Re2Cl8]2-‐ .
• Interes&ng structures s&mulated research; lack of syntheses frustrated research. Recent systema&c syntheses have led to the explosion in cluster chemistry.
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STRUCTURE AND BONDING (1) TYPES OF CLUSTERS
(i) π-‐Donor clusters: early transi&on metals (oxida&on state +2 or +3) with π-‐donor ligands (O2–, S2–, Cl–, Br–, I–, OR–, etc.). Typically triangular or octahedral geometries.
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Electron dona&ng characteris&cs of common π-‐donor bridging ligands
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Examples: [Nb6Cl12]2+, [Ta6Cl12]2+ octahedral M6, edge-‐bridging Cl (1) [Mo6Cl8]4+, [W6Cl8]4+ octahedral M6, face-‐capping Cl (2) [Re3Cl12]3-‐ triangular Re3, 3 x μ-‐Cl, 9 x terminal Cl (3)
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• COs frequency as a ligand for stabilizing low oxidaAon state clusters arises because of its flexibility -‐ coordinaAon as terminal, μ-‐ and μ3-‐ are all common and, because they are all 2e donors (unlike Cl), intramolecular exchange processes frequently have very low acAvaAon energies. • H-‐ is not a π-‐acceptor ligand, but there are many carbonyl clusters that also have H-‐ as a ligand, so these clusters are usually considered together.
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Ligand bonding modes • see more extensive discussion on bonding modes of CO, hydrido and acetylenic ligands (later in course). • Cl ligand is a 1 e donor when terminal, or uses a lone pair to become a 3e donor when edge-‐bridging, or a 5e donor using an addiAonal lone pair when face bridging.
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Bonding modes and electron count for some common ligands
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(2) CLUSTER GEOMETRY – • high symmetry -‐ usually define regular polyhedra as triangle, tetrahedra, octahedra, etc • metal posiAons usually define deltahedra, i.e. Triangular faces; other arrangements, e.g. square-‐planar, trigonal prismaAc, square anAprisms occur to lesser extent
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• many represent fragments of close-‐packed arrays of metal atoms, hcp ccp or bcc
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• form inters&&al complexes containing hydrogen, carbon, nitrogen, sulfur, phosphorus, and arsenic, as well as other metal atoms in the case of high-‐nuclearity clusters, in the intersAces
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3 metal atom clusters • great majority are triangular. Structural differences require only small changes in total energies of molecules e.g. Fe3(μ-‐CO)2(CO)10 and Ru3(CO)12.
• linear e.g [Mn3(CO)14]-‐ or bent. Bond opening on a triangular cluster results in a linear geometry
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4 metal atom clusters: • many are tetrahedral with 6 M–M bonds, e.g. CpWIr3(CO)11
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• bu)erflies can be regarded as resulAng from a bond opening on the tetrahedron e.g. CpWIr3(CO)8(F3CC≡CCF3)2
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• interplanar angle in bu)erflies can be near 180°, as in [Re4(CO)16]2-‐ • Fe4(μ4-‐C)(μ-‐CO)(CO)12 contains a carbide atom in the middle of the cluster, approximately colinear with the wing-‐Ap atoms, providing an excellent model for a carbon atom on a metal surface.
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• square-‐planar geometries -‐ for carbonyl clusters, onen contain a capping ligand e.g. Fe4(μ4-‐NEt)(μ3-‐η2-‐ONEt)(μ-‐CO)3(CO)8
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• bond rupture in a bu)erfly arrangement leads to a spiked triangle geometry, as in Re4(μ-‐H)3H(CO)15
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Examples of prepara&ons
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Ouch!
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n+1 n+1
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Os6(CO)18
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n = 3
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EAN PSPET
18x3 = 54-‐6 = 48 48-‐(12x3) = 12 à 6 pairs (arachno) 18x4 = 72-‐12 = 60 60-‐ (12x4) = 12à 6 pairs (nido) 18x5 = 90 – 18 = 72 72-‐(12x5) = 12 à 6 pairs (closo) 18x6 = 108 -‐24 = 84 n + 2
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CHEM 3032 – Cluster Chemistry
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