Transition Elements Properties of the Transition Elements

Transition Elements 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 5/23/2018

Transition Elements 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 Main –Group Transition Elements Main-group elements change from metal to non-metal across a period All transition elements are metals Most main-group ionic compounds are colorless and diamagnetic (non-magnetic) Many transition metal compounds are highly colored and paramagnetic 5/23/2018

Transition Elements Properties of Transition Elements
Recall: The “A” (Main Group) elements make up the “s” and “p” blocks Transition Elements make up the “d” block (B group) “f” block elements (Inner Transition Elements) As ions, transition metals (elements) provide fascinating insights into chemical bonding and structure Transition metals play an important role in living organisms 5/23/2018

Transition Elements 5/23/2018

(5 orbitals per period x 2 electrons per orbital x 4 Periods
Transition Elements 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 l = 2  [ml = ] (5 orbitals per period x 2 electrons per orbital x 4 Periods = 40 Elements 5/23/2018

Transition Elements 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] 4s23d Cu [Ar] 4s23d9 Actual: Cr [Ar] 4s13d 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) 5/23/2018

Transition Elements Note Aufbau build up exceptions for “Cr” & “Cu”
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Transition Elements [noble gas] ns2 (n-2)f14(n-1)dx, with n = 6 -7
The “Inner Transition” elements Lie between the 1st and 2nd members of the “d-block” elements in Periods 6 & 7 (n=6 & n=7) Condensed f-block ground-state electron configuration (Periods 6 & 7): [noble gas] ns2 (n-2)f14(n-1)dx, with n = 6 -7 The 28 “f” orbitals are filled as follows: l = 3  [ml = ] 7 orbitals per period x 2 electrons per orbital x 2 periods = 28 Elements 5/23/2018

Ex. Mn2+ and Fe3+ are both d5 ions
Transition Elements 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 Mn2+ [Ar] 3d54s2 → [Ar] 3d5 + 2e- Fe3+ [Ar] 3d64s2 → [Ar] 3d5 + 3e- Both Ions have pale colors in aqueous solutions Both form complex ions with similar magnetic properties 5/23/2018

Practice Problem Write condensed electron configurations for the following ions: Zr V3+ Mo3+ Vanadium (V) – 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 (d2 ion) b. V 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- 5/23/2018

Transition Elements Trends of Transition Elements Across a Period
Transition elements exhibit smaller, less regular changes in Size Electronegativity First Ionization Energy than the Main Group Elements in the same group 5/23/2018

Transition Elements Atomic Size
General overall decrease across a period for both Main group and Transition group elements 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 Main group Transition Metals Main group 5/23/2018

Transition Elements 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 5/23/2018

Transition Elements 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 5/23/2018

“Lanthanide Contraction”
Transition Elements 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, starting in period 6 with buried “4f” sublevel orbitals, appear between the 4d orbitals in period 5 and the 5d orbitals in period 6 – Aufbau buildup sequence 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” 5/23/2018

Sublevels not utilized for any element in the current Period Table
Transition Elements ml = 0 l=0 (1s) n=1 n=2 l=0 (2s) l=1 (2p) n=3 l=0 (3s) l=2 (3d) l=1 (3p) n=4 ml = 0 l=0 (4s) l=1 (4p) l=2 (4d) l=3 (4f) Note: n > 7 & l > 3 Sublevels not utilized for any element in the current Period Table n=5 ml = 0 l=0 (5s) l=1 (5p) l=2 (5d) l=3 (5f) n=6,7 ml = 0 l=0 (6s,7s) l=1 (6p,7p) l=2 (6d) l=3 (6f) 5/23/2018

Transition Elements Order of Sublevel Orbital Filling
Main Group Non-metals Main Group Metals Transition Metals Inner Transition Metals Order of Sublevel Orbital Filling 5/23/2018

Transition Elements 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 5/23/2018

Transition Elements Trends Within a Group (relative to Main-group elements) Ionization Energy – Energy required to remove an electron from a gaseous atom or ion Main-group elements increase in size down a group, decreasing the Zeff , 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), makes it more difficult to remove a valence electron, resulting in a general increase in the first ionization energy down a group 5/23/2018

Transition Elements Trends Within a Group (relative to Main-group elements) Density Atomic size (volume) is inversely related to density (As size increases density decreases) Transition element density across a period initially increases, then levels off, finally dips at end of series From Period 5 to Period 6 the density increases dramatically because atomic volumes change little 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) 5/23/2018

