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Coordination Chemistry Bonding in transition-metal complexes Crystal field theory: an electrostatic model - + - - The metal ion will be positive and therefore attract the negatively charged ligands But there are electrons in the metal orbitals, which will experience repulsions with the negatively charged ligands Ligand/d orbital interactions Orbitals point at ligands (maximum repulsion) Orbitals point between ligands (less pronounced repulsion) The two effects of the crystal field Splitting of d orbitals in an octahedral field eg 3/5 Do Do 2/5 Do t2g Do is the crystal field splitting E(t2g) = -0.4Do x 3 = -1.2Do E(eg) = +0.6Do x 2 = +1.2Do The magnitude of the splitting (ligand effect) Strong field Weak field The spectrochemical series CO, CN- > phen > NO2- > en > NH3 > NCS- > H2O > F- > RCO2- > OH- > Cl- > Br- > I- The magnitude of the splitting (metal ion effect) Strong field Weak field D increases with increasing formal charge on the metal ion D increases on going down the periodic table Placing electrons in d orbitals Strong field Weak field Strong field Weak field d1 d2 d3 d4 When the 4th electron is assigned it will either go into the higher energy eg orbital at an energy cost of D0 or be paired at an energy cost of P, the pairing energy. d4 Strong field = Low spin (2 unpaired) Weak field = High spin (4 unpaired) P < Do P > Do Notes: the pairing energy, P, is made up of two parts. 1) Coulombic repulsion energy caused by having two electrons in same orbital Pairing Energy, P The pairing energy, P, is made up of two parts. 1) Coulombic repulsion energy caused by having two electrons in same orbital. Destabilizing energy contribution of Pc for each doubly occupied orbital. 2) Exchange stabilizing energy for each pair of electrons having the same spin and same energy. Stabilizing contribution of Pe for each pair having same spin and same energy P = sum of all Pc and Pe interactions Placing electrons in d orbitals d5 1 u.e. 5 u.e. d6 0 u.e. 4 u.e. d8 2 u.e. 2 u.e. d7 1 u.e. 3 u.e. d9 1 u.e. 1 u.e. d10 0 u.e. 0 u.e. Positive favors high spin. Neg favors low spin. Spectrochemical Series Purely s ligands: D: en > NH3 (order of proton basicity) donating which decreases splitting and causes high spin: D: H2O > F > RCO2 > OH > Cl > Br > I (also proton basicity) Adding in water, hydroxide and carboxylate D: H2O > F > RCO2 > OH > Cl > Br > I accepting ligands increase splitting and may be low spin D: CO, CN-, > phenanthroline > NO2- > NCS- Splitting of d orbitals in a tetrahedral field t2 Dt e Dt = 4/9Do Always weak field (high spin) A crystal-field aproach: from octahedral to tetrahedral L L L M L L M L L L L L Less repulsions along the axes where ligands are missing Magnetic properties of metal complexes Diamagnetic complexes very small repulsive interaction with external magnetic field no unpaired electrons Paramagnetic complexes attractive interaction with external magnetic field some unpaired electrons s n(n 2) Measured magnetic moments include contributions from both spin and orbital spin. In the first transition series complexes the orbital contribution is small and usually ignored.