Coordination And Transition Metal Chemistry Flashcards
coordination complexes
compounds formed by lewis acid-base interactions, a coordinate covalent vond
complexes
metal atom or ion bonded to ligands. They don't dissociate into ions, they act as a single molecule of ion
coordination geometries for transition metals
#4 -- tetrahedral and square planar#6--octahedral
common ligands
- all ligands donate a pair of electrons to an empty elemental orbital of sigma-symmetry- some also pi-backbond
pi-backbond
- accept transition metal d-electron density, typically into empty ligand antibonding orbitals- filled metal pi-symmetry orbital donates into empty pi-LUMO of ligand (frequently antibonding within the ligand)
k (kappa) notation3
- denotes atom by which ambidentate ligand bond to M, or the non-contiguous atoms of a monodentate ligand
macrocyclic effect
combined multidentate chelate effect and pre-organization of macrocycle
innocent ligands
ligand that coordinated (spectator ligand) and does not directly attack metal center
non-innocent
ligand that reacts with the metal center
sigma-donor ligands
- halides, oxo, hydroxo, NH3, OH2, CH3, S, RO-, amine, amido
pi-donor ligands
- NO, CO, C2H4, CNR, C5H5 -
3 factors of coordination numbers
1. central atom/ion size; specific for a given metal in a particular formal oxidation state2. ligand-ligand steric interactions3. central atom/ion-ligand electronic interactions
chelate ligands
- bidentate, polydentate ligands are chelating agents
chelate effect
- increased thermodynamic stability, positive reaction entropy in chelate ligand substitution of monodentate ligands, and larger rate of formation for a meta complex because of tethered free end of mono-coordinated chelate
transition metal properties
- partially filled d orbitals --> multiple oxidation states- s-orbitals are first filled, except for Cr, Cu, but first emptied for cation- unpaired electrons ==> paramagnetic (magnetism)- colors from light absorption, excitation of electron to higher energy d-orbitals- coordination compounds with 4-9 ligands
why use carbonyl ligands in compound with negative oxidation states?
pi-backbonding stabilized negative oxidation states
electrostatic model of crystal fields & d-orbitals
- electron-electron repulsion between ligands and d-orbitals is energetically unfavorable- energy gap betwwen the two orbital sets depends on M and to a larger extend, on the ligand
ligand effect on d-orbital splitting
- small energy gap, less E to promote electrons into higher E orbital than to pair them -- high spin- large energy gap, the E cost is lowest when electrons are paired in lower energy orbitals--low spin
ligand field stablization
energy gained by placing electrons in lower energy orbitals
labile
complexes that undergo rapid ligand exchange (kinetically)
inert
slow exchange of ligands (kinetically)
d-orbital splittings in Td and D4h ligand fields
- ligands affect dxy, dxz, dyz orbitals on diagonals; dx2y2 and dz2 are along the z and x, y axes, so they are less perturbed- common for high oxidation state early transition metals and 1st row late transition metals- ligands affect dx2-y2 more and affect dxz, dyz least because of nodes in xy place
thermodynamic stability
- propertry of a compound with a large formation constant (Kf) and thus a large negatice Gf. - these compounds want to form
kinetic stability
- tendency to exchange ligands very quickly in solution- the ligands come on and off very rapidly
photochemistry
- characterized by the absorption of a photon of light followed by a chemical reaction (often electron transfer)