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 Instructor: Prof. Dianne Xiao TA: Leo Porter-Zasada Due: 5 PM, Friday, December 11 1) The following compounds are listed in the order of decreasing ligand exchange rates. The observed rate constants are listed below each compound. [Mn(OH2)6] 2+ [Fe(OH2)6] 3+ [V(OH2)6] 2+ [Cr(OH2)6] 3+ kexchange (s–1 ) 2.1x107 1.6x102 8.7x101 2.4x10–6 a. Why does [Fe(OH2)6] 3+ undergo faster ligand exchange than [V(OH2)6] 2+? Fe3+ is d5 , HS, and has two electrons in �* antibonding orbitals. V2+ is d3 with no electrons in antibonding orbitals, so the V–OH2 bonds are stronger and much slower to dissociate. b. Why does [V(OH2)6] 2+ undergo faster ligand exchange than [Cr(OH2)6] 3+? Neither has electrons in �* antibonding orbitals, and both are d3 . However, Cr3+ is in a higher oxidation state than V2+, so H2O dissociates more slowly due to a greater electrostatic attraction with the nucleus. c. Why does [Fe(OH2)6] 3+ undergo ligand exchange nearly four orders of magnitude faster than [Ru(OH2)6] 3+? Fe3+ is d5 high-spin and has two electrons in �* antibonding orbitals, while Ru3+ is d5 low-spin because it is a 2nd row transition metal (better spatial overlap with ligands, larger ∆!). Ru3+ has no electrons in �*, and the stronger Ru–OH2 bonds lead to slower ligand dissociation.