Transition Metals and Coordination Chemistry Chapter 19 Transition Metals and Coordination Chemistry

The Differences between Main Group Metals and Transition Metals Transition metals are more electronegative than the main group metals. The main group metals tend to form salts. The transition metals form similar compounds, but they are more likely than main group metals to form complexes. NaCl(s)→ Na+(aq)+Cl-(aq) CrCl3(s) + 6 NH3(l) →CrCl3 · 6 NH3(s) Violet Yellow

Electron Configurations Sc: [Ar]4s23d1 Ti: [Ar]4s23d2 V: [Ar]4s23d3 Cr: [Ar]4s13d5 Mn: [Ar]4s23d5 Fe:[Ar]4s23d6 Co:[Ar]4s23d7 Ni: [Ar]4s23d8 Cu: [Ar]4s13d10 Zn:[Ar]4s23d10

Half Filled Set of 3d Orbitals Cr: [Ar]4s13d5 Cu: [Ar]4s13d10 The orbital energies are not constant for a given atom but depend on the way that the other orbitals in the atom are occupied. Because the 4s and 3d orbitals have similar energies, the 4s23dn and 4s13dn+1. configurations have similar energies. For most elements, 4s23dn is lower in energy, but for Cr and for Cu the 4s13dn+1 is more stable.

Oxidation States Co:[Ar]4s23d7 Co2+: [Ar]3d7 Co3+:[Ar]3d6 The discussion of the relative energies of the atomic orbitals suggests that the 4s orbital has a lower energy than the 3d orbitals. Thus, we might expect cobalt to lose electrons from the higher energy 3d orbitals, but this is not what is observed. In general, electrons are removed from the valence-shell s orbitals before they are removed from valence d orbitals when transition metals are ionized.

The 4d and 5d Transition Series

Lanthanide Contraction Since the 4f orbitals are buried in the interior of these atoms, the additional electrons do not add to the atomic size. The increasing nuclear charge causes the radii of lanthanide elements (Z=58-71) to decrease significantly going from left to right.

Coordination Number

Ligands A ligands is a neutral molecule or ion having a lone pair that can be used to form a bond to a metal ion. Because a ligand donates an electron pair to an empty orbital on a metal ion, the formation of a metal-ligand bond (coordinate covalent bond) can be described as the interaction between a Lewis base (the ligand) and a Lewis acid (the metal ion).

Isomerism

Structural Isomerism Coordination Isomers Isomers involving exchanges of ligands between complex cation and complex anion of the same compound. [Co(NH3)6][Cr(CN)6] & [Co(CN)6][Cr(NH3)6] [Ni(C2H4)3][Co(SCN)4] & [Ni(SCN)4][Co(C2H4)3] [Cr(NH3)5SO4]Br& [Cr(NH3)5Br]SO4

Structural Isomerism Linkage Isomers Isomers in which a particular ligand bonds to a metal ion through different donor atoms. [Co(NH3)5ONO]Cl2&[Co(NH3)5NO2]Cl2

[Co(NH3)5NO2]Cl2 [Co(NH3)4ONO]Cl2

[Co(NH3)5NO2]2+ [Co(NH3)5ONO]2+

Stereo-isomerism Geometric Isomers/cis-trans Isomers Stereoisomers: Molecules have the same molecular formula and the same connectivity of atoms, but differ only in the three-dimensional arrangement of those atoms in space. Geometric Isomers: Atoms or groups of atoms can assume different positions around a rigid ring or bond.

Stereo-isomerism Optical Isomer Optical isomerism is a form of isomerism whereby the different 2 isomers are the same in every way except being non-superimposable mirror images(*) of each other.

The two structures are nonsuperimposable mirror images The two structures are nonsuperimposable mirror images. They are like a right hand and a left hand.

Simple substances which show optical isomerism exist as two isomers known as enantiomers. A molecule which has no plane of symmetry is described as chiral. The carbon atom with the four different groups attached which causes this lack of symmetry is described as a chiral center. chiral center

One enantiomer will rotate the light a set number of degrees to the right. This is called the Dextrorotator (from the Latin dexter, "right"右旋) isomer or (+) isomer. The other enantiomer will rotate the plane polarized light the same number of set degrees in the opposite left direction. This isomer is said to be a Levorotatory (from the Latin laevus, "left“ 左旋) isomer or (-) isomer.

