Lecture 5: p-Acceptor Ligands and Biology CO, N2 and O2 complexes Schedule Last Week: Electronic spectroscopy Interelectron repulsion, covalency and spin-orbit coupling Lecture 4: Re-cap Lecture 5: p-Acceptor Ligands and Biology CO, N2 and O2 complexes Demos: charles law (2.3), CO2 density (2.1), star wars (4.3) Brief mention of 3 states of matter. Lecture 6: M-M bonding Multiple bonds and metal clusters

Summary of Course – week 5 Complexes of p-acceptor ligands be able to explain synergic (s-donation, p-back donation) model for bonding in M-CO and M-N2 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, N2 and NO complexes: 18 e- rule Resources Slides for lectures 5-6 Winter, Chapter 6.5-6.7 and 6.10-6.11(basic) Shriver and Atkins “Inorganic Chemistry” Chapter 21.1-5, 21.18 (4th Edition) Housecroft and Sharpe “Inorganic Chemistry” Chapter 23.2 (2nd Edition)

Summary of the Last Lecture Electronic spectroscopy Be able to explain number of bands Be able to obtain Doct from spectrum for d1, d3, d4, d6, d7, d8 and d9 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 p-acceptor ligands

Molecular Orbitals for O2 and CO 2ps 2ps 2pp 2pp 2p 2s 2p 2p 2p 2pp 2ps 2s 2s 2s O O2 O O CO C JKB Lecture 5 slides 8-9

Molecular Orbitals for O2 and CO bond order = 2 (O=O double bond) Two singly occupied 2ppg antibonding orbitals CO: bond order = 3 (C≡O triple bond) HOMO is dominated by C 2pz (~ C “lone pair”) LUMOs are dominated by C 2px and 2py:

Metal Carbonyl Complexes 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 self-enhancing: synergic JKB Lecture 5 slide 10

Metal Carbonyl Complexes M-CO: synergic: s and p 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 s-donation strengthens M-C bond p-back donation strengthens M-C bond and weakens C≡O carbonyls are found for low-oxidation state metals only (+2 or less) carbonyls almost always obey the 18e rule JKB Lecture 5 slide 10

Metal Carbonyl Complexes – Vibrations M-CO – effect of bonding mode: s-donation strengthens M-C bond p-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: vco = 2143 cm-1 1850–2120 cm-1 1750–1850 cm-1 1620–1730 cm-1

Metal Carbonyl Complexes – Vibrations M-CO – effect of charge: s-donation strengthens M-C bond p-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: vco = 2143 cm-1 Mn(CO)6+: 2090 cm-1 Ni(CO)4: 2060 cm-1 Cr(CO)6: 2000 cm-1 Co(CO)4-: 1890 cm-1 V(CO)6-: 1860 cm-1 Fe(CO)42-: 1790 cm-1

Metal Carbonyl Complexes – Vibrations M-CO – effect of other ligands: s-donation strengthens M-C bond p-back donation strengthens M-C bond and weakens C≡O C≡O stretching frequency is reduced from value in free CO in LnM(CO)m complexes, weak p-acceptor ligands increase M  CO back-donation free CO: vco = 2143 cm-1 L: good p-acceptor Mo(CO)6: 2005 cm-1 (PF3)3Mo(CO)3: 2055, 2090 cm-1 (PCl3)3Mo(CO)3: 1991, 2040 cm-1 (P(OMe)3)3Mo(CO)3: 1888, 1977 cm-1 (CH3CN)3Mo(CO)3: 1783, 1915 cm-1 L: poor p-acceptor

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

Metal Carbonyl Complexes – Vibrations M-CO – symmetry of the molecule: octahedral M(CO)6 vCO 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 M-CO – symmetry of the molecule: cis-[M(CO)4Cl2] trans-[M(CO)4Cl2] vco: 1 IR 2 Raman no common bands – rule of mutual exclusion vco: 4 IR (1 very weak) 4 Raman (1 very weak) some common bands

