DOCSLIB.ORG

"Non-VSEPR" Structures and Bonding in D0 Systems

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
REVIEWS ªNon-VSEPRº Structures and Bonding in d0 Systems Martin Kaupp* Dedicated to the memory of Marco Häser (1961 ± 1997) Under certain circumstances, metal denum or tungsten enzymes), or mate- d) p bonding. Suggestions are made complexes with a formal d0 electronic rials science (e.g. ferroelectric as to which of the factors are the configuration may exhibit structures perovskites or zirconia). Moreover, dominant ones in certain situations. that violate the traditional structure their electronic structure without for- In heteroleptic complexes, the compe- models, such as the VSEPR concept or mally nonbonding d orbitals makes tition of s and p bonding of the various simple ionic pictures. Some examples them unique starting points for a gen- ligands controls the structures in a of such behavior, such as the bent gas- eral understanding of structure, bond- complicated fashion. Some guidelines phase structures of some alkaline earth ing, and reactivity of transition metal are provided that should help to better dihalides, or the trigonal prismatic compounds. Here we attempt to pro- understand the interrelations. Bents coordination of some early transition vide a comprehensive view, both of the rule is of only very limited use in these metal chalcogenides or pnictides, have types of deviations of d0 and related types of systems, because of the para- been known for a long time. However, complexes from regular coordination mount influence of p bonding. Finally, the number of molecular examples for arrangements, and of the theoretical computed and measured structures of ªnon-VSEPRº structures has in- framework that allows their rationali- multinuclear complexes are discussed, creased dramatically during the past zation. Many computational and ex- including possible consequences for decade, in particular in the realm of perimental examples are provided, the properties of bulk solids. organometallic chemistry. At the same with an emphasis on homoleptic mon- time, various theoretical models have onuclear complexes. Then the factors Keywords: bond theory ´ d0 com- been discussed, sometimes controver- that control the structures are dis- plexes ´ density functional calculations ´ sially, to explain the observed, unusual cussed in detail. They are a) metal transition metals ´ VSEPR Model structures. Many d0 systems are impor- d orbital participation in s bonding, tant in homogeneous and heterogene- b) polarization of the outermost ous catalysis, biocatalysis (e.g. molyb- core shells, c) ligand repulsion, and 1. Introduction central metal (and also with d1 and d2 configurations) have since been found, both experimentally and computationally, The development of improved models for chemical bonding which do not conform to the simple structural models. These has always benefited from specific cases in which the existing interesting ªnon-VSEPRº d0 systems and the electronic models fail. One well-known example is the failure of the factors that control their shapes are the topic of this article. widely used valence-shell electron-pair-repulsion (VSEPR[1] ) Figure 1 shows the part of the periodic table, in which model and also of simple ionic models to predict the bent metals with a formal d0 configuration may be found. The structures of some of the molecular dihalides of the heavy configuration assignment is based on the usual oxidation state alkaline earth metals (Ca, Sr, and Ba) in the gas phase.[2±10] formalism, which assumes a heterolytic distribution of the Many more complexes with a formal d0 configuration of the bonding electrons in favor of the ligands.[11] The number of different metals for which such a configuration is possible is [*] Prof. Dr. M. Kaupp remarkable indeed, and ranges from the heavier members of Institut für Anorganische Chemie ªmainº Groups 1 and 2 through the genuine transition metals Universität Würzburg Groups 3 ± 8 to the lanthanides, and even to the later members Am Hubland, 97074 Würzburg (Germany) [12, 13] 0 Fax : (49)931-888-7135 of the actinides. Many compounds with a formal d E-mail: [email protected] configuration are of practical importance, for example, as Angew. Chem. Int. Ed. 2001, 40, 3534 ± 3565 WILEY-VCH Verlag GmbH, D-69451 Weinheim, 2001 1433-7851/01/4019-3535 $ 17.50+.50/0 3535 REVIEWS M. Kaupp is known about the electronic origin of the structural preferences (mainly in Section 3). We will provide many experimental examples (mainly in Sections 2, 4, and 5). Our emphasis, however, will be on the interrelation between structure and bonding. It should be noted that the interaction between experimental and quan- tum-chemical studies has been tremendously fruitful in this area of research on d0 systems. In particular, ab initio and density functional theory (DFT) computations have been shown in a considerable variety of cases to be at a stage, where rather detailed, qualitative and quantitative, structural pre- Figure 1. Range of occurrence of systems with formal d0 configuration dictions can be made. Some of the aspects of the unusual throughout the periodic table. Shaded areas indicate the range found. structures of d0 complexes have already been mentioned in Darker shading indicates increasing covalency. other contexts in previous reviews (see, e.g., refs. [10a, 16 ± 18]). d1 Complexes and low-spin d2 complexes frequently exhibit distortions very similar to those of their d0 analogues, catalysts, in important materials (see Section 5), or in bio- although the single or double occupation of one nonbonding logical systems.