Coordination Chemistry: Bonding Theories Molecular Orbital Theory Chapter 20
Review of the Previous Lecture 1. Discussed magnetism in coordination chemistry and the different classification of compounds with magnetic properties 2. Evaluated the magnetic susceptibility of compounds as it depends on temperature 3. Described the property of spin crossover 4. Briefly surveyed tools to characterized the magnetic properties of compounds 2
1. Introduction to Molecular Orbital Theory Unlike crystal field theory, molecular orbital theory accounts for covalency in M-L bonding Electrons shared by metal ions and ligands The identity of the ligand is important in the sharing of these electrons Let s examine how MOT helps us to account for and π interactions. 3
2. The Spectrochemical Series I - < Br - < [NCS] - < Cl - < F - < [OH] - < [ox] 2- ~ H 2 O< [NCS] - < NH 3 < en < [CN] - ~ CO π donors σ donors π acceptors Weak field ligands Small Δ High spin π donors Ligands increasing Δ oct Strong field ligands Large Δ Low spin π acceptors If splitting of the d orbitals resulted simply from the effect of point charges then anionic ligands would exert the greatest effect on the magnitude of Δ. OH - would be expected to induce a stronger field than H 2 O but does not 4
3. interactions Let s use the octahedral geometry (C.N. = 6) as our point of reference: z x y Use vectors aligned with the internuclear axes of the 6 M-L bonds as your basis set to examine interactions in coordination compounds. 5
3. interactions Use group theory to identify the symmetry of the metal atomic orbitals and the ligand group orbitals that will be involved in bonding. z Have 6 vectors to represent 6 bonds, therefore expect your reducible representation to be composed of 6 irreducible representations Point Group: O h x y red ( )= a 1g + e g + t 1u 6 irreducible representations 6
3A. Metal atomic orbitals engaged in interactions z a 1g :sorbital e g :d z 2 ;d x 2-y2 x y t 1u :p x,p y,p z For a 1 st row transition metal, the orbitals would be from the 3d, 4s, and 4p orbitals. Of these, the d xy, d yz, and d xz do not engage in bonding because they are of the t 2g symmetry Nonbonding orbitals 7
3B. Ligand group orbitals engaged in interactions z The ligand group orbitals will have the a 1g, e g, t 1u symmetries and there will be a total of six. x y Will be defined by atomic orbitals from the ligands that engage in bonding For instance, if L = hydrogen, then s orbitals if L = H 2 O, then sp 3 hybrid orbitals Let s consider the ligand group orbitals as a set of lobes that will overlap with the metal atomic orbital lobes. 8
a 1g Symmetry Metal Atomic Orbital Ligand Group Orbital The a 1g metal atomic orbital and LGO will generate one bonding molecular orbital and one antibonding molecular orbital 9
a 1g Symmetry Bonding Molecular Orbital Antibonding Molecular Orbital Zero Nodes One Spherical Node 10
t 1u Symmetry Metal Atomic Orbital Ligand Group Orbital The t 1u metal atomic orbitals and LGOs will generate three bonding molecular orbitals and three antibonding molecular orbitals 11
t 1u Symmetry Bonding Molecular Orbital Metal Atomic Orbital One Nodal Plane Ligand Group Orbital Antibonding Molecular Orbital Three Nodal Planes 12
e g Symmetry Metal Atomic Orbital Ligand Group Orbital The e g metal atomic orbitals and LGOs will generate two bonding molecular orbitals and two antibonding molecular orbitals 13
e g Symmetry Bonding Molecular Orbital Metal Atomic Orbital Two Nodal Planes Ligand Group Orbital Antibonding Molecular Orbital Two Nodal Planes, One Nodal Cylinder 14
e g Symmetry Bonding Molecular Orbital Metal Atomic Orbital Two Nodal Cylinders Ligand Group Orbital Antibonding Molecular Orbital Three Nodal Cylinders 15
3C. Molecular Orbital Diagram for σ interaction The 6 metal atomic orbitals interact with the 6 LGOs : 12 molecular orbitals 6 bonding molecular orbitals 6 antibonding molecular orbitals Antibonding MOs Bonding MOs 16
3C. Molecular Orbital Diagram for σ interaction The 6 metal atomic orbitals interact with the 6 LGOs : 12 molecular orbitals 6 bonding molecular orbitals 6 antibonding molecular orbitals Two nodes One node No nodes 17
3C. Molecular Orbital Diagram for σ interaction The 6 metal atomic orbitals interact with the 6 LGOs : 12 molecular orbitals 6 bonding molecular orbitals 6 antibonding molecular orbitals The t 2g metal atomic orbitals are nonbonding d xy,d yz,andd xz orbitals 18
3C. Molecular Orbital Diagram for σ interaction Each of the 6 ligands contributes 2 electrons for a total of 12 electrons: The 12 ligand electrons fill the bonding molecular orbitals (a 1g, t 1u,e g ) The metal-ligand interactions stabilize the 12 ligand electrons 12 e - 19
