Chapter 23 Sections 2-4 Transition Metal Complexes © 2012 Pearson Education, Inc.

Complexes Commonly, transition metals can have molecules or ions that bond to them. These give rise to complex ions or coordination compounds. © 2012 Pearson Education, Inc. Transition Metals

Ligands The molecules or ions that bind to the central metal are called ligands (from the Latin ligare, meaning “to bind”). © 2012 Pearson Education, Inc. Transition Metals

A.Yes. Ammonia is the Lewis acid. B.Yes. The metal cation is the Lewis acid. C.No. Ammonia is a neutral molecule and cannot can not accept electron density from a Lewis base. D.No. The metal cation cannot accept electron density from a Lewis base.

Coordination One of the properties that has led to the fascination with complexes and transition metals is the wide range of stunning colors found in them. © 2012 Pearson Education, Inc. Transition Metals Werner Theory The Swiss chemist Alfred Werner deduced that there was a difference between the oxidation number of a metal and the number of ligands it took on, which he called the coordination number. Thus, although the first two complexes in the table above each have 3 chlorines, in the first all three serve as anions, while in the second one of the chlorines is tightly bound to the cobalt and the other two are counterions.

Sample Exercise 23.1 Identifying the Coordination Sphere of a Complex Palladium(II) tends to form complexes with coordination number 4. A compound has the composition PdCl 2 · 3 NH 3. (a) Write the formula for this compound that best shows the coordination structure. (b) When an aqueous solution of the compound is treated with excess AgNO3(aq), how many moles of AgCl(s) are formed per mole of PdCl 2 · 3 NH 3 ? (a) By analogy to the ammonia complexes of cobalt(III) shown in Figure 23.7, we predict that the three NH 3 are ligands attached to the Pd(II) ion. The fourth ligand around Pd(II) is one chloride ion. The second chloride ion is not a ligand; it serves only as a counterion (a noncoordinating ion that balances charge) in the compound. We conclude that the formula showing the structure best is [Pd(NH 3 ) 3 Cl]Cl. (b) Because only the non-ligand can react, we expect to produce 1 mol of AgCl(s) per mole of complex. The balanced equation is [Pd(NH 3 ) 3 Cl]Cl(aq) + AgNO 3 (aq) → [Pd(NH 3 ) 3 Cl]NO 3 (aq) + AgCl(s) This is a metathesis reaction (Section 4.2) in which one of the cations is the [Pd(NH 3 ) 3 Cl] + complex ion. Practice Exercise Predict the number of ions produced per formula unit in an aqueous solution of CoCl 2 · 6 H 2 O. Answer: three: [Co(H 2 O) 6 ] 2+ and two Cl –

The Metal–Ligand Bond The reaction between a metal and a ligand is a reaction between a Lewis acid (the metal) and Lewis base (the ligand). The new complex has distinct physical and chemical properties. © 2012 Pearson Education, Inc. Transition Metals

A.[Fe(H 2 O) 4 ] 3+ (aq) + SCN – (aq)  [Fe(H 2 O) 3 NCS] 2+ (aq) + H 2 O(l) B.[Fe(H 2 O) 4 ] 3+ (aq) + S 2+ (aq) + CN – (aq)  [Fe(H 2 O) 5 NCS] 2+ (aq) + H 2 O(l) C.Fe 3+ (aq) + 3SCN – (aq)  FeCN 3 (aq) + 3S – (aq) D.[Fe(H 2 O) 6 ] 3+ (aq) + SCN – (aq)  [Fe(H 2 O) 5 NCS] 2+ (aq) + H 2 O(l)

Coordination Numbers The number of atoms directly bonded to the metal atom in a complex is the coordination number. © 2012 Pearson Education, Inc. Transition Metals CompoundCoordination number [Ag(NH 3 ) 2 ] + 2 [Co(NH 3 ) 6 ]Cl 3 6 [Co(NH 3 ) 5 Cl]Cl 2 6 trans -[Co(NH 3 ) 4 Cl 2 ]Cl6 cis -[Co(NH 3 ) 5 Cl 2 ]Cl6

Sample Exercise 23.2Determining the Oxidation Number of a Metal in a Complex What is the oxidation number of the metal in [Rh(NH 3 ) 5 Cl](NO 3 ) 2 ? Solve The NO 3 group is the nitrate anion, which has a 1– charge. The NH 3 ligands carry zero charge, and the Cl is a coordinated chloride ion, which has a 1– charge. The sum of all the charges must be zero: x + 5(0) + (–1) + 2(–1) = 0 [Rh(NH 3 ) 5 Cl](NO 3 ) 2 The oxidation number of rhodium, x, must therefore be +3. Practice Exercise What is the charge of the complex formed by a platinum(II) metal ion surrounded by two ammonia molecules and two bromide ions? Answer: zero