Transition Elements Trends are unlike those for the Main-group elements in several ways 2nd & 3rd members of a transition group are nearly same size Electronegativity increases down a transition group 1st ionization energies are highest at the bottom of transition group Densities increase down a transition group (mass increases faster than density 5/23/2018

Transition Elements Chemical Properties of the Transition Elements
Atomic & physical properties of Transitions elements are similar to Main group elements 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 as valence electrons in bonding – Transition metals can have multiple oxidation states 5/23/2018

Transition Elements Note: All 3 d5 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 Manganese (Mn) dx Electronic Configuration d5 [Ar] 4s2 3d5 +1 [Ar] 4s1 3d5 +2 [Ar] 3d5 +3 d4 [Ar] 3d4 +4 d3 [Ar] 3d3 +5 d2 [Ar] 3d2 +6 d1 [Ar] 3d1 +7 d0 [Ar] 4s 3d 4p Note: All 3 d5 Ex. MnO2 ; O.N. Mn +4 Ex. MnO4- ; O.N. Mn +7 5/23/2018

Transition Elements 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 5/23/2018

Transition Elements 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 5/23/2018

Transition Elements 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) Cr – Cr6+ – P – 2.1 (acidic oxides) 5/23/2018

Transition Elements 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 = V, 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 = 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 5/23/2018

Transition Elements 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 5/23/2018

Transition Elements 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 5/23/2018

Transition Elements 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 Potential (Eo) also increases (becomes more positive) Chromium is stronger reducing agent 5/23/2018

Transition Elements 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 5/23/2018

Sample Problem Finding the Number of Unpaired Electrons The alloy SmCo5 forms a permanent magnet because both Samarium and Cobalt have 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 17) 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 5/23/2018

(Thorium (Th); Z=90 - Lawrencium; Z=103)
Transition Elements The Actinides: (Thorium (Th); Z= Lawrencium; Z=103) All Actinides are Radioactive (Alpha (4He2) Decay Only Thorium & Uranium occur in nature Share very similar chemical & physical properties Silvery and chemically reactive Principal oxidation state is +3, similar to lanthanides 5/23/2018

Transition Elements 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 Electron Configuration ([Ar] 4s1 3d5) with 6 valence electrons occurs in all possible positive oxidation states Important ions Cr2+, Cr3+, Cr6+ 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) 5/23/2018

Transition Elements 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) 5/23/2018

Transition Elements Highlights of Selected Transition Metals
Chromium (con’t) Chromium (VI) - Cr6+ (Deep Red) CrO3 is covalent and acidic Dissolves in water to form 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) 5/23/2018

Mn(s) + 2H+(aq) → Mn2+(aq) + H2(g) Eo = 1.18 V
Transition Elements 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) 5/23/2018

Transition Elements All Manganese species with oxidation states greater than +2 act as oxidizing agents (gaining the electrons lost by the atoms being oxidized) Mn7+O4-(aq) + 4H e- → Mn4+O2(s) + 2H2O(l) Eo = 1.68 Mn7+O4-(aq) + 2H2O + 3e- → Mn4+O2(s) + 4OH Eo = 0.59 (Mn7+O4- is a much stronger oxidizing agent in acid solution than in basic solution – note difference in Eo values) Oxidation State Manganese (Mn) dx Electronic Configuration d5 [Ar] 4s2 3d5 +1 [Ar] 4s1 3d5 +2 [Ar] 3d5 +3 d4 [Ar] 3d4 +4 d3 [Ar] 3d3 +5 d2 [Ar] 3d2 +6 d1 [Ar] 3d1 +7 d0 [Ar] 4s 3d 4p 5/23/2018

Transition Elements Manganese
Unlike Cr2+ & Fe2+, the Mn2+ (3d5) ion resists oxidation in air Recall: half-filled (-1/2 spin 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 5/23/2018

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 5/23/2018

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 [Co(NH3)6]Cl3(s) Octahedral Geometry Central Atom Ligands Counter Ions 5/23/2018

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 (single, double, triple bonds) Ligands are viewed as Lewis Bases (donate electron pairs), although rare cases are known involving Lewis acidic ligands 5/23/2018

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 5/23/2018

Complex ions Geometry – Depends on Coordination No. & Nature of Metal Ion Metal ion CN Shape dx Cu+ 2 Linear d10 Ag+ Au+ Ni2+ 4 Octahedral Sq Planar d8 Pd2+ Pt2+ Cu2+ d9 Cu3+ Tetrahedral Zn2+ Cd2+ Mn2+ d5 Ti3+ 6 Octahedral d1 V2+ d3 Cr3+ Fe3+ Co3+ d6 d1 d8 d3 d9 d5 d10 d6 5/23/2018

