"Transition Elements Their Coordination Compounds"
Transition Elements & Their Coordination Compounds Properties of the Transition Elements The Inner Transition Elements Highlights of Selected Transition Elements Coordination Compounds Theoretical Basis for the Bonding and Properties of Complexes 6/14/2010 1 Transition Elements & Their Coordination Compounds Main-Group vs Transition Elements Most important uses of Main-Group elements involve the compounds made up of these elements Transition Elements are highly useful in their elemental or uncombined form Transition Elements make up the “d” block (B group) and the “f” block elements in the periodic chart As ions, transition metals (elements) provide fascinating insights into chemical bonding and structure Transition metals play an important role in living organisms 6/14/2010 2 Transition Elements & Their Coordination Compounds Properties of Transition Elements Main –Group Transition Elements Main-group elements change from All transition elements are metals metal to non-metal across a period Most main-group ionic compounds Many transition metal compounds are colorless and diamagnetic (non- are highly colored and paramagnetic magnetic) 6/14/2010 3 Transition Elements & Their Coordination Compounds Electron Configurations of the Transition Metals In the Periodic Table, the transition metals, designated “d-block (B-Group)” elements, are located in: 40 elements in 4 series within Periods 4 -7 Lie between the last ns-block elements in group [2A(2)] (Ca – Ra) and the first np-block elements in group [(3A(13)] (Ga & element 113 (unnamed) Each series represents the filling of the 5 d orbitals for each period [ml = -2 -1 0 +1 +2] (total of 10 electrons, 2 in each orbital) or 10 x 4 = 40 elements The “Inner Transition” elements lie between the 1st and 2nd members of the “d-block” elements in Periods 6 & 7 (n=6 & n=7), where the 28 “f” orbitals are filled [ml= -3 -2 -1 0 +1 +2 +3] (7 orbitals per period x 2 electrons per orbital x 2 periods = 28 6/14/2010 4 Transition Elements & Their Coordination Compounds 6/14/2010 5 Transition Elements & Their Coordination Compounds Condensed d-block ground-state electron configuration: [noble gas] ns2(n-1)dx, with n = 4 -7; x= 1-10 (several aufbau build-up exceptions) Partial (valence shell) electron configuration ns2(n-1)dx Recall: Chromium (Cr) and Copper (Cu) are exceptions to the above aufbau configuration setup Expected: Cr [Ar] 4s23d4 Cu [Ar] 4s23d9 Actual: Cr [Ar] 4s13d5 Cu [Ar] 4s13d10 Reasons: change in relative energies of 4s & 3d orbitals and the unusual stability of ½ filled and filled sublevels (level 4 relative to level 3) Condensed f-block ground-state electron configuration (Periods 6 & 7): [noble gas] ns2 (n-2)f14(n-1)dx, with n = 6 -7 6/14/2010 6 Transition Elements & Their Coordination Compounds Transition Metal Ions Form through the loss of the “ns” electrons before the (n-1)d electrons Ex. Ti2+ [Ar] 3d2 4s2 → [Ar] 3d2 + 2e- (not [Ar] 4s2) (Ti2+ also called d2 ion) Ions of different transition metals with the same electron configuration often have similar properties Ex. Mn2+ and Fe3+ are both d5 ions Both Ions have pale colors in aqueous solutions Both form complex ions with similar magnetic properties 6/14/2010 7 Transition Elements & Their Coordination Compounds Note Aufbau build up exceptions for “Cr” & “Cu” 6/14/2010 8 Practice Problem Write condensed electron configurations for the following ions: Zr V3+ Mo3+ Vanadium – Period 4; Zirconium (Zr) & Molybdenum (Mo) – Period 5 General Configuration: ns2(n-1)dx a. Zr is 2nd element in the 4d series: [Kr] 5s24d2 b. Va is the 3rd element in the 3d series: [Ar] 4s23d3 “ns” electrons lost first In forming V3+, 3 electrons lost – two 4s & one 3d V3+ = [Ar] 4s23d3 → [Ar] 3d2 (d2 ion) + 3e- c. Mo lies below Cr in Period 5, Group 6B(6): [kr] 5s1 4d5 Note: Same electron configuration exception as Cr Mo3+ = [Kr] 5s1 4d5 → [Kr] 4d3 (d3 ion) + 3 e- 6/14/2010 9 Transition Elements & Their Coordination Compounds Trends of Transition Elements Across a Period Transition elements exhibit smaller, less regular changes in size, electronegativity, and first ionization energy Atomic Size General overall decrease across a period As the “d” orbitals are filled across a period, the change in atomic size within the transition elements evens out because the increased nuclear charge shields the outer electrons preventing them from spreading out Transition Metals 6/14/2010 10 Transition Elements & Their Coordination Compounds Electronegativity Electronegativity generally increases across period Change in electronegativity within a series (period) is relatively small in keeping with the relatively small change in size Small electronegativity change in transition elements is in contrast with the steeper increase between the main group elements across a period Magnitude of Electronegativity in transition elements is similar to the larger main-group metals Transition Metals 6/14/2010 11 Transition Elements & Their Coordination Compounds Ionization Energy Ionization Energy of Period 4 Main-group elements rise steeply from left to right as the electrons become more difficult to remove from the poorly shielded increasing nuclear charge, i.e., no “d” electrons In the transition metals, however, the first ionization energies increase relatively little because of the effective shielding by the inner “d” electrons reducing the effect of the increased nuclear charge Transition Metals 6/14/2010 12 Transition Elements & Their Coordination Compounds Trends Within (down) a Group (relative to main-group elements) Vertical trends