[14, 15] Moreover, as d0 complexes lack non- d orbital typically reduces the tendency to distort. We will bonding d electrons by definition, one may view them as the concentrate on d0 complexes but will occasionally also simplest transition metal compounds and thus as convenient mention d1 and d2 examples. starting points for a better understanding of transition metal chemistry in general. It is therefore worthwhile having a closer look at the performance of simple models in describing 2. Examples of Homoleptic Complexes with bonding and the molecular shapes of such systems. ªNon-VSEPR Structuresº Very different views have been put forward on how to explain the ªnon-VSEPR structuresº of certain d0 systems. In this Section we will give some examples, both from These range from modified ionic models through ªextended experimental and from computational studies, for homoleptic VSEPRº approaches to molecular-orbital (MO) and valence- d0 systems that depart from the expected ªVSEPR struc- bond (VB) models emphasizing the covalent bonding con- turesº. The literature available has grown tremendously tributions. Based on the results of a large number of quantum- during the past few years. We will therefore not attempt to chemical calculations during the past years by many research review all the compounds studied but rather try to cover all groups, herein we will attempt to provide a unified view of the the important types of systems. The examples will be grouped factors that control the structures of d0 systems. Initially we in the order of increasing coordination numbers. will focus mainly on the simplest mononuclear, homoleptic complexes. Recently there has been increased interest in more complicated heteroleptic, as well as in multinuclear com- 2.1. Bent Dicoordinate Complexes plexes; preliminary discussions will be given in Sections 4 and 5, respectively, of the new considerations that come into play Let us start with the well-known case of the gaseous, for such systems. Rather than giving a comprehensive choice monomeric alkaline earth dihalides. Table 1 summarizes the of examples, we will try to be systematic with respect to what current knowledge (see ref. [10] for more details) about the Martin Kaupp, born 1962 in Stuttgart, studied Chemistry in Stuttgart and Cincinnati. After his diploma thesis 1989 in Stuttgart in theoretical chemistry with Hermann Stoll, he moved to Erlangen, where he received his doctorate 1992 performing quantum-chemical studies with Paul von Rague Schleyer. Following a stay 1992/1993 at the Max-Planck-Institut für Festkörperforschung in Stuttgart with Hans-Georg von Schnering, further postdoctoral work was carried out 1993/1994 with Dennis Salahub in MontreÂal. After the return to Stuttgart, the completion of the Habilitation in theoretical chemistry followed in 1997. Since the end of 1999, he is a Professor at Universität Würzburg. Research interests cover a wide- range of theoretical and computational chemistry. The topic of the present review reflects a general interest in structure, bonding, and reactivity throughout the entire periodic table. A further central field of research is the development and application of density functional methods for the calculation and interpretation of NMR and EPR parameters, extending their applicability to compounds of heavier elements. In this context, but also in many other areas of chemistry, he is is particularly interested in the influence of relativistic effects, and in their quantum-chemical treatment. 3536 Angew. Chem. Int. Ed. 2001, 40, 3534 ± 3565 d0 Complexes REVIEWS Table 1. Preferred structures of the monomeric alkaline earth dihalides matrix-isolation infrared (IR) and density functional study [a] [MX2]. has found bent structures also for almost all lanthanide X: F Cl Br I dihydrides.[21] M: A computational comparison of a range of [MX2] com- [6] Be linear linear linear linear plexes (M Ca, Sr, Ba; X Li, BeH, CH3,NH2 , OH, F) has Mg linear linear linear linear indicated that the dimethyl complexes [M(CH3)2] also exhibit Ca quasilinear quasilinear quasilinear quasilinear an even larger tendency to bend than the corresponding Sr bent quasilinear quasilinear quasilinear difluorides, for reasons we will discuss in Section 3.8. Indeed, (DE 6.6) [22] lin Eaborn et al. have prepared a number of unsolvated [MR2] Ba bent bent bent quasilinear complexes (M Ca, Eu, Yb) with very bulky alkyl ligands (DElin 20.9) (DElin 6.8) (DElin 4.5) (R C[Si(CH3)3]3) and structurally characterized them by 1 [a] Linearization energies DElin [kJ mol ] are from Configuration inter- X-ray diffraction.
