Chapter 21 Transition Metals and Coordination Chemistry Some History In the 19 th century, chemists started to prepare colored compounds containing transition metals and other substances like ammonia, chloride, water and cyanide They were very interesting because their formulae gave little clue as to how they were bonded together Co(NH 3 ) 6 Cl 3 Sophus Mads Jørgensen (1837-1914) Chain Theory Metal ammonia complexes contain chains of ammonia molecules connected to metal with chlorides at the end of each chain Alfred Werner (1866-1919) Coordination Theory Octahedral arrangement of ammonia molecules around metal forming the complex ion Co(NH 3 ) 6 3+ along with three Cl - ions 3Cl- 1
Complex Ions A complex ion is a charged species consisting of a metal ion (most commonly a transition metal) surrounded by a number of molecules or ions called ligands A ligand is a Lewis base having a lone pair of electrons that can be donated to an empty orbital on the metal ion (acting as a Lewis acid) forming a coordinate covalent bond Coordination Number The number of ligands attached to the metal ion is called the coordination number The most common coordination numbers are 6, 4 and 2 The most common ligands are H 2 O, NH 3, Cl - and CN - The charge on a complex ion is the sum of the charges of the metal ion and the ligands Examples: CoCl 4 2- = Co 2+ + 4Cl - Ni(NH 3 ) 6 2+ = Ni 2+ + 6NH 3 The ligands do not all have to be the same Cu(NH 3 ) 4 (H 2 O) 2 2+ Examples: Co(H 2 O) 6 2+, CoCl 4 2-, Ag(NH 3 ) 2 + Octahedral Complexes Complex Ion Shapes Coordination number 6 complexes will always be octahedral Coordination number 2 complexes will always be linear Tetrahedral Complexes Coordination number 4 complex can be tetrahedral or square planar Square Planar Complexes VSEPR Theory and Complex Ions Valence Shell Electron Pair Repulsion (VSEPR) theory works for six and two coordinate complex ions but cannot be used to predict whether a four coordinate complex will be tetrahedral or square planar! Linear Complexes [H 3 N: Ag :NH 3 ] + [Ag(NH 3 ) 2 ] + 2
Coordination Compounds A coordination compound consists of a complex ion along with one or more counterions which are anions or cations required to produce a compound with no net charge Square brackets are placed around the complex ion to indicate the composition of the complex ion Colors of Coordination Compounds Many coordination compounds are very colorful since the transition metal ions absorb light in the visible spectrum Counterions are shown outside the brackets [Co(NH 3 ) 5 Cl]Cl 2 = Co(NH 3 ) 5 Cl 2+ + 2Cl - A coordination compound behaves like any other ionic compound when dissolved in water forming independent cations and anions: [Co(NH 3 ) 5 Cl]Cl 2 (s) HO 2 Co(NH 3 ) 5 Cl 2+ (aq) + 2Cl - (aq) Ligand Types Ligands that can form one bond to a metal ion are called monodenate or unidentate ( single toothed ) Etheylenediaminetetraacetate (EDTA) A hexadenatate ligand! Ligands with more than one atom with a lone pair that can bond to a metal are said to chelating ligands or chelates ( clawed ) A ligand that can form two bonds to a metal ion is called a bidentate ligand Ligands that can form more than two bonds are called polydentate Common Ligands Naming Coordination Compounds 3
Isomerism in Coordination Compounds Occurs when two or more complex ions contain exactly the same types and numbers of atoms (hence same formula) but the arrangement of ligands differ leading to different physical and/or chemical properties Linkage Isomerism The composition of the complex is the same but one or more of the ligands attaches with a different atom Geometrical Isomerism Occurs when ligands occupy different spatial positions around the central atom [Co(NH 3 ) 4 (NO 2 )Cl]Cl cis trans [Co(NH 3 ) 4 (ONO)Cl]Cl trans cis 4
Review of Electronic Configurations of Transition Metal Atoms Iron? 1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d 6 or [Ar] 4s 2 3d 6 Chromium? Expect? 1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d 4 or [Ar]4s 2 3d 4 Find: 1s 2 2s 2 2p 6 3s 2 3p 6 4s 1 3d 5 or [Ar]4s 1 3d 5 Half-filled 3d level is very stable! Gadolinium? 1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d 10 4p 6 5s 2 4d 10 5p 6 6s 2 5d 1 4f 7 or [Xe]6s 2 5d 1 4f 7 5
In neutral transition elements, the s- and d-orbitals have very similar energies Electronic Configurations of Transition Metal Ions However, in transition metal ions the energy of the d-orbitals is significantly lower than that of the s-orbitals so that the electrons remaining after the ion forms occupy the lower energy d-orbitals. Fe 3+ is [Ar]3d 5 not [Ar]4s 2 3d 3 Bonding in Coordination Compounds Can be considered a Lewis acid-base reaction where lone pairs on the ligands are shared with orbitals on the metal ion 6