Sample Exercise 23.3 Determining the Formula of a Complex Ion A complex ion contains a chromium(III) bound to four water molecules and to two chloride ions.What is the formula and charge of this ion? (0) + 2(–1) = +1 Cr(H 2 O) 4 Cl 2 The charge on the ion is 1+, [Cr(H 2 O) 4 Cl 2 ] +. Practice Exercise Write the formula for the complex described in the Practice Exercise accompanying Sample Exercise Answer: [Pt(NH 3 ) 2 Br 2 ]

Coordination Numbers The coordination number of a metal depends upon the size of the metal and the size of the ligands. While iron(III) can bind to 6 fluorides, it can only accommodate 4 of the larger chlorides. © 2012 Pearson Education, Inc. Transition Metals

NH 3 —Zn—NH 3 NH 3 —Pt—NH 3 A.109.5°90° B.120 ° 109.5° C.180°120° D.120°90°

Geometries:C.N. = 4C.N. = 6 A.OctahedralTetrahedral B.TetrahedralOctahedral C. OctahedralSquare planar D. Square planarTetrahedral

A.Two for both B. Three for both C. Four for both D. Six for both

A.Bidentate ligand B.Monodentate ligand

Common Ligands The table above contains some ligands commonly found in complexes. © 2012 Pearson Education, Inc. Transition Metals Monodentate ligands coordinate to one site on the metal, bidentate to two, and so forth.

Common Ligands Bi  and polydentate ligands are also called chelating agents. © 2012 Pearson Education, Inc. Transition Metals

Chelates in Biological Systems There are many transition metals that are vital to human life. Several of these are bound to chelating agents. © 2012 Pearson Education, Inc. Transition Metals

Chelates in Biological Systems For instance, the iron in hemoglobin carries O 2 and CO 2 through the blood. © 2012 Pearson Education, Inc. Transition Metals

A. On the protein group B. On the heme group C. On the NH group D. Where O 2 binds

Chelates in Biological Systems For instance, the iron in hemoglobin carries O 2 and CO 2 through the blood. Carbon monoxide and cyanide are poisonous because they will bind more tightly to the iron than will oxygen. © 2012 Pearson Education, Inc. Transition Metals

A.420 nm B. 500 nm C. 610 nm D. 650 nm

A.It has a planar structure that enables the C-H bonds to absorb light in the visible region. B.The C-C bonds absorb light in the ultraviolet region. C.It has conjugated double bonds that absorb light in the visible region. D.It has C-H bonds that absorb light in the infrared region.

Nomenclature in Coordination Chemistry 1.In naming complexes that are salts, the name of the cation is given before the name of the anion. © 2012 Pearson Education, Inc. Transition Metals

Nomenclature in Coordination Chemistry 2.In naming complex ions or molecules, the ligands are named before the metal. Ligands are listed in alphabetical order, regardless of their charges. © 2012 Pearson Education, Inc. Transition Metals

Nomenclature in Coordination Chemistry 3.The names of anionic ligands end in the letter o, but electrically neutral ligands ordinarily bear the name of the molecules. © 2012 Pearson Education, Inc. Transition Metals

Nomenclature in Coordination Chemistry 4.Greek prefixes (di-, tri-, tetra-, etc.) are used to indicate the number of each kind of ligand when more than one is present. If the ligand contains a Greek prefix or is polydentate, the prefixes bis-, tris-, tetrakis-, etc. are used and the ligand name is placed in parentheses. © 2012 Pearson Education, Inc. Transition Metals

Nomenclature in Coordination Chemistry 5.If the complex is an anion, its name ends in -ate. 6.The oxidation number of the metal is given in parentheses in Roman numerals following the name of the metal. © 2012 Pearson Education, Inc. Transition Metals

Sample Exercise 23.4 Naming Coordination Compounds Name the compounds (a) [Cr(H 2 O) 4 Cl 2 ]Cl, (b) K 4 [Ni(CN) 4 ] (0) + 2(–1) + (–1) = 0 [Cr(H 2 O) 4 Cl 2 ]Cl tetraaquadichlorochromium(III) chloride 4(+1) (–1) = 0 K 4 [Ni(CN) 4 ] potassium tetracyanonickelate(0) Solve (a) The ligands are four water molecules— tetraaqua—and two chloride ions—dichloro. By assigning all the oxidation numbers we know for this molecule, we see that the oxidation number of Cr is +3: Thus, we have chromium(III). Finally, the anion is chloride. The name of the compound is (b) The complex has four cyanide ion ligands, CN –, which means tetracyano, and the oxidation state of the nickel is zero: Because the complex is an anion, the metal is indicated as nickelate(0). Putting these parts together and naming the cation first, we have Practice Exercise Name the compounds (a) [Mo(NH 3 ) 3 Br 3 ]NO 3, (b) (NH 4 ) 2 [CuBr 4 ]. (c) Write the formula for sodium diaquabis(oxalato)ruthenate(III). Answer: (a) triamminetribromomolybdenum(IV) nitrate, (b) ammonium tetrabromocuprate(II) (c) Na[Ru(H 2 O) 2 (C 2 O4) 2 ]