Schedule Lecture 4: Re-cap Lecture 5:  -Acceptor Ligands and Biology CO, N 2 and O 2 complexes Lecture 6: M-M bonding Multiple bonds and metal clusters Last Week: Electronic spectroscopy Interelectron repulsion, covalency and spin-orbit coupling

Summary of Course – week 5 Complexes of  -acceptor ligands be able to explain synergic (  -donation,  -back donation) model for bonding in M-CO and M-N 2 complexes be able to explain reduction in CO stretching frequency in complex be able to explain changes in CO stretching frequency with metal charge and with ligands electron counting in CO, N 2 and NO complexes: 18 e - rule Resources Slides for lectures 5-6 Winter, Chapter and (basic) Shriver and Atkins “Inorganic Chemistry” Chapter , (4 th Edition) Housecroft and Sharpe “Inorganic Chemistry” Chapter 23.2 (2 nd Edition)

Slide 4/25 Summary of the Last Lecture Electronic spectroscopy Be able to explain number of bands Be able to obtain  oct from spectrum for d 1, d 3, d 4, d 6, d 7, d 8 and d 9 Selection rules Be able to predict relative intensity of spin-allowed vs spin forbidden, octahedral vs tetrahedral and ligand-field vs charge-transfer transitions Today Bonding and vibrational spectroscopy in complexes containing  -acceptor ligands

Slide 5/25 2p  2s 2p 2s 2p 2s 2p JKB Lecture 5 slides 8-9 Molecular Orbitals for O 2 and CO 2p  2p  2p  OOO2O2 2p 2s OCCO 2p 

Slide 6/25 Molecular Orbitals for O 2 and CO O 2 :  bond order = 2 (O=O double bond)  Two singly occupied 2p  g antibonding orbitals CO:  bond order = 3 (C≡O triple bond)  HOMO is dominated by C 2p z (~ C “ lone pair ” )  LUMOs are dominated by C 2p x and 2p y :

Slide 7/25 Metal Carbonyl Complexes CO:  bond order = 3 (C≡O triple bond)  donation from HOMO into empty metal d-orbital: increases e - density on metal  back donation from filled metal orbitals into LUMOs decreases e- density on metal JKB Lecture 5 slide 10 self-enhancing: synergic

Slide 8/25 M-CO:  synergic:  and  bonding are both weak in the absence of each other  therefore requires d electrons on metal and non-contracted d-orbitals to overlap with CO orbitals   -donation strengthens M-C bond   -back donation strengthens M-C bond and weakens C≡O Metal Carbonyl Complexes JKB Lecture 5 slide 10 carbonyls are found for low-oxidation state metals only (+2 or less) carbonyls almost always obey the 18e rule

Slide 9/25 Metal Carbonyl Complexes – Vibrations M-CO – effect of bonding mode:   -donation strengthens M-C bond   -back donation strengthens M-C bond and weakens C≡O  C≡O stretching frequency is reduced from value in free CO  more metals = more back donation: free CO: v co = 2143 cm –2120 cm –1850 cm –1730 cm -1

Slide 10/25 Mn(CO) 6 + : 2090 cm -1 Metal Carbonyl Complexes – Vibrations M-CO – effect of charge:   -donation strengthens M-C bond   -back donation strengthens M-C bond and weakens C≡O  C≡O stretching frequency is reduced from value in free CO  positive charge on complex contracts d-orbitals = less back bonding  negative charge on complex expands d-orbitals = more back bonding free CO: v co = 2143 cm -1 Ni(CO) 4 : 2060 cm -1 Cr(CO) 6 : 2000 cm -1 V(CO) 6  : 1860 cm -1 Co(CO) 4  : 1890 cm -1 Fe(CO) 4 2  : 1790 cm -1

Slide 11/25 Mo(CO) 6 : 2005 cm -1 (PF 3 ) 3 Mo(CO) 3 : 2055, 2090 cm -1 (PCl 3 ) 3 Mo(CO) 3 : 1991, 2040 cm -1 (P(OMe) 3 ) 3 Mo(CO) 3 : 1888, 1977 cm -1 (CH 3 CN) 3 Mo(CO) 3 : 1783, 1915 cm -1 Metal Carbonyl Complexes – Vibrations M-CO – effect of other ligands:   -donation strengthens M-C bond   -back donation strengthens M-C bond and weakens C≡O  C≡O stretching frequency is reduced from value in free CO  in L n M(CO) m complexes, weak  -acceptor ligands increase M  CO back-donation free CO: v co = 2143 cm -1 L: good  -acceptor L: poor  -acceptor