differ from those of the main-group elements Atomic Size Increases, as expected, from Period 4 to 5 No increase from Period 5 to 6 Lanthanides with buried “4f” sublevel orbitals appear between the 4d (period 5) and 5d (period 6) series An element in Period 6 is separated from the one above it in Period 5 by 32 electrons (ten 4d, six 5p, two 6s, and fourteen 4f) The extra shrinking that results from the increased nuclear charge due to the addition of the fourteen 4f electrons is called the: “Lanthanide Contraction” 6/14/2010 13 Transition Elements & Their Coordination Compounds n=1 n=2 n=3 l=0 l=0 l=1 l=0 l=1 l=2 (1s) (2s) (2p) (3s) (3p) (3d) ml = 0 0 -1 0 +1 0 -1 0 +1 -2 -1 0 +1 +2 n=4 l=0 l=1 l=2 l=3 Note: (4s) (4p) (4d) (4f) n>7&l>3 -1 0 +1 ml = 0 -2 -1 0 +1 +2 -3 -2 -1 0 +1 +2 +3 Sublevels not utilized for n=5 any element in the current l=0 l=1 l=2 l=3 Period Table (5s) (5p) (5d) (5f) ml = 0 -1 0 +1 -2 -1 0 +1 +2 -3 -2 -1 0 +1 +2 +3 n=6,7 l=0 l=1 l=2 l=3 (6s,7s) (6p,7p) (6d) (6f) ml = 0 -1 0 +1 -2 -1 0 +1 +2 -3 -2 -1 0 +1 +2 +3 6/14/2010 14 Transition Elements & Their Coordination Compounds Main Group Metals Main Group Non-metals Transition Metals Inner Transition Metals Order of Sublevel Orbital Filling 6/14/2010 15 Transition Elements & Their Coordination Compounds Trends Within a Group (relative to main-group elements) Electronegativity (EN) – Relative ability of an atom in a covalent bond to attract shared electrons EN of main-group elements decreases down group greater size means less attraction by nucleus Greater Reactivity EN in transition elements is opposite the trend in main-group elements EN increases from period 4 to period 5 No change from period 5 to period 6, since the change in volume is small and Zeff increases (f orbital electrons) Transition metals exhibit more covalent bonding and attract electrons more strongly than main-group metals The EN values in the heavy metals exceed those of most metalloids, forming salt-like compounds, such as CsAu and the Au- ion 6/14/2010 16 Transition Elements & Their Coordination Compounds Trends Within a Group (relative to main-group elements) Ionization Energy Main-group elements increase in size down a group, decreasing the 1st ionization energy, making it relatively easier to remove the outer electrons The relatively small increase in size of transition metals, combined with the relatively large increase in nuclear charge (Zeff), results in a general increase in the first ionization energy down a group 6/14/2010 17 Transition Elements & Their Coordination Compounds Trends Within a Group (relative to main-group elements) Density Atomic size (volume) is inversely related to density Across a period densities increase In transition metals the density down a group increases dramatically because atomic volumes change little from Period 5 to Period 6 while nuclear mass increases significantly Period 6 series contains some of the densest elements known: Tungsten, Rhenium, Osmium, Iridium, Platinum, Gold (Density 20 times greater than water, 2 times more dense than lead) 6/14/2010 18 Transition Elements & Their Coordination Compounds Vertical (down group) trends in key properties within the transition elements. 6/14/2010 19 Transition Elements & Their Coordination Compounds Chemical Properties of the Transition Elements Similar to atomic & physical properties, the chemical properties of transition elements are very different from main group elements Oxidation States Main-group elements display one, or at most two, oxidation states The ns & (n-1)d electrons in transition elements are very close in energy All or most can be used in bonding leading to multiple oxidation states 6/14/2010 20 Transition Elements & Their Coordination Compounds Oxidation State (Number) Magnitude of charge an atom in a covalent compound would have if its shared electrons were held completely by the atom that attracts them more strongly Oxidation State Electronic dx 4s 3d 4p Manganese (Mn) Configuration 0 d5 [Ar] 4s2 3d5 +1 d5 [Ar] 4s1 3d5 Note: All 3 d5 +2 d5 [Ar] 3d5 +3 d4 [Ar] 3d4 +4 d3 [Ar] 3d3 Ex. MnO2 ; O.N. Mn +4 +5 d2 [Ar] 3d2 +6 d1 [Ar] 3d1 +7 d0 [Ar] Ex. MnO4- ; O.N. Mn +7 6/14/2010 21 Transition Elements & Their Coordination Compounds Metallic Behavior Atomic size and oxidation state have a major effect on the nature of bonding in transition metal compounds Transition elements in their lower oxidation states behave more like metals – Oxides more basic Transition elements in their higher oxidation states exhibit more covalent bonding – Oxides more acidic Ex. TiCl2 (Ti2+) is an ionic solid TiCl4 (Ti4+) is a molecular liquid 6/14/2010 22 Transition Elements & Their Coordination Compounds Metallic Behavior In the higher oxidation states: The atoms have fewer electrons The nuclear charge pulls remaining electrons closer, decreasing the volume and increasing the density The charge density (ratio of the ion‟s charge to its volume) increases The increase in charge density leads to more polarization of the electron clouds in non-metals The bonding becomes more covalent The stronger the covalent bond, the less metallic The oxides, therefore, become less basic Ex. TiO (Ti2+) is weakly basic in water TiO2 (Ti4+) is amphoteric, reacting with both acid and base 6/14/2010 23 Transition Elements & Their Coordination Compounds Electronegativity, Oxidation State, Acidity/Basicity Why does oxide acidity increase with oxidation state? Metal with a higher oxidation state is more positively charged Attraction of electrons is increased, i.e., electronegativity increases Effective Electronegativity = Valence State