Octahedral Complexes eg t2g

Strong Field and Low Spin The splitting of d orbital energies explains the color and magnetism of complex ions. If the splitting produced by the ligands is very large, a situation called strong field case, the electrons will pair in the low energy t2g orbitals. The strong field case is also called low spin case. This gives a diamagnetic complex in which all electrons are pairs. △0>P

Weak Field and High Spin If the splitting produced by the ligands is small, the electrons will occupy all five orbitals before pairing occurs called weak field case. The weak field case is also called high spin case. In this case, the complex is paramagnetic. △0<P

Octahedral transition-metal ions with d1, d2, or d3 configurations

Octahedral transition-metal ions with d4, d5, d6, and d7 configurations

For octahedral d8, d9, and d10 complexes , there is only one way to write satisfactory configurations.

weak field case strong field case with paramagnetic with diamagnetic

The Color of Complexes Very commonly for the first transition series, the energy corresponds to that of visible light, so that d-d transitions are the cause of the delicate colours of so many of the complexes.

Charge Effect of Metal Ions As the metal ion charge increases, the ligands are drawn closer to the metal ion because of its increased charge density. As the ligands move closer, they cause greater splitting of the d orbitals, thereby producing a larger Δ value. The magnitude of Δ for a given ligand increases as the charge on the metal ion increases. NH3-Co+2 (weak field) NH3-Co+3 (strong field)

Spectrochemical Series I- < Br- < SCN- ~Cl- < F- < OH- ~ ONO- < C2O42- < H2O< NCS- < EDTA4- < NH3 ~ pyr ~ en < phen < CN- ~ CO Mn2+ < Ni2+ < Co2+ < Fe2+ < V 2+ < Fe3+ < Co3+ < Mn3+ < Mo3+ < Rh3+ < Ru3+ < Pd4+ < Ir3+ < Pt4+ pyr: pyridine phen: phenol

Tetrahedral complex

Energy Splitting of Tetrahedral Complex Because a tetrahedral complex has fewer ligands, the magnitude of the splitting is smaller. The difference between the energies of the t2g and eg orbitals in a tetrahedral complex (Δt) is slightly less than half as large as the splitting in analogous octahedral complexes (Δo). Δt = 4/9Δo

Square Planar and Linear Complex

Ligand Field Theory Ligand field theory can be considered an extension of crystal field theory such that all levels of covalent interactions can be incorporated into the model. Treatment of the bonding in LFT is generally done using molecular orbital theory.

Molecular Orbital Model eg

Molecular Orbital of Complex The dz2, dx2-y2, 4s, 4px, 4py and 4pz orbitals will be involved in the MOs in the s complex ions. The dxz, dyz and dxy orbitals (the t2g set) of the metal ion do not overlap with ligand orbitals. They are called nonbonding orbitals. The eg* orbitals is relatively little contribution from ligand orbitals. This lack of mixing is caused by the large energy difference between the ligand orbitals and the metals ion 3d orbitals.

The Effect of Weak Field Ligands A ligand with a electronegative donor atom will have lone pair orbitals of very low energy (the electrons are very firmly bound to the ligand); these orbitals do not mix very thoroughly with the metal ion orbitals. This will result in a small difference between the t2g and eg* orbitals.

The Effect of Strong Field Ligands The strong field ligands produce larger degree of mixing between the orbitals of ligands and metal ions This gives a relatively large amount of d-orbital splitting, and low spin case results.

Biological Importance of Coordination Complexes－Hemoglobin The principal electron transfer molecules in the respiratory chain are iron-containing species called cytochromes, consisting of two main part: an iron complex called heme and a protein. (cytochromes= heme+ protein) A metal ion coordinated to a rather complicated planar ligand is called a porphyrin. The various porphyrin molecules act as tetradentate ligands for many metal ions, including iron, cobalt and magnesium

Chlorophyll

Myoglobin Iron plays a principal role in the transport and storage of oxygen in mammalian blood and tissues. Oxygen is stored using a molecule called myoglobin, which contains a heme complex and a protein. In myoglobin, the Fe+2 ion is coordinated to four nitrogen atoms of porphyrin ring and to one nitrigen atom of the protein chain. Since Fe+2 ion is normally six-coordinate, this leaves one position open for attachment of an O2 molecule. Fe2+ N Histidine O2

Heme和Myoglobin的連結 Hb + 4O2 <=> Hb(O2)4 Hemoglobin Oxyhemoglobin

Myoglobin molecule

Hemoglobin The transport of O2 in the blood is carried out by hemoglobin, a molecule consisting of four myoglobin molecules units.

Conformation change when heme is oxidized 氧氣在和中心鐵離子結合之後，會造成所連接的多肽鍵構型產生改變。 尚未和氧結合時：非平面 和氧結合後：平面(由於電子之間的相互排斥力) 這種構型改變的結果，會導致整體的Hemoglobin產生變化，使得尚未接到氧氣的Heme變成容易接上氧氣的構型。

Normal red blood cell (right) and a sickle cell, both magnified 18,000 times.