Load more

Recommended publications
  • Crystal Structures
    Crystal Structures Academic Resource Center Crystallinity: Repeating or periodic array over large atomic distances. 3-D pattern in which each atom is bonded to its nearest neighbors Crystal structure: the manner in which atoms, ions, or molecules are spatially arranged. Unit cell: small repeating entity of the atomic structure. The basic building block of the crystal structure. It defines the entire crystal structure with the atom positions within. Lattice: 3D array of points coinciding with atom positions (center of spheres) Metallic Crystal Structures FCC (face centered cubic): Atoms are arranged at the corners and center of each cube face of the cell. FCC continued Close packed Plane: On each face of the cube Atoms are assumed to touch along face diagonals. 4 atoms in one unit cell. a 2R 2 BCC: Body Centered Cubic • Atoms are arranged at the corners of the cube with another atom at the cube center. BCC continued • Close Packed Plane cuts the unit cube in half diagonally • 2 atoms in one unit cell 4R a 3 Hexagonal Close Packed (HCP) • Cell of an HCP lattice is visualized as a top and bottom plane of 7 atoms, forming a regular hexagon around a central atom. In between these planes is a half- hexagon of 3 atoms. • There are two lattice parameters in HCP, a and c, representing the basal and height parameters Volume respectively. 6 atoms per unit cell Coordination number – the number of nearest neighbor atoms or ions surrounding an atom or ion. For FCC and HCP systems, the coordination number is 12. For BCC it’s 8.
    [Show full text]
  • Crystal Structure of a Material Is Way in Which Atoms, Ions, Molecules Are Spatially Arranged in 3-D Space
    Crystalline Structures – The Basics •Crystal structure of a material is way in which atoms, ions, molecules are spatially arranged in 3-D space. •Crystal structure = lattice (unit cell geometry) + basis (atom, ion, or molecule positions placed on lattice points within the unit cell). •A lattice is used in context when describing crystalline structures, means a 3-D array of points in space. Every lattice point must have identical surroundings. •Unit cell: smallest repetitive volume •Each crystal structure is built by stacking which contains the complete lattice unit cells and placing objects (motifs, pattern of a crystal. A unit cell is chosen basis) on the lattice points: to represent the highest level of geometric symmetry of the crystal structure. It’s the basic structural unit or building block of crystal structure. 7 crystal systems in 3-D 14 crystal lattices in 3-D a, b, and c are the lattice constants 1 a, b, g are the interaxial angles Metallic Crystal Structures (the simplest) •Recall, that a) coulombic attraction between delocalized valence electrons and positively charged cores is isotropic (non-directional), b) typically, only one element is present, so all atomic radii are the same, c) nearest neighbor distances tend to be small, and d) electron cloud shields cores from each other. •For these reasons, metallic bonding leads to close packed, dense crystal structures that maximize space filling and coordination number (number of nearest neighbors). •Most elemental metals crystallize in the FCC (face-centered cubic), BCC (body-centered cubic, or HCP (hexagonal close packed) structures: Room temperature crystal structure Crystal structure just before it melts 2 Recall: Simple Cubic (SC) Structure • Rare due to low packing density (only a-Po has this structure) • Close-packed directions are cube edges.