3C. Molecular Orbital Diagram for σ interaction The 6 LGOs create an octahedral field: oct is defined by the separation in the nonbonding metal atomic orbitals t 2g and the antibonding molecular orbitals e g * The metal d orbital electrons will fill in these orbitals 20
3C. Molecular Orbital Diagram for σ interaction * 3d The metal-ligand interactions stabilize the metal d electrons. Recall CFSE. 21
3D. The 18 electron rule The most stable metal-ligand interactions in octahedral complexes are those that result in the filling of the metal and ligand electrons into the bonding molecular orbitals and the nonbonding t 2g metal atomic orbitals. Altogether, these 9 orbitals accept 18 electrons 22
4. π interactions Consider ligand orbitals that can engage in π interactions with metals: M L M L M L d π p π d π d π d π π * 23
The Spectrochemical Series I - < Br - < [NCS] - < Cl - < F - < [OH] - < [ox] 2- ~ H 2 O< [NCS] - < NH 3 < en < [CN] - ~ CO π donors σ donors π acceptors M L d π p π 24
4A. Let s consider p ligand orbitals involved in and π interactions with metals Consider each of the ligand p orbitals that can engage in and π interactions with metals: 1 p orbital along the internuclear axis for interactions 2 p orbitals perpendicular to the internuclear axis for π interactions z L L x L L M L L y M M interactions π interactions 25
4B. Use group theory to examine the π interactions with metals Choose a basis set to define the symmetry of the ligand group orbitals that can engage in π interactions 12 vectors indicate that the reducible representation will be defined by 12 irreducible representations Point Group: O h red (π) = t 2g + t 2u + t 1u + t 1g z 12 irreducible representations M y M x 26
4C. Focus on the t 2g orbital symmetry We will focus on the t 2g orbital symmetry because this symmetry represented the nonbonding metal atomic orbitals in the molecular orbital diagram for only interactions. These orbitals can engage in π interactions. d xy,d yz,andd xz orbitals z M y M x 27
4D. Factors to consider regarding the energy of the t 2g ligand group orbitals The energy of the t 2g ligand group orbitals will be greater or lower than the metal atomic orbitals depending on The electronegativity difference between the ligands and the metal Whether the LGOs are electron occupied z M y M x 28
4E. Explaining the origins of the weak field ligands Consider the octahedral complex [CoF 6 ] 3- : Co 3+ ;d 6 F - is more electronegative than Co 3+ The t 2g LGOs will be lower in energy than the t 2g metal atomic orbitals When these orbitals interact, they will form 3 bonding molecular orbitals (t 2g ) and 3 antibonding molecular orbitals (t 2g* ) 29
4E. Explaining the origins of the weak field ligands Consider the complex [CoF 6 ] 3- : 6F - : Total of 36 electrons in the 18 p orbitals 12 electrons used for interactions The remaining 24 electrons can engage in π interactions, of which 6 of these belong z to the t 2g LGOs M y M x interactions π interactions 30
Molecular Orbital Diagram including π interaction with Weak Field Ligands 31
Molecular Orbital Diagram including π interaction with Weak Field Ligands 32
Molecular Orbital Diagram including π interaction with Weak Field Ligands [CoF 6 ] 3- High spin, S = 2 Co 3+ ; d 6 33
4E. Explaining the origins of the strong field ligands I - < Br - < [NCS] - < Cl - < F - < [OH] - < [ox] 2- ~ H 2 O< [NCS] - < NH 3 < en < [CN] - ~ CO π donors σ donors π acceptors M L Consider the octahedral complex [Co(CO) 6 ] 3+ : Co 3+ ;d 6 d π π * 34
Molecular Orbital Diagram for CO LUMO t 2g symmetry The t 2g LGOs are higher in energy than the t 2g metal atomic orbitals These orbitals are unoccupied and can accept electrons from the metal 35
Molecular Orbital Diagram including π interaction with Strong Field Ligands 36
Molecular Orbital Diagram including π interaction with Strong Field Ligands 37
Molecular Orbital Diagram including π interaction with Strong Field Ligands [Co(CO) 6 ] 3+ Co 3+ ; d 6 Low spin, S = 0 38
4F. Revisiting the 18 electron rule The most stable metalligand interactions in octahedral complexes are those that result in the filling of the metal and ligand electrons into the 9 bonding molecular orbitals Altogether, these 9 orbitals accept 18 electrons 39
4G. π backbonding with π acceptor ligands M d π π * The ligand donates electron density to the metal through bonds The metal donates electron density to the ligand through π bonds 40
4G. π backbonding with π acceptor ligands π backbonding weakens the CO bond because electron density is moved into its π * molecular orbital: Ths effect is more pronounced depending on the electron donation capacity of another ligand positioned trans to the CO ligand Electron withdrawing Strengthens CO bond; Increased υ CO Trans influence trans-l M Electron donating d π π * Weakens CO bond; Decreased υ CO 41