Metal Carbonyl Complexes – Vibrations M-CO – symmetry of the molecule: fac-[M(CO)4Cl2] mer-[M(CO)4Cl2] vco: 3 IR (1 week) 3 Raman (1 week) some common bands vco: 2 IR (which overlap) 2 Raman (which overlap) some common bands

Molecular Orbitals for O2 2ps 2ps 2pp 2pp 2p 2s 2p 2p 2p 2pp 2ps 2s 2s 2s O O2 O O CO C JKB Lecture 5 slides 8-9

2H2(g) + O2(g)  2H2O(l) combH = -484 kJ mol-1 Spin-Triplet O2 O2 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”) 2H2(g) + O2(g)  2H2O(l) combH = -484 kJ mol-1 spin inhibited O H O H H-H O=O spin-selection rules prevents “spin-flip” transition in O2 being important so reaction is not initiated by sunlight initiation happens via a spark or a catalyst

4Fe2+ + O2 + 2H2O + 8OH-  4Fe(OH)3  2Fe2O3 + 6H2O O2 Transport Complexes Almost all reactions between O2 and metal complexes are irreversible: 4Fe2+ + O2 + 2H2O + 8OH-  4Fe(OH)3  2Fe2O3 + 6H2O Transport system for O2 in animals must: carry O2 in its ground state form (with two unpaired electrons) capture gas phase O2 transport it via the circulatory system release it completely to intermediate storage site Transport system for O2 in animals must: not react irreversibly with O2 be highly efficient and cope with changes in supply and demand have a lower affinity for O2 than the storage system Very important equation - one that you need to remember surely this should depend on the gas ? … No, as we’ll see soon

O2 Transport Complexes In humans, transport system (haemoglobin) and storage system (myoglobin) are both Fe(II) complexes: myoglobin haemoglobin affinity of myoglobin > affinity of haemoglobin affinity of haemoglobin increases as O2 pressure grows – cooperative effect Very important equation - one that you need to remember surely this should depend on the gas ? … No, as we’ll see soon muscle lungs

Haemoglobin and Myoglobin - Structures Haemoglobin consists of 4 haem groups, myoglobin consists of 1 haem group: distal histidine residue Very important equation - one that you need to remember surely this should depend on the gas ? … No, as we’ll see soon proximal histidine residue

Haemoglobin and Myoglobin - Function Unoxygenated protein contains high spin Fe(II) d6: distal histidine residue Oxygenated protein contains low spin Fe(III) d5 and O2-: Very important equation - one that you need to remember surely this should depend on the gas ? … No, as we’ll see soon Unpaired electron on Fe(III) is weakly coupled to unpaired electron on O2-: complex is diamagnetic proximal histidine residue

Haemoglobin and Myoglobin - Function weak H-bond? enforced bending distal histidine residue distal histidine residue Very important equation - one that you need to remember surely this should depend on the gas ? … No, as we’ll see soon proximal histidine residue proximal histidine residue partial prevention of (irreversible) CO attachment

Haemoglobin – Cooperative Effect proximal histidine residue Unoxygenated protein contain high spin Fe(II) d6: 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) d5 which fits into ring Very important equation - one that you need to remember surely this should depend on the gas ? … No, as we’ll see soon The motion of the proximal group is transferred through protein structure to the next deoxygenated haem group decreasing its activation energy for O2 attachment proximal histidine residue

Summary By now you should be able to.... Explain that metal-carbonyl bonding is due to synergic OC  M s-donation and M  CO p-back donation Explain that the reduction in vco stretching frequency is related to the extent of back-bonding Appreciate that the number of vCO in IR and Raman can be used to work out structure Explain that haemoglobin and myoglobin bind weakly to O2 allowing transport and storage of highly reactive molecule Next lecture N2 complexes and Metal-Metal bonding

Practice