    [Show full text]
  • Tetrahedral Coordination with Lone Pairs
    TETRAHEDRAL COORDINATION WITH LONE PAIRS In the examples we have discussed so far, the shape of the molecule is defined by the coordination geometry; thus the carbon in methane is tetrahedrally coordinated, and there is a hydrogen at each corner of the tetrahedron, so the molecular shape is also tetrahedral. It is common practice to represent bonding patterns by "generic" formulas such as AX4, AX2E2, etc., in which "X" stands for bonding pairs and "E" denotes lone pairs. (This convention is known as the "AXE method") The bonding geometry will not be tetrahedral when the valence shell of the central atom contains nonbonding electrons, however. The reason is that the nonbonding electrons are also in orbitals that occupy space and repel the other orbitals. This means that in figuring the coordination number around the central atom, we must count both the bonded atoms and the nonbonding pairs. The water molecule: AX2E2 In the water molecule, the central atom is O, and the Lewis electron dot formula predicts that there will be two pairs of nonbonding electrons. The oxygen atom will therefore be tetrahedrally coordinated, meaning that it sits at the center of the tetrahedron as shown below. Two of the coordination positions are occupied by the shared electron-pairs that constitute the O–H bonds, and the other two by the non-bonding pairs. Thus although the oxygen atom is tetrahedrally coordinated, the bonding geometry (shape) of the H2O molecule is described as bent. There is an important difference between bonding and non-bonding electron orbitals. Because a nonbonding orbital has no atomic nucleus at its far end to draw the electron cloud toward it, the charge in such an orbital will be concentrated closer to the central atom.
    [Show full text]
  • Oxide Crystal Structures: the Basics
    Oxide crystal structures: The basics Ram Seshadri Materials Department and Department of Chemistry & Biochemistry Materials Research Laboratory, University of California, Santa Barbara CA 93106 USA [email protected] Originally created for the: ICMR mini-School at UCSB: Computational tools for functional oxide materials – An introduction for experimentalists This lecture 1. Brief description of oxide crystal structures (simple and complex) a. Ionic radii and Pauling’s rules b. Electrostatic valence c. Bond valence, and bond valence sums Why do certain combinations of atoms take on specific structures? My bookshelf H. D. Megaw O. Muller & R. Roy I. D. Brown B. G. Hyde & S. Andersson Software: ICSD + VESTA K. Momma and F. Izumi, VESTA 3 for three-dimensional visualization of crystal, volumetric and morphology data, J. Appl. Cryst. 44 (2011) 1272–1276. [doi:10.1107/S0021889811038970] Crystal structures of simple oxides [containing a single cation site] Crystal structures of simple oxides [containing a single cation site] N.B.: CoO is simple, Co3O4 is not. ZnCo2O4 is certainly not ! Co3O4 and ZnCo2O4 are complex oxides. Graphs of connectivity in crystals: Atoms are nodes and edges (the lines that connect nodes) indicate short (near-neighbor) distances. CO2: The molecular structure is O=C=O. The graph is: Each C connected to 2 O, each O connected to a 1 C OsO4: The structure comprises isolated tetrahedra (molecular). The graph is below: Each Os connected to 4 O and each O to 1 Os Crystal structures of simple oxides of monovalent ions: A2O Cu2O Linear coordination is unusual. Found usually in Cu+ and Ag+.
    [Show full text]
  • Inorganic Chemistry
    Inorganic Chemistry Chemistry 120 Fall 2005 Prof. Seth Cohen Some Important Features of Metal Ions Electronic configuration • Oxidation State/Charge. •Size. • Coordination number. • Coordination geometry. • “Soft vs. Hard”. • Lability. • Electrochemistry. • Ligand environment. 1 Oxidation State/Charge • The oxidation state describes the charge on the metal center and number of valence electrons • Group - Oxidation Number = d electron count 3 4 5 6 7 8 9 10 11 12 Size • Size of the metal ions follows periodic trends. • Higher positive charge generally means a smaller ion. • Higher on the periodic table means a smaller ion. • Ions of the same charge decrease in radius going across a row, e.g. Ca2+>Mn2+>Zn2+. • Of course, ion size will effect coordination number. 2 Coordination Number • Coordination Number = the number of donor atoms bound to the metal center. • The coordination number may or may not be equal to the number of ligands bound to the metal. • Different metal ions, depending on oxidation state prefer different coordination numbers. • Ligand size and electronic structure can also effect coordination number. Common Coordination Numbers • Low coordination numbers (n =2,3) are fairly rare, in biological systems a few examples are Cu(I) metallochaperones and Hg(II) metalloregulatory proteins. • Four-coordinate is fairly common in complexes of Cu(I/II), Zn(II), Co(II), as well as in biologically less relevant metal ions such as Pd(II) and Pt(II). • Five-coordinate is also fairly common, particularly for Fe(II). • Six-coordinate is the most common and important coordination number for most transition metal ions. • Higher coordination numbers are found in some 2nd and 3rd row transition metals, larger alkali metals, and lanthanides and actinides.
