MetalLigand Bonding

E A Moore, Rob Janes

• Publisher: Royal Society of Chemistry
• Year: 2019
• Print ISBN: 9780854049790
• eBook ISBN: 9781788018289
• Edition: 1
• Topic: Physical Sciences
• Subtopic: Higher Education

To appreciate the chemistry and physical properties of complexes of the transition series, an understanding of metal-ligand interactions applied to complexes of the d-block is needed. Metal Ligand Bonding aims to provide this through an accessible, detailed, non-mathematical approach. Initial chapters detail the crystal-field model, using it to describe the use of magnetic measurements to distinguish complexes with different electronic configurations and geometries. Subsequent chapters look at the molecular orbital theory of transition metal complexes using a pictorial approach. Bonding in octahedral complexes is explored and electronic spectra and magnetic properties are given extensive coverage. The material addressed in this book forms the foundation of undergraduate lecture courses on d-block chemistry and facilitates learning through various key features, including: full colour diagrams; in-text questions with answers; revision exercises and clearly defined learning outcomes to encourage a reflective approach to study; an associated website; and experimental data and observations from everyday life. A basic knowledge of atomic and molecular orbitals as applied to main group elements is assumed.

INTRODUCTION 1

The attribute of transition-metal ions on which this book focuses is their possession of partially occupied d orbitals. Across the fourth row of the Periodic Table, an electron enters the 4s sub-shell at potassium, and a second fills it at calcium. Then, from scandium to zinc, the 3d sub-shell is progressively filled. For the neutral atoms, the energies of the 3d and 4s orbitals are very close, and it is the exchange energy stabilisation † associated with half-filled and filled shells that gives rise to configuration irregularities at chromium and copper, respectively. This is shown in Table 1.1, where [Ar] represents the argon core electrons.

Table 1.1 Electronic configurations of the free atoms of the first transition series and zinc

Element	Configuration
Sc	[Ar]3d^14s^2
Ti	[Ar]3d^24s^2
V	[Ar]3d^34s^2
Cr	[Arpd^5s^1
Mn	[Ar]3d^54s^2
Fe	[Ar]3d^64s^2
Co	[Ar]3d^74s^2
Ni	[Ar]3d^84s^2
Cu	[Artfd^10s^1
Zn	[Ar]3d^104s^2

When transition-metal atoms form cations, the 4s electrons are lost first. On ionisation, the 3d orbitals are significantly more stabilised (that is, drop to lower energy) than the 4s would be. This stems from the fact that the 3d electrons are not shielded from the nucleus as well as the 4s electrons. Therefore, the +2 and +3 ions have electronic configurations of [Ar]3dⁿ (or ls²2s²2p⁶3s²3p⁶3dⁿ). The electronic configurations of the +2 and +3 ions, which we shall refer to frequently, are shown in Table 1.2.

Table 1.2 Electronic configurations of the dipositive ions and tripositive ions of the first transition series, zinc and gallium

Configuration	M^2+	M^3+
[Ar]3d^1	^aSc^2+	Ti^3+
[Ar]3d^2	Ti^2+	V^3+
[Ar]3d^3	V^2+	Cr^3+
[Ar]3d^4	Cr^2+	Mn^3+
[Ar]3d^5	Mn^2+	Fe^3+
[Ar]3d^6	Fe^2+	Co^3+
[Ar]3d^7	Co^2+	Ni^3+
[Ar]3d^8	Ni^2+	Cu^3+
[Ar]3d^9	Cu^2+	—
[Ar]3d^10	Zn^2+	Ga^3+

a Compounds of scandium(U) are very rare.

One of the characteristic features of the chemistry of the transition elements is the formation of a vast number of complexes such as [Ti(H₂O)₆]³⁺, Ni(CO)₄ and [CoCl(NH₃)₄(H₂O)]²⁺. These are molecules that consist of a central metal atom or ion, to which is bonded a number of molecules or ions by coordinate-covalent bonds. We refer to the latter as ligands, and the number of electron pairs donated to the metal is its coordination number.

What is the coordination number of the metal in the following complexes?

(a) [Ti(H₂O)₆]³⁺; (b) Ni(CO)₄ (c) [CoCl (NH₃)₄(H₂O)]²⁺.

(a) 6; (b) 4; (c) 6.

Possibly the most striking property of transition-metal complexes is the wide range of colours they exhibit. This tells us that part of the visible region of the electromagnetic spectrum is being absorbed by the molecule. But what energy changes are actually occurring at the molecular level? There are also intriguing variations in the magnetic behaviour of transition-metal complexes. For example, although they both contain central Fe²⁺ ions, [Fe(H₂O)₆]²⁺ is paramagnetic (it is attracted into a magnetic field), but [Fe(CN)₆]^4– is diamagnetic (it is weakly repelled by a magnetic field). In this book, we shall look at some bonding theories to help us explain these, and other, observations.

Our starting point is crystal-field theory (Section 2). This is a delightfully simple approach, which, provides us with a remarkable insight into the chemical and physical properties of complexes of d-block metals. However, there are cases where this model is inadequate, and where molecular orbital theory is more appropriate. In developing a theory of bonding in transition-metal complexes, our starting poin...

Table of contents

• Cover
• Title
• Copyright
• Contents
• 1 Introduction
• 2 Crystal-Field Theory
• 3 Electronic Spectra of Octahedral Complexes
• 4 Substituted and Distorted Octahedral Complexes, and Square-Planar Complexes
• 5 Tetrahedral Complexes
• 6 Magnetism and the Magnetic Properties of Transition-Metal Complexes
• 7 Summary of Crystal-Field Theory
• 8 Molecular Orbital Theory of Transition-Metal Complexes
• 9 Bonding in Octahedral Complexes
• 10 Bonding in Complexes of D4h Symmetry
• 11 Bonding in Tetrahedral Complexes
• 12 Complexes of other Symmetry
• 13 Complexes with Two Metal Atoms
• 14 Charge-Transfer Bands in the Electronic Spectra of Transition-Metal Complexes
• 15 Revision Exercise: Complexes of Cobalt(II) and Cobalt(III)
• Appendix Flow Chart for Determining the Symmetry Point Group of an Object
• Learning Outcomes
• Answers to Questions
• Answers to Revision Exercise Questions
• Acknowledgements
• Index