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) 5/23/2018

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 5/23/2018

Donor Atom The Ligands contains one or more Donor atoms that have electron pairs to donate to the Central Atom 5/23/2018

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 5/23/2018

Formulas and Names of Coordination Compounds 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., [ ] 5/23/2018

Formulas and Names of Coordination Compounds Coordination Compound Formulas Example # 1 Two compound cations (K+) – Total Charge +2 Ion Central Metal Cation (Co2+) – Total Charge +2 Neutral Ligands (2 NH3) – Total Charge 0 Counter Ions (4 Cl-) – Total Charge -4 Net Charge on Complex Ion – [Co(NH3)2Cl4]-2 5/23/2018

Formulas and Names of Coordination Compounds Coordination Compound 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- 5/23/2018

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+(NH3)5Br-]2[Fe2+(CN-)6] 2 x (3 -1) = = -4 5/23/2018

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), C=O (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 Tetra ammine di chloro cobalt(III)chloride m 5/23/2018

Formulas and Names of Coordination Compounds Names of Some Neutral and Anionic Ligands Symbol Fe Cu Pb Ag Au Sn Names of Some Metals Ions in Complex Anions Di Bis II Tri Tris III Tetra Tetrakis IV Penta pentakis V Hexa Hexakis VI Septa Septakis VII Numerical Prefixes used In Complex Anions 5/23/2018

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 would contain the following ligand name bis(ethylenediamine) 5/23/2018

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 [Co3+(NH3)4Cl-2]Cl- Tetra ammine di chloro cobalt(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 ammine penta chloro platinate(IV) Na4[FeBr6] Na+4[Fe2+Br-6] Sodium hexa bromo ferrate(II) 5/23/2018

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” (net charge -3) 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 5/23/2018

Practice Problem [Co3+(en)2Cl-2]+
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 Dichloro bis (ethylene diamine)cobalt(III) nitrate 5/23/2018

Practice Problem [Pt(NH3)4BrCl]Cl2 What is the formula of:
Tetra ammine bromo chlroro platinum(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 5/23/2018

Practice Problem What is the formula of Hexa ammine cobalt(III) tetra chloro ferrate(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 5/23/2018

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) [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) 5/23/2018

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 5/23/2018

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 5/23/2018

Constitutional (Structural) Isomers Stereo Isomers Optical Isomers (enantiomers) Occur when a molecule and its mirror image can not 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 in the cis compound gives structure III, which is not the same as structure II, its mirror image, Image I & Image III are optical isomers Rotating structure I in the trans compound gives structure III,which is the same as structure II, its mirror image, The trans compound does not have any mirror images 5/23/2018

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 Two different monodentate ligands Geometric Isomers Each isomer is superimposable on the mirror image – no optical isomerism trans cis 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 5/23/2018

Theoretical Basis for the Bonding and Properties of Complexes Questions How do Metal Ligand 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 5/23/2018

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 5/23/2018

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 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 Paramagnetic Unpaired e- 5/23/2018

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 after one 3d, one 4s and two 4p orbitals combine to form the four dsp2 hybrid orbitals Paramagnetic Unpaired e- 5/23/2018

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 5/23/2018

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 5/23/2018

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” 5/23/2018

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 5/23/2018

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- 5/23/2018

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 5/23/2018

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 - 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 Crystal Field Splitting Energy Forming Complex 5/23/2018

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 5/23/2018

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 5/23/2018

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 5/23/2018

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 5/23/2018

Practice Problem Rank the following ions in terms of the relative value of  and of the energy of visible light absorbed [Ti(H2O)6] Ti(NH3)6] 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+ 5/23/2018

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 5/23/2018

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) 5/23/2018

Crystal Field Theory Explanation of Magnetic Properties Weak-Field ligands and High-Spin complexes Ex. [Mn(H2O)6] 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 5/23/2018

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, i.e., those with fewer unpaired electrons Generally Diamagnetic Fewer unpaired electrons 5/23/2018

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: d 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 5/23/2018

Crystal Field Theory Explaining Magnetic Properties high spin: weak-field ligand low spin: strong-field ligand high spin: weak-field ligand low spin: strong-field ligand 5/23/2018

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) 5/23/2018

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 by a tetrahedral field of ligands Splitting of d-orbital energies by a square planar field of ligands. 5/23/2018

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) 5/23/2018

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” 5/23/2018

Transition Elements Properties of the Transition Elements

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Transition Elements Properties of the Transition Elements

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