coordination number and geometry

coordination sphere

transition metals form bonds with multiple ligands to form a coordination sphere

what can coordination spheres consist of

different numbers and geometries of ligands depending on the metal ion and size/shape of the ligands

what does ligand-ligand repulsion tend to do

limit the number of bonds made

CN 2

known as linear with 180 degree angle

normally seen for monovalent d10 metals

example compounds

tollens’ reagent, sodium aurothiosulphate and merbromin

CN = 3

a rare coordination number and usually due to bulky ligands causing steric hinderance around the metal centre - all known are usually trigonal planar

intermediate coordination CN 4

2 major geometries: tetrahedral and square planar

tetrahedral (Td)

compounds like this are fairly common

Td is a cubic group with each of the 4 ligands on the opposite corner of a square on each face

Td ions and compounds 2 classes

• small, highly charged metal

• larger ligand, lower metal charge

square planar

fairly uncommon and found usually only in d8 metal ions

CN = 5

trigonal bipyramidal and square-based pyramidal

trigonal pyramidal

found for a number of metals with different dn configurations

square based pyramidal

Tetradentate ligands can induce sbp geometry in 3d metals, but some metal ions such as V, Mo and W in high oxidation states can be stable as sbp

CN = 6

octahedral and trigonal prismatic

octahedral

the most common geometry for transition metals - a cubic group

where does Oh coordination stem from

the interaction of the ligand lone pairs with the dx^2-y^2 and dz^2 orbitals

octahedral distortions

tetragonal, rhombic and trigonal

tetragonal

distortion along one L-M-L axis only

can be compressed or elongated - common in d4 and d9

rhombic

compression in one axis and elongation another

trigonal

elongation vector is between 2 axes

trigonal prismatic

Uncommon and usually driven by ligand shape and sterics. This occurs in later transition metals, especially W and Mo

high coordindation

CN 7 and 8

CN 7

pentagonal bipyramid, capped octahedron and capped trigonal prism

CN 8

cubic, square antiprism and dodecahedral

where are CN 7 and 8 common

lanthanides and later transition metals

pentagonal

a plane of 5 M-L and 2 axial M-L bonds

capped octahedron

an octahedron with an additional ligand