Electronegativity EN Cr – 1.6 Al – 1.5 (basic oxide) Cr3+ – 1.7 Cr6+ – 2.3 P – 2.1 (acidic oxides) 6/14/2010 24 Transition Elements & Their Coordination Compounds Metallic Behavior Reduction Strength (Redox) Standard Electrode Potential, Eo , generally decreases across a period As the value of Eo becomes more negative, i.e., at the beginning of the series, the ability of the species to act as a reducing agent increases. Thus, Ti2+, Eo = -01.63V, is a stronger reducing agent than Ni2+, Eo = -0.25V All species with a negative value of Eo can reduce H+ 2H+(aq) + 2e- H2(g) Eo = 0.0V) Note: Cu2+ (Eo = +0.34 V) cannot reduce H+ The magnitude of the Eo values between two species, and the relative degree of surface oxidation, determines the level of reactivity of the oxidation/reduction reaction in water, steam, or acid solution 6/14/2010 25 Transition Elements & Their Coordination Compounds Color in Transition Elements Most Main-Group Ionic Compounds are colorless Metal ions have a filled outer shell With only much higher energy orbitals available to receive an “excited” electron, the ion does not absorb visible light The partially filled “d” orbitals of the transition metals can absorb visible wavelengths and move to slightly higher energy “d” levels 6/14/2010 26 Transition Elements & Their Coordination Compounds Magnetism in Transition Elements Magnetic properties are related to electron sublevel occupancy A “Paramagnetic” substance has atoms or ions with “unpaired” electrons A “Diamagnetic” substance has atoms or ions with only “paired” electrons Most Main-Group metal ions are diamagnetic (filled outer shells) Many Transition metal compounds are paramagnetic because of unpaired electron in the “d” subshells 6/14/2010 27 Transition Elements & Their Coordination Compounds Chemical Behavior Within a Group Main_Group The decrease in Ionization Energy (IE) going down a group results in “increased reactivtiy” Transition metals Ionization Energy increases down group The Standard Electrode also increases (becomes more positive) Chromium is stronger reducing agent 6/14/2010 28 Transition Elements & Their Coordination Compounds The Inner Transition Elements Lanthanides (Rare Earth Elements) (Cerium (Ce); Z = 58 – Lutetium (Lu); Z = 71) Silvery, high melting point (800 – 1600oC) metals Small variations in chemical properties makes them difficult to separate Occur naturally in the +3 oxidation state as M3+ ions of very similar radii Most lanthanides have the ground-state electron configuration filling the “f” subshell level [Xe] 6s2 4fx 5d0 x varies across series (Period) Exceptions – Ce, Gd, Lu have single e- in 5d orbital 6/14/2010 29 Sample Problem Finding the Number of Unpaired Electrons The alloy SmCo5 forms a permanent magnet because both Samarium and Cobalt and unpaired electrons How many unpaired electrons are in the Sm atom (Z=62)? Ans: Samarium is the eighth element after Xe (Noble Shell) [Xe] 6s2 4f6 Two (2) electrons go in the 6s sublevel In general, the 4f sublevel fills before the 5d sublevel (slide 15) Recall previous slide - only Ce, Gd, Lu have 5d electrons Remaining 6 electrons go into the 4f orbitals 6s 4f 5d 6p Six unpaired electrons 6/14/2010 30 Transition Elements & Their Coordination Compounds The Actinides (Thorium (Th); Z=90 - Lawrencium; Z=103) All Actinides are Radioactive Only Thorium & Uranium occur in nature Share very similar chemical & physical properties Silvery and chemically reactive Principal oxidation state is +3, similar to lanthanides 6/14/2010 31 Transition Elements & Their Coordination Compounds Highlights of Selected Transition Metals Period 4 – Chromium & Manganese Chromium Silvery, shiny metal with many colorful compounds Cr2O3 acts as protective coating on easily corroded (oxidized) metals, such as iron “Stainless” steels contain as much as 18 % Cr, making them highly resistant to corrosion Chromium – ([Ar] 4s2 3d5) with 6 valence electrons occurs in all possible positive oxidation states Cr2+, Cr3+, Cr6+ are most important Non-metallic character and oxide acidity increase with metal oxidation state Cr2+ potential reducing agent (Cr loses additional electrons) Cr6+ potential oxidizing agent (Cr gains electrons) 6/14/2010 32 Transition Elements & Their Coordination Compounds Highlights of Selected Transition Metals Chromium Chromium (II) – Cr2+ CrO is basic and largely ionic Forms insoluble hydroxide in neutral or basic solution Dissolves in acid to yield Cr2+ ion and water CrO(s) + 2H+ → Cr2+ (aq) + H2O(l) Chromium(III) – Cr3+ Cr2O3 is amphoteric, similar properties as Aluminum Dissolves in acid to yield violet Cr3+ ion Cr2O3(s) 6H+(aq) → 2Cr3+(aq) + 3H2O(l) Reacts with base to form the green Cr(OH)4- ion Cr2O3(s) + 3H2O + OH- → 2Cr(OH)4-(aq) 6/14/2010 33 Transition Elements & Their Coordination Compounds Highlights of Selected Transition Metals Chromium (con‟t) Chromium (VI) - Cr6+ (Deep Red) CrO3 is covalent and acidic Dissolves in water to from Chromic Acid (H2CrO4) CrO3(s) + H2O(l) → H2CrO4(aq) H2CrO4 yields yellow Chromate ion (CrO42-) in base H2CrO4(aq) + 2OH(l) → CrO42-(aq) + 2H2O(l) Chromate ion forms orange dichromate (Cr2O72-) ion in acid 2CrO42-(aq) + 2H+(aq) ⇆ Cr2O72-(aq) H2O(l) 6/14/2010 34 Transition Elements & Their Coordination Compounds Highlights of Selected Transition Metals Manganese Hard and Shiny Like Vanadium & Chromium used to make steel alloys Chemistry of Manganese is similar to Chromium Metal reduces H+ from acids to form Mn2+ ion Mn(s) + 2H+(aq) → Mn2+(aq) + H2(g) Eo = 1.18 V Manganese can use all its valence electrons (several oxidation states) to form compounds Mn2+ Mn4+ Mn7+ most