    [Show full text]
  • Fifty Years of the VSEPR Model R.J
    Available online at www.sciencedirect.com Coordination Chemistry Reviews 252 (2008) 1315–1327 Review Fifty years of the VSEPR model R.J. Gillespie Department of Chemistry, McMaster University, Hamilton, Ont. L8S 4M1, Canada Received 26 April 2007; accepted 21 July 2007 Available online 27 July 2007 Contents 1. Introduction ........................................................................................................... 1315 2. The VSEPR model ..................................................................................................... 1316 3. Force constants and the VSEPR model ................................................................................... 1318 4. Linnett’s double quartet model .......................................................................................... 1318 5. Exceptions to the VSEPR model, ligand–ligand repulsion and the ligand close packing (LCP) model ........................... 1318 6. Analysis of the electron density.......................................................................................... 1321 7. Molecules of the transition metals ....................................................................................... 1325 8. Teaching the VSEPR model ............................................................................................. 1326 9. Summary and conclusions .............................................................................................. 1326 Acknowledgements ...................................................................................................
    [Show full text]
  • Searching Coordination Compounds
    CAS ONLINEB Available on STN Internationalm The Scientific & Technical Information Network SEARCHING COORDINATION COMPOUNDS December 1986 Chemical Abstracts Service A Division of the American Chemical Society 2540 Olentangy River Road P.O. Box 3012 Columbus, OH 43210 Copyright O 1986 American Chemical Society Quoting or copying of material from this publication for educational purposes is encouraged. providing acknowledgment is made of the source of such material. SEARCHING COORDINATION COMPOUNDS prepared by Adrienne W. Kozlowski Professor of Chemistry Central Connecticut State University while on sabbatical leave as a Visiting Educator, Chemical Abstracts Service Table of Contents Topic PKEFACE ............................s.~........................ 1 CHAPTER 1: INTRODUCTION TO SEARCHING IN CAS ONLINE ............... 1 What is Substructure Searching? ............................... 1 The Basic Commands .............................................. 2 CHAPTEK 2: INTKOOUCTION TO COORDINATION COPPOUNDS ................ 5 Definitions and Terminology ..................................... 5 Ligand Characteristics.......................................... 6 Metal Characteristics .................................... ... 8 CHAPTEK 3: STKUCTUKING AND REGISTKATION POLICIES FOR COORDINATION COMPOUNDS .............................................11 Policies for Structuring Coordination Compounds ................. Ligands .................................................... Ligand Structures........................................... Metal-Ligand
    [Show full text]
  • Ten-Coordinate Lanthanide [Ln(HL)(L)] Complexes (Ln = Dy, Ho, Er, Tb) with Pentadentate N3O2-Type Schiff-Base Ligands: Synthesis, Structure and Magnetism
    magnetochemistry Article Ten-Coordinate Lanthanide [Ln(HL)(L)] Complexes (Ln = Dy, Ho, Er, Tb) with Pentadentate N3O2-Type Schiff-Base Ligands: Synthesis, Structure and Magnetism Tamara A. Bazhenova 1, Ilya A. Yakushev 1,2,3 , Konstantin A. Lyssenko 4 , Olga V. Maximova 1,5,6, Vladimir S. Mironov 1,7,* , Yuriy V. Manakin 1 , Alexey B. Kornev 1, Alexander N. Vasiliev 5,8 and Eduard B. Yagubskii 1,* 1 Institute of Problems of Chemical Physics, 142432 Chernogolovka, Russia; [email protected] (T.A.B.); [email protected] (I.A.Y.); [email protected] (O.V.M.); [email protected] (Y.V.M.); [email protected] (A.B.K.) 2 Kurnakov Institute of General and Inorganic Chemistry, 119991 Moscow, Russia 3 National Research Center “Kurchatov Institute”, 123182 Moscow, Russia 4 Department of Chemistry, Lomonosov Moscow State University, 119991 Moscow, Russia; [email protected] 5 Faculty of Physics, Lomonosov Moscow State University, 119991 Moscow, Russia; [email protected] 6 National University “MISiS”, 119049 Moscow, Russia 7 Shubnikov Institute of Crystallography of Federal Scientific