Slide 12/25 Metal Carbonyl Complexes – Vibrations M-CO – symmetry of the molecule:  octahedral M(CO) 6 dipole moment change? no yes no

Slide 13/25 Metal Carbonyl Complexes – Vibrations M-CO – symmetry of the molecule:  octahedral M(CO) 6 v CO 1 IR 2 Raman rule of mutual exclusion: for molecules with a centre of inversion, no vibrations are both IR and Raman active

Metal Carbonyl Complexes – Vibrations v co : 4 IR (1 very weak) 4 Raman (1 very weak) some common bands  trans-[M(CO) 4 Cl 2 ] v co : 1 IR 2 Raman no common bands – rule of mutual exclusion M-CO – symmetry of the molecule:  cis-[M(CO) 4 Cl 2 ]

Metal Carbonyl Complexes – Vibrations v co : 2 IR (which overlap) 2 Raman (which overlap) some common bands v co : 3 IR (1 week) 3 Raman (1 week) some common bands M-CO – symmetry of the molecule:  fac-[M(CO) 4 Cl 2 ]  mer-[M(CO) 4 Cl 2 ]

2p  2s 2p 2s 2p 2s 2p JKB Lecture 5 slides 8-9 Molecular Orbitals for O 2 2p  2p  2p  OOO2O2 2p 2s OCCO 2p 

spin inhibited Spin-Triplet O 2 O 2 in the atmosphere is the result of continuous photosynthesis  it is a potentially highly toxic in the presence of fuels (carbohydrates etc)  however, it is metastable because of the 2 unpaired electrons (“triplet”) 2H 2 (g) + O 2 (g)  2H 2 O(l)  comb H = -484 kJ mol -1 H-H O=O O H H O H H  spin-selection rules prevents “spin-flip” transition in O 2 being important so reaction is not initiated by sunlight  initiation happens via a spark or a catalyst

Slide 18/25 O 2 Transport Complexes Almost all reactions between O 2 and metal complexes are irreversible: 4Fe 2+ + O 2 + 2H 2 O + 8OH -  4Fe(OH) 3  2Fe 2 O 3 + 6H 2 O Transport system for O 2 in animals must:  carry O 2 in its ground state form (with two unpaired electrons)  capture gas phase O 2  transport it via the circulatory system  release it completely to intermediate storage site Transport system for O 2 in animals must:  not react irreversibly with O 2  be highly efficient and cope with changes in supply and demand  have a lower affinity for O 2 than the storage system

Slide 19/25 O 2 Transport Complexes In humans, transport system (haemoglobin) and storage system (myoglobin) are both Fe(II) complexes: myoglobin haemoglobin muscle lungs affinity of myoglobin > affinity of haemoglobin affinity of haemoglobin increases as O 2 pressure grows – cooperative effect

Slide 20/25 Haemoglobin and Myoglobin - Structures Haemoglobin consists of 4 haem groups, myoglobin consists of 1 haem group: proximal histidine residue distal histidine residue

Slide 21/25 Haemoglobin and Myoglobin - Function Unoxygenated protein contains high spin Fe(II) d 6 : proximal histidine residue distal histidine residue Oxygenated protein contains low spin Fe(III) d 5 and O 2  : Unpaired electron on Fe(III) is weakly coupled to unpaired electron on O 2    complex is diamagnetic

Slide 22/25 Haemoglobin and Myoglobin - Function proximal histidine residue distal histidine residue weak H- bond? proximal histidine residue distal histidine residue enforced bending partial prevention of (irreversible) CO attachment

Slide 23/25 Unoxygenated protein contain high spin Fe(II) d 6 : High spin ion has is too large to fit in haem ring and actually sits slightly below it Oxygenated protein contains smaller low spin Fe(III) d 5 which fits into ring Haemoglobin – Cooperative Effect proximal histidine residue The motion of the proximal group is transferred through protein structure to the next deoxygenated haem group decreasing its activation energy for O 2 attachment

Slide 24/25 Summary By now you should be able to.... Explain that metal-carbonyl bonding is due to synergic OC  M  -donation and M  CO  -back donation Explain that the reduction in v co stretching frequency is related to the extent of back-bonding Appreciate that the number of v CO in IR and Raman can be used to work out structure Explain that haemoglobin and myoglobin bind weakly to O 2 allowing transport and storage of highly reactive molecule Next lecture N 2 complexes and Metal-Metal bonding

Slide 25/25 Practice