important As oxidation state rises from +2 to +7, the valence state electronegativity increases and the oxides of Mn change from basic to acidic Mn(II)O (basic) Mn(III)2O3 (amphoteric) Mn(IV)O2 (insoluble) Mn(VII)2O7 (acidic) 6/14/2010 35 Transition Elements & Their Coordination Compounds All Manganese species with oxidation states greater than +2 acts as oxidizing agents (gains electrons causing other atoms to lose electrons) MnO4-(aq) + 4H+ + 3e- → MnO2(s) + 2H2O(l) Eo = 1.68 MnO4-(aq) + 2H2O + 3e- → MnO2(s) + 4OH- Eo = 0.59 (MnO4- is a much stronger oxidizing agent in acid solution than in basic solution – note difference in Eo values) Oxidation State Electronic dx 4s 3d 4p Manganese (Mn) Configuration 0 d5 [Ar] 4s2 3d5 +1 d5 [Ar] 4s1 3d5 +2 d5 [Ar] 3d5 +3 d4 [Ar] 3d4 +4 d3 [Ar] 3d3 +5 d2 [Ar] 3d2 +6 d1 [Ar] 3d1 +7 d0 [Ar] 6/14/2010 36 Transition Elements & Their Coordination Compounds Manganese Unlike Cr2+ & Fe2+, the Mn2+ ion resists oxidation in air Recall: half-filled (-1/2 spins electrons missing) & filled sublevels are more stable than partially filled sublevels Cr2+ is a d4 species and readily loses a 3d electron to form the d3 ion Cr3+, which is more stable Fe2+ is a d6 species and removing a 3d electron yields the stable, half-filled d5 configuration of Fe3+ Removing an electron from Mn2+ disrupts the more stable d5 configuration 6/14/2010 37 Transition Elements & Their Coordination Compounds Coordination Compounds (Complexes) Most distinctive aspect of transition metal chemistry Complex – Substances that contain at least one complex ion Complex ion – Species consisting of a “central metal cation” (either a main-group or transition metal) that is bonded to molecules and/or anions called “Ligands” The Complex ion is typically associated with other (counter) ions to maintain neutrality A coordination compound behaves like an electrolyte in water Complex ion and counter ion separate Complex ion behaves like a polyatomic ion – the ligands and central atom remain attached 6/14/2010 38 Transition Elements & Their Coordination Compounds Components of Coordination Compound When solid complex dissolves in water, the complex ion and the counter ions separate, but ligands remain bound to central atom Central Ligands Counter Atom Ions 6/14/2010 39 Transition Elements & Their Coordination Compounds Complex ions A complex ion is described by the metal ion and the number and types of ligands attached to it The bonding between metal and ligand generally involves formal donation of one or more of the ligand's electron pairs The metal-ligand bonding can range from covalent to more ionic Furthermore, the metal-ligand bond order can range from one to three. Ligands are viewed as Lewis Bases, although rare cases are known involving Lewis acidic ligands 6/14/2010 40 Transition Elements & Their Coordination Compounds Complex ions The complex ion structure is related to three characteristics: Coordination Numbers The number of ligand atoms that are bonded directly to the central metal ion Coordination number is specific for a given metal ion in a particular oxidation state and compound Coordination number in [Co(NH3)6]3+ is 6 The most common coordination number in complex ions is 6, but 2 and 4 are common, with a few higher 6/14/2010 41 Transition Elements & Their Coordination Compounds Complex ions Geometry – Depends on Coordination No. & Nature of Metal Ion Metal ion CN Shape dx Cu+ 2 Linear d10 Ag+ 2 Linear d10 Au+ 2 Linear d10 Ni2+ 4 Octahedral Sq Planar d8 Pd2+ 4 Octahedral Sq Planar d8 Pt2+ 4 Octahedral Sq Planar d8 Cu2+ 4 Octahedral Sq Planar d9 Cu3+ 4 Tetrahedral d8 Zn2+ 4 Tetrahedral d10 Cd2+ 4 Tetrahedral d10 Mn2+ 4 Tetrahedral d5 Ti3+ 6 Octahedral d1 V2+ 6 Octahedral d3 Cr3+ 6 Octahedral d3 d1 d8 Mn2+ 6 Octahedral d5 d3 d9 Fe3+ 6 Octahedral d5 d5 d10 Co3+ 6 Octahedral d6 d6 6/14/2010 42 Transition Elements & Their Coordination Compounds Complex Ions Donor Atoms per Ligand The Ligands of complex ions are “molecules” or “anions” with one or more donor atoms that each donate a lone pair of electrons to the metal ion to form a covalent bond Atoms with lone pairs of electrons often come from Groups 5A, 6A, or 7A (main-group elements) 6/14/2010 43 Transition Elements & Their Coordination Compounds Complex Ions Ligands are classified in terms of the number of donor atoms (teeth) that each uses to bond to the central metal ion Monodentate Ligands use a “single” donor atom Bidentate Ligands have two donor atoms Polydentate Ligands have more than two donor atoms 6/14/2010 44 Transition Elements & Their Coordination Compounds Complex Ions Chelates (Greek “chela” – crab‟s claw) Bidentate and Polydentate ligands give rise to “rings” in the complex ion Ex: Ethylene Diamine (abbreviated (en) in formulas) (:N – C – C – N:) forms a 5-member ring, with the two electron donating N atoms bonding to the metal atom Such ligands seem to grab the metal ion like claws Ethylenediaminetetraacetate (EDTA) Used in treating heavy-metal poisoning, by acting as a scavenger of lead and other heavy-metal ions, removing them from blood and other body fluids 6/14/2010 45 Transition Elements & Their Coordination Compounds Formulas and Names of Coordination Compounds Three important rules for writing formulas of coordinate compounds The cation is written before the anion The charge of the cation(s) is balanced by the charge of the anions In the complex ion, neutral ligands are written before anionic ligands The entire ion is placed in brackets, i.e., [ ] 6/14/2010 46 Transition Elements & Their Coordination Compounds Formulas and Names of Coordination Compounds Coordination