Research Centre “Crystallography and Photonics” RAS, 119333 Moscow, Russia 8 National Research South Ural State University, 454080 Chelyabinsk, Russia * Correspondence: [email protected] (V.S.M.); [email protected] (E.B.Y.) Received: 26 October 2020; Accepted: 9 November 2020; Published: 13 November 2020 Abstract: A series of five neutral mononuclear lanthanide complexes [Ln(HL)(L)] (Ln = Dy3+, 3+ 3+ 3+ H Ho Er and Tb ) with rigid pentadentate N3O2-type Schiff base ligands, H2L (1-Dy, OCH3 3-Ho, 4-Er and 6-Tb complexes) or H2L ,(2-Dy complex) has been synthesized by reaction H of two equivalents of 1,10-(pyridine-2,6-diyl)bis(ethan-1-yl-1-ylidene))dibenzohydrazine (H2L , [H2DAPBH]) or 1,10-(pyridine-2,6-diyl)bis(ethan-1-yl-1-ylidene))di-4-methoxybenzohydrazine OCH3 (H2L , [H2DAPMBH]) with common lanthanide salts.
    [Show full text]
  • Coordination Number 2
    Coordination Numbers and Geometry Page 1 of 11 Coordination Numbers and Geometry Lecture 2. CHEM1902 (C 10K) Coordination Chemistry The total number of points of attachment to the central element is termed the coordination number and this can vary from 2 to as many as 16, but is usually 6. In simple terms, the coordination number of a complex is influenced by the relative sizes of the metal ion and the ligands and by electronic factors, such as charge which is dependent on the electronic configuration of the metal ion. These competing effects are described by the term ionic potential which is defined as the charge to radius ratio. Based on this, it can be seen that the bigger the charge on the central ion, the more attraction there will be for negatively charged ligands, however at the same time, the bigger the charge the smaller the ion becomes which then limits the number of groups able to coordinate. Coordination Number 2 + The linear [Ag(NH 3)2] ion This arrangement is not very common for first row transition metal ion complexes and some of the best known examples are for Silver(I). In this case we have a low charge and an ion at the right hand side of the d- block indicating smaller size For instance, a method often employed for the detection of chloride ions involves the formation of the linear diamminesilver(I) complex. The first step is: Ag+ + Cl- → AgCl (white ppt) and to ensure that the precipitate is really the chloride salt, two further Although [Ag(en)]ClO 4 involves a tests must be done: normally bidentate ligand, in this case the structure is polymeric and AgCl + 2 NH [Ag(NH ) ]+ 3 → 3 2 the silver ion still retains a CN=2.
    [Show full text]
  • Bicapped Tetrahedral, Trigonal Prismatic, and Octahedral Alternatives in Main and Transition Group Six-Coordination
    2484 Bicapped Tetrahedral, Trigonal Prismatic, and Octahedral Alternatives in Main and Transition Group Six-Coordination Roald Hoffmann,*laJames M. Howell,lb and Angelo R. RossilC Contribution from the Departments of Chemistry, Cornell University, Ithaca, New York 14853, Brooklyn College, City University of New York. Brooklyn, New York, 1121 0, and The University of Connecticut, Storrs, Connecticut 06268. Received June 30, 1975 Abstract: We examine theoretically the bicapped tetrahedron and trigonal prism as alternatives to the common octahedron for six-coordinate main and transition group compounds. The pronounced preference of six-coordinate sulfur for an octahe- dral geometry is traced by a molecular orbital analysis to a pair of nonbonding levels, whose higher energy in the nonoctahe- dral geometries is due to the molecular orbital equivalent of a ligand-ligand repulsion. For transition element complexes we draw a correlation diagram for the trigonal twist interrelating octahedral and trigonal prismatic extremes. A possible prefer- ence for the trigonal prism in systems with few d electrons is discussed, as well as a strategy for lowering the energy of the trigonal prism by symmetry-conditioned 7r bonding. The bicapped tetrahedron geometry for transition metal complexes can be stabilized by a substitution pattern ML4D2 where D is a good u donor and M is a d6 metal center. In main and transition group six-coordinate compounds logical, or group theoretical analysis of rearrangement the octahedral geometry predominates. There is, however, a modes or basic permutational sets. However, the solution of growing class of trigonal prismatic complexes and struc- the mathematical problem leaves the question of the physi- tures intermediate between the octahedron and trigonal cal mechanism of a rearrangement still unsolved.
