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Bonding in Molecules Lecture 4 d orbitals Bonding in Hypervalent Molecules XeF2 (‘hypervalent’ = 10 electrons on Xe) Possible explanations? sp3d hybridisation? Ionic resonance forms? Total bond order 4 (2 per C=O bond) Total bond order = 1.0 (0.5 per Xe-F bond) d orbitals improve bonding, but we can explain the stability of XeF2 without them They do not participate to the extent implied by a formal sp3d hybridisation model unrealistic more realistic SF6 sp3d2 hybrids? ionic resonance? S-F bond order = 4/6 MO diagram for octahedral AH6 E E E E Total bond order = 4 E =4/6 per A-H bond Non-bonding 1eg electrons localised entirely on H (so ligand has to be electronegative) allowing nd to participate in bonding Total bond order = 6 E =1 per A-H bond 1eg electrons delocalised over A and H d orbitals improve bonding, but we can explain the stability of SF6 without them They do not participate to the extent implied by a formal sp3d2 hybridisation model [C(AuPR3)6]2+ (an analogue of [CH6]2+): 8 valence electrons sp3d2 hybrids? ionic resonance? Very unlikely! 1eg filled 1eg empty Bonding in Transition Metal Complexes Main group: ns < np < nd Transition metal: nd < (n+1)s < (n+1)p S Cr metallic radius for Co = 125 pm (••••••••) rmax 145 pm 4πr2ψ2 rmax 40 pm Distance from nucleus Overlap with 3d is small (bonding dominated by overlap with 4s) Overlap improves down group so bond strengths increase 3d < 4d < 5d Contrast main group where bond strengths decrease 2p < 3p < 4p < 5p Overlap as a function of orbital size: A SAA A Cr Mo W O S Se orbital size σ-only MO diagram for ML6 Symmetry analysis σ ‘lone pairs’ on ligands σ-only MO diagram for ML6 E σ-only MO diagram for ML6 E σ-only MO diagram for ML6 E σ-only MO diagram for ML6 E σ-only MO diagram for ML6 E σ-only MO diagram for ML6 Note a1g very strongly antibonding. So unlike bare atoms, complexes never have electrons in ‘s’ orbitals: Cr is 4s13d5 but Cr in Cr(CO)6 is (t2g)6(eg)0(a1g)0 i.e. 4s03d6 E σ-only MO diagram for ML6 6 non-bonding electrons 12 bonding electrons (‘18 electron’ rule) E σ-only MO diagram for ML6 E Region we focus on in crystal/ligand field theory (∆O) High-spin/Low-spin (see Transition metals lecture course) High-spin if ∆O < spin pairing energy Low-spin is ∆O > spin pairing energy Typical values: 1st row TM P = 15000-25000 cm-1 2nd/3rd row TM P smaller What controls the magnitude of ∆o? the spectrochemical series ∆o ∆E 1 ∆o ∝ ∆E