Compounds Formulas Example # 1 K2[Co(NH3)2Cl4] Two compound cations (K+) – Total Charge +2 Ion Central Metal Cation (Co2+) – Total Charge +2 Neutral Ligands (2 NH3) – Total Charge 0 Charged Ligands (4 Cl-) – Total Charge -4 Net Charge on Complex Ion – - 2 [Co(NH3)2Cl4]-2 Net Cation Charge – +2 K+2[Co2+(NH3)2Cl-4] 6/14/2010 47 Transition Elements & Their Coordination Compounds Formulas and Names of Coordination Compounds Coordination Compounds Formulas Example # 2 – Complex Ion and Counter Ion [Co(NH3)4Cl2]Cl Counter Ion (Cl-) (not part of complex ion) – Total charge -1 Complex Ion - Neutral Ligands (4 NH3) – Total Charge 0 Complex Ion - Anion Ligands (2 Cl-) – Total Charge -2 Complex Ion - [Co(NH3)4Cl2]+ – Total Charge +1 Complex Ion - Central Metal Atom (Co) – Total Charge +3 [Co3+(NH3)4Cl-2]+Cl- 6/14/2010 48 Transition Elements & Their Coordination Compounds Formulas and Names of Coordination Compounds Example #3 – Complex Cation and Complex Anion [Co(NH3)5Br]2[Fe(CN)6] Complex Cation - [Co(NH3)5Br]2+ Complex Cation Central Atom (Co+3) – Total charge +3 Complex Cation Neutral Ligands (5 NH3) – Total Charge 0 Complex Cation Anionic Ligand (Br-) – Total Charge -1 Complex Anion ([Fe(CN)6]4-) – Total Charge -4 Complex Anion Central Cation (Fe2+) – Total Charge +2 Complex Anion Ligand (6 CN-1) – Total Charge -6 [Co3+(NH ) Br-] [Fe2+(CN-) ] 3 5 2 6 6/14/2010 2 x (3 -1) = 4 2 - 6 = -4 49 Transition Elements & Their Coordination Compounds Formulas and Names of Coordination Compounds Naming Coordination Compounds Rules The Cation is named before the Anion Within the Complex Ion, the Ligands are named, in alphabetical order, before the metal ion Neutral Ligands generally have the molecule name, with exceptions Ex NH3 (ammine), H2O (aqua), CO (carbonyl) Anionic Ligands drop the –ide and add –o after the root name Ex. Cl- becomes “chloro” A numerical prefix indicates the number of ligands of a particular type Ex di (2), tri (3), tetra (4) [Co(NH3)4Cl2]Cl Tetraamminedichlorocobalt(III)chloride 6/14/2010 50 Transition Elements & Their Coordination Compounds Formulas and Names of Coordination Compounds Names of Some Neutral and Anionic Ligands Symbol Fe Cu Names of Some Metals Ions Pb in Complex Anions Ag Au Sn Di Bis II Tri Tris III Numerical Prefixes used Tetra Tetrakis IV In Complex Anions Penta pentakis V Hexa Hexakis VI Septa Septakis VII 6/14/2010 51 Transition Elements & Their Coordination Compounds Formulas and Names of Coordination Compounds Naming Coordination Compounds Rules Some ligand names already contain a numerical prefix Ethylenediamine In these cases the number of ligands is indicated by such terms as: bis (2), tris(3), tetrakis(4) A compound with two ethylene ligands the following it its name bis(ethylenediamine) 6/14/2010 52 Transition Elements & Their Coordination Compounds Formulas and Names of Coordination Compounds Naming Coordination Compounds Rules The oxidation state of the central metal ion is given by a Roman numeral (in parentheses) only if the metal ion can have more than one state, as in the compound [Co(NH3)4Cl2]Cl Tetraamminedichlorocobalt(III)chloride If the complex ion is an anion, drop the ending of the Central metal name and add “–ate” K[Pt(NH3)Cl5] K+[Pt4+(NH3)Cl-5]- potassium amminepentachloroplatinate(IV) Na4[FeBr6] Na+4[Fe2+Br-6] sodium hexabromoferrate(II) 6/14/2010 53 Practice Problem What is the systematic name of Na3[AlF6]? Ans: Complex ion – [AlF6]3- Ligands 6 (hexa) F- ions (fluoro) Complex ion is an “anion” End of metal ion Aluminum must be changed to –ate Complex ion name – hexafluoroaluminate Aluminum has only the +3 oxidation state so Roman numerals are not required Na3+ is the positive counter ion; it is separated from the complex anion by a space Na3[AlF6] Sodium hexfluoroaluminate 6/14/2010 54 Practice Problem What is the systematic name of [Co(en)2Cl2]NO3? Ans: Listed alphabetically, there are two Cl- (dichloro) and two “en” [bis(ethylenediamine] ligands Note: Alphabetically refers to the root chemical names: Chloro & Ethylenediamine The “Complex ion” is a “Cation,” with a charge of +1 [Co3+(en)2Cl-2]+ The metal name in a complex ion is unchanged - cobalt Because cobalt can have several oxidation states, its charge must be specified - Cobalt (III) One Nitrate ion (NO-3) balances the +1 complex cation dichlorobis(ethylenediamine)cobalt(III) nitrate 6/14/2010 55 Practice Problem What is the formula of: tetraamminebromochlroroplatinum(IV) chloride Ans: The central atom of the complex cation is written first Platinate(IV) Pt4+ The ligands follow in alphabetical order of root chemical name Tetraammine (NH3) Bromo (Br-) Chloro (Cl-) Complex ion formula - [Pt(NH3)4BrCl]2+ [Pt4+(NH3)4Br-Cl-]2+ To balance the +2 charge of the complex cation, 2 Cl- counter ions are required [Pt(NH3)4BrCl]Cl2 6/14/2010 56 Practice Problem What is the formula of hexaamminecobalt(III) tetrachloroferrate(III) Ans: Compound consists of two complex ions Complex Cation – Six hexammine (NH3) & cobalt(III) (Co3+) Complex Cation – [Co(NH3)6]3+ [Co3+(NH3)6]3+ Complex Anion – tetrachloro - 4 Cl- Complex Anion – ferrate(III) - Fe3+ Complex Anion – [FeCl-4]- Complex cation – balanced by 3 complex anions Coordinate Compound – [Co(NH3)6][FeCl4]-3 6/14/2010 57 Transition Elements & Their Coordination Compounds Isomerism in Coordination Compounds Isomers are compounds with the same chemical formula but different properties Constitutional (Structural) Isomers Two compounds with the same formula, but with atoms connected differently Two Types Coordination Isomers – Composition of the complex ion changes but not the compound Ex. Ligand