    [Show full text]
  • Lecture 2--2016
    Chem 462 Lecture 2--2016 z [MLnXm] Classification of Ligands Metal Oxidation States Electron Counting Overview of Transition Metal Complexes 1.The coordinate covalent or dative bond applies in L:M 2.Lewis bases are called LIGANDS—all serve as σ-donors some are π-donors as well, and some are π-acceptors 3. Specific coordination number and geometries depend on metal and number of d-electrons 4. HSAB theory useful z [MLnXm] a) Hard bases stabilize high oxidation states b) Soft bases stabilize low oxidation states Classification of Ligands: The L, X, Z Approach Malcolm Green : The CBC Method (or Covalent Bond Classification) used extensively in organometallic chemistry. L ligands are derived from charge-neutral precursors: NH3, amines, N-heterocycles such as pyridine, PR3, CO, alkenes etc. X ligands are derived from anionic precursors: halides, hydroxide, alkoxide alkyls—species that are one-electron neutral ligands, but two electron 4- donors as anionic ligands. EDTA is classified as an L2X4 ligand, features four anions and two neutral donor sites. C5H5 is classified an L2X ligand. Z ligands are RARE. They accept two electrons from the metal center. They donate none. The “ligand” is a Lewis Acid that accepts electrons rather than the Lewis Bases of the X and L ligands that donate electrons. Here, z = charge on the complex unit. z [MLnXm] octahedral Square pyramidal Trigonal bi- pyramidal Tetrahedral Range of Oxidation States in 3d Transition Metals Summary: Classification of Ligands, II: type of donor orbitals involved: σ; σ + π; σ + π*; π+ π* Ligands, Classification I, continued Electron Counting and the famous Sidgewick epiphany Two methods: 1) Neutral ligand (no ligand carries a charge—therefore X ligands are one electron donors, L ligands are 2 electron donors.) 2) Both L and X are 2-electron donor ligand (ionic method) Applying the Ionic or 2-electron donor method: Oxidative addition of H2 to chloro carbonyl bis triphenylphosphine Iridium(I) yields Chloro-dihydrido-carbonyl bis-triphenylphosphine Iridium(III).
    [Show full text]
  • The Transition Metals
    The Transition Metals • d electrons in group 3 are readily removed via ionization . • d electrons in group 11 are stable and generally form part of the core electron configuration. Transition Metal Valence Orbitals • nd orbitals • (n + 1)s and (n + 1)p orbitals • 2 2 2 dx -dy and dz (e g) lobes located on the axes • dxy, dxz, dyz lobes (t 2g) located between axes • orbitals oriented orthogonal wrt each other creating unique possibilities for ligand overlap. • Total of 9 valence orbitals available for bonding (2 x 9 = 18 valence electrons!) • For an σ bonding only Oh complex, 6 σ bonds are formed and the remaining d orbitals are non-bonding . • It's these non-bonding d orbitals that give TM complexes many of their unique properties • for free (gas phase) transition metals: (n+1)s is below (n)d in energy (recall: n = principal quantum #). • for complexed transition metals: the (n)d levels are below the (n+1)s and thus get filled first. (note that group # = d electron count) • for oxidized metals, subtract the oxidation state from the group #. Geometry of Transition Metals Coordination Geometry – arrangement of ligands around metal centre Valence Shell Electron Pair Repulsion (VSEPR) theory is generally not applicable to transition metals complexes (ligands still repel each other as in VSEPR theory) For example, a different geometry would be expected for metals of different d electron count 3+ 2 [V(OH 2)6] d 3+ 4 [Mn(OH 2)6] d all octahedral geometry ! 3+ 6 [Co(OH 2)6] d Coordination geometry is , in most cases , independent of ground state electronic configuration Steric: M-L bonds are arranged to have the maximum possible separation around the M.
    [Show full text]