and counter ion exchange positions [Pt(NH3)4Cl2](NO2)2 [Pt(NH3)4(NO2)2]Cl2 Ex. Two sets of ligands reversed [Cr(NH3)6][Co(CN)6] [Co(NH3)6][Cr(CN)6] (NH3 is ligand of Cr3+ in one compound and of Co3+ in the other) 6/14/2010 58 Transition Elements & Their Coordination Compounds Constitutional (Structural) Isomers Linkage Isomers Composition of the complex ion remains the same, but the attachment of the ligand donor atom changes Some ligands can bind to the metal ion through either of two donor atoms Ex. pentaamminenitrocobalt(III) chloride [Co(NH3)5(NO2]Cl2 pentaamminenitritocobalt(III) chloride [Co(NH3)5(ONO]Cl2 Ex. Cyanate ion can attach via lone pair of electrons on the Oxygen atom (NCO:) or the Nitrogen atom (isocyanato (OCN:) Other examples of alternate electron donor pairs for Linkage IsomerS 6/14/2010 59 Transition Elements & Their Coordination Compounds Constitutional (Structural) Isomers Stereo Isomers Compounds that have the same atomic connections but different spatial arrangements of the atoms Geometric Isomers (cis-trans isomers [diastereomers]) Atoms or groups of atoms arranged differently in space relative to the “Central” metal 6/14/2010 60 Transition Elements & Their Coordination Compounds Constitutional (Structural) Isomers Stereo Isomers Optical Isomers (enantiomers) Occur when a molecule and its mirror image canot be superimposed Optical isomers have distinct physical properties like other types of isomers, with one exception – the direction in which they rotate the plane of polarized light Optical isomerism in an octahedral complex ion Rotating structure I Rotating structure I in the cis in the trans compound gives compound gives structure III, which structure III,which is not the same as is the same as structure II, its structure II, its mirror image, mirror image, Image I & Image III The trans are optical isomers compound does not have any mirror 6/14/2010 images 61 Practice Problem Draw all stereo isomers for the following [Pt(NH3)2Br2] Cr(en)3]3+ (en = H2NCH2CH2NH2) Pt(II) complex is Square Planar Geometry Br NH3 H3N Br Two different monodentate ligands Pt Pt Geometric Isomers H3N Br Each isomer is superimposable on the H3N Br mirror image – no optical isomerism tran ci s s Ethylenediamine is a bidentate ligand The Cr3+ has a coordination number of 6 and an octahedral geometry, similar to Co3+ The three bidendate ions are identical No geometric isomerism This complex ion has a nonsuperimposable mirror image Optical Isomerism does occur 6/14/2010 62 Transition Elements & Their Coordination Compounds Theoretical Basis for the Bonding and Properties of Complexes Questions How do Metal Ligands bonds form Why certain geometries are preferred Why are complexes often brightly colored Why are complexes often paramagnetic – attracted to a magnetic field as a result of their electron pairs being unpaired 6/14/2010 63 Transition Elements & Their Coordination Compounds Theoretical Basis for the Bonding and Properties of Complexes Application of Valence Bond Theory to Complex Ions In the formation of a complex ion, the filled ligand orbital overlaps the empty metal-ion orbital The Ligand (Lewis Base) donates the electron pair and the metal-ion (Lewis Acid) accepts it to form one of the covalent bonds of the complex ion (Lewis adduct) When one atom in a bond donates both electrons the bond is referred to as a ”coordinate covalent bond” The number and type of metal-ion hybrid orbitals occupied by ligand lone pairs determine the geometry of the complex ion 6/14/2010 64 Transition Elements & Their Coordination Compounds Application of Valence Bond Theory to Complex Ions Octahedral Complexes (six electron groups about central atom) Ex. Hexaamminechromium(III) ion [CrNH3)6]3+ Six hybrid orbitals are needed to make the ion The six lowest energy orbitals of the Cr3+ ion Two 3d, one 4s, three 4p mix and become six equivalent d2sp3 hybrid orbitals that point to the corners of an octahedron The six d2sp3 hybrid orbitals are filled with the six electron pairs from the six NH3 ligands Note the lowest 6 energy levels for Cr3+ involve both n=3 & n=4 sublevels The 3d orbitals are of lower energy than the 4s Paramagnetic Unpaired e- and 4p orbitals The hybrid designation, d2sp3, follows this order If all the orbitals had the same “n” value, the order would have been sp3d2 6/14/2010 65 Transition Elements & Their Coordination Compounds Application of Valence Bond Theory to Complex Ions Square Planar Complexes (four electron groups about central atom) Metal ions with a d8 configuration usually form square planar complexes In the [Ni(CN)4]2- ion, the model proposes one 3d, one 4s, two 4p for Ni2+ to from four dsp2 hybrid orbitals pointing the corners of a square accepting one electron pair from each of the four CN- orbitals Note the filling of the first 4 unhybridized 3d orbitals Paramagnetic after one 3d, one 4s and Unpaired e- two 4p orbitals combine to form the four dsp2 hybrid orbitals 6/14/2010 66 Transition Elements & Their Coordination Compounds Application of Valence Bond Theory to Complex Ions Tetrahedral Complexes (four electron groups about central atom) Metal ions that have a filled d sublevel, such as Zn+2 [Ar] 3d10 often form Tetrahedral complexes In the [Zn(OH)4]2- ion, the model proposes the lowest available Zn2+ orbitals one 4s, three 4p mix to become four sp3 hybrid orbitals that point to the corners of a tetrahedron, occupied by four lone pairs, one from each of the four OH- ligands Diamagnetic 6/14/2010 67 Transition Elements & Their Coordination Compounds Crystal Field Theory Valence Bond Theory pictures and rationalizes bonding and shape of molecules VB theory gives little insight into the colors of coordination compounds and can be ambiguous with regard to magnetic properites Crystal Field Theory explains color and magnetism Highlights the “effects” on the d-orbital energies of the metal ion as the ligands approach 6/14/2010 68 Transition Elements & Their Coordination Compounds Crystal Field Theory What is Color? White light is electromagnetic radiation consisting of “all” wavelengths () in the “visible” range Objects appear “colored” in white light because they absorb certain wavelengths and reflect or transmit others Opaque objects reflect light Clear objects transmit light If the object absorbs all visible wavelengths, it appears “black” If the object reflects all visible wavelengths, it appears “white” 6/14/2010 69 Transition Elements & Their Coordination Compounds Crystal Field Theory What is Color? Each color has a “complimentary” color An object has a particular color for two reasons It reflects (or transmits) light of that color or It absorbs light of the “complimentary” color Ex. If an object absorbs only red (compliment of green), it is interpreted as “green” Colors with approximate wavelength ranges Complimentary colors, such as red and green, lie opposite each other 6/14/2010 70 Transition Elements & Their Coordination Compounds Crystal Field Theory In CF Theory, the properties of complexes result from the splitting of d-orbital energies Split d-orbital energies arise from “electrostatic” interactions between the positively charged metal ion cation and the negative charge of the ligands The negative charge of the ligand is either partial as in a polar neutral ligand like NH3, or full, as in an anionic ligand like Cl- 6/14/2010 71 Transition Elements & Their Coordination Compounds Crystal Field Theory The ligands approach the metal ion along the mutually perpendicular x, y, and z axes (octahedral orientation), minimizing the overall energy of the system B & C Lobes of the dx2-y2 and dz2 orbitals lie directly in line with the approaching ligands and have stronger repulsions D, E, F lobes of the dxy, dxz, and dyz orbitals lie “between” the approaching ligands, so the repulsion are weaker 6/14/2010 72 Transition Elements & Their Coordination Compounds Crystal Field Theory An energy diagram of the orbitals shows all five d orbitals are higher in energy in the forming complex than in the free metal ion, because of the repulsions from the approaching ligands Crystal Field Splitting Energy Forming Complex Crystal Field Splitting Energy - The d orbital energies are “split” with the two dx2-y2 and dz2 orbitals (eg orbital set) higher in energy than the dxy, dxz, and dyz orbitals (t2g orbital set) Strong-field ligands, such as CN- lead to larger splitting energy Weak-field ligands such as H2O lead to smaller splitting energy 6/14/2010 73 Transition Elements & Their Coordination Compounds Crystal Field Theory Explaining the Colors of Transition Metals Diversity in colors is determined by the energy difference () between the t2g and eg orbital sets in complex ions When the ions absorbs light in the visible range, electrons move from the lower energy t2g level to the higher eg level, i.e., they are “excited” and jump to a higher energy level E electron = Ephoton = hv = hc/ The substance has a “color” because only certain wavelengths of the incoming white light are absorbed 6/14/2010 74 Transition Elements & Their Coordination Compounds Crystal Field Theory Example – Consider the [Ti(H2O)6]3+ ion – Purple in aqueous solution Hydrated Ti3+ is a d1 ion, with the d electron in one of the three lower energy t2g orbitals The energy difference (A) between the t2g and eg orbitals corresponds to the energy of photons spanning the green and yellow range These colors are absorbed and the electron jumps to one of the eg orbitals Red, blue, and violet light are transmitted as purple 6/14/2010 75 Transition Elements & Their Coordination Compounds Crystal Field Theory For a given “ligand”, the color depends on the oxidation state of the metal ion – the number of “d” orbital electrons available A solution of [V(H2O)6]2+ ion is violet A solution of [V(H2O)6]3+ ion is yellow For a given “metal”, the color depends on the ligand [Cr(NH3)6]3+ (yellow-orange) [Cr(NH3)5]2+ (Purple) Even a single ligand is enough to change the color 6/14/2010 76 Transition Elements & Their Coordination Compounds Crystal Field Theory Spectrochemical Series The Spectrochemical Series is a ranking of ligands with regard to their ability to split d-orbital energies For a given ligand, the color depends on the oxidation state of the metal ion For a given metal ion, the color depends on the ligand As the crystal field strength of the ligand increases, the splitting energy () increases (shorter wavelengths of light must be absorbed to excite the electrons 6/14/2010 77 Practice Problem Rank the following ions in terms of the relative value of and of the energy of visible light absorbed [Ti(H2O)6]3+ Ti(NH3)6]3+ Ti(CN)6]3+ Ans: Oxidation State of Ti is +3 in all formulas From the spectrochemical series table, the ligand strength is in the order: CN- > NH3 > H2O Relative size of , thus, the energy of light absorbed is Ti(CN)6]3+ > Ti(NH3)6]3+ > [Ti(H2O)6]3+ 6/14/2010 78 Transition Elements & Their Coordination Compounds Explaining the Magnetic Properties of Transition Metal Complexes The splitting of energy levels influence magnetic properties Affects the number of unpaired electrons in the metal ion “d” orbitals According to Hund‟s rules, electrons occupy orbitals one at a time as long as orbitals of “equal energy” are available When “all” lower energy orbitals are “half-filled (all +½ spin state)”, the next electron can Enter a half-filled orbital and pair up (with a –½ spin state electron) by overcoming a repulsive pairing energy (Epairing) or Enter an empty, higher energy orbital by overcoming the crystal field splitting energy () The relative sizes of Epairing and () determine the occupancy of the d orbitals 6/14/2010 79 Transition Elements & Their Coordination Compounds Crystal Field Theory Explanation of Magnetic Properties The occupancy of “d” orbitals, in turn, determines the number of unpaired electrons, thus, the paramagnetic behavior of the ion Ex. Mn2+ ion ([Ar] 3d5) has 5 unpaired electrons in 3d orbitals of equal energy In an octahedral field of ligands, the orbital energies split The orbital occupancy is affected in two ways: Weak-Field ligands (low ) and High-Spin complexes Strong-Field ligands (high ) and Low-Spin complexes (from spectrochemical series) 6/14/2010 80 Transition Elements & Their Coordination Compounds Crystal Field Theory Explanation of Magnetic Properties Weak-Field ligands and High-Spin complexes Ex. [Mn(H2O)6]2+ Mn2+ ([Ar] 3d5) A weak-field ligand, such as H2O, has a “small” crystal field splitting energy () It takes less energy for “d” electrons to move to the “eg” set (remaining unpaired) rather than pairing up in the “t2g” set with its higher repulsive pairing energy (Epairing) Thus, the number of unpaired electrons in a weak-field ligand complex is the same as in the free ion Weak-Field Ligands create high-spin complexes, those with a maximum of unpaired electrons Generally Paramagnetic 6/14/2010 81 Transition Elements & Their Coordination Compounds Crystal Field Theory Explanation of Magnetic Properties Strong-Field Ligands and Low-Spin Complexes Ex. [Mn(CN)6]4- Strong-Field Ligands, such CN-, cause large crystal field splitting of the d-orbital energies, i.e., higher () () is larger than (Epairing) Thus, it takes less energy to pair up in the “t2g“ set than would be required to move up to the “eg” set The number of unpaired electrons in a Strong-Field Ligand complex is less than in the free ion Strong-Field ligands create low-spin complexes, Fewer i.e., those with fewer unpaired electrons unpaired electrons Generally Diamagnetic 6/14/2010 82 Transition Elements & Their Coordination Compounds Crystal Field Theory Explaining Magnetic Properties Orbital diagrams for the d1 through d9 ions in octahedral complexes show that both high-spin and low-spin options are possible only for: d4 d5 d6 d7 ions With three “lower” energy t2g orbitals available, the d1, d2, d3 ions always form high-spin (unpaired) complexes because there is no need to pair up Similarly, d8 & d9 ions always form high-spin complexes because the 3 orbital t2g set is filled with 6 electrons (3 pairs) The two t2g orbitals must have either two d8 or one d9 unpaired electron 6/14/2010 83 Transition Elements & Their Coordination Compounds Crystal Field Theory Explaining Magnetic Properties high spin: low spin: high spin: low spin: weak-field strong- weak-field strong- ligand field ligand ligand field ligand 6/14/2010 84 Practice Problem Iron(II) forms an essential complex in hemoglobin For each of the two octahedral complex ions [Fe(H2O)6]2+ [Fe(CN)6]4- Draw an orbital splitting diagram, predict the number of unpaired electrons, and identify the ion as low-spin or high spin Ans: Fe2+ has the [Ar] 3d6 configuration H2O produces smaller crystal field splitting () than CN- The [Fe(H2O)6]2+ has 4 unpaired electrons (high spin) The [Fe(CN)6]4- has no unpaired electrons (low spin) 6/14/2010 85 Transition Elements & Their Coordination Compounds Crystal Field Theory Four electron groups about the central atom Four ligands around a metal ion also cause d-orbital splitting, but the magnitude and pattern of the splitting depend on the whether the ligands are in a “tetrahedral” or “square planar” arrangement Tetrahedral – AX4 Octahedral – AX4E2 (2 ligands along “z” axis removed) Splitting of d-orbital energies Splitting of d-orbital energies by 6/14/2010 by a tetrahedral field of ligands a square planar field of ligands. 86 Transition Elements & Their Coordination Compounds Crystal Field Theory (Splitting) Tetrahedral Complexes Ligands approach corners of a tetrahedron None of the five metal ion “d” orbitals is directly in the path of the approaching ligands Minimal repulsions arise if ligands approach the dxy, dyz, and dyz orbitals closer than if they approach the dx2-y2 and dz2 orbitals (opposite of octahedral case) Thus, the dxy, dyz, and dyz orbitals experience more electron repulsion and become higher energy Splitting energy of d-orbital energies is “less” in a tetrahedral complex than in an octahedral complex tetrahedral < octahedral Only high-spin tetrahedral complexes are known because the magnitude of () is small (weak) 6/14/2010 87 Transition Elements & Their Coordination Compounds Crystal Field Theory (Splitting) Square Planar Complexes Consider an Ocatahedral geometry with the two z axis ligands removed, no z-axis interactions take place Thus, the dz2, dxz an dyz orbital energies decrease The two „d” orbitals in the xy plane (dxy, dx2-y2) interact most strongly with the approaching ligands The (dxy, dx2-y2) orbital has its lobes directly on the x,y axis and thus has a higher energy than the dxy orbital Square Planar complexes are generally strong field – low spin and generally diamagnetic D8 metals ions such as [PdCl4]2- have 4 pairs of the electrons filling the lowest energy levels and are thus, “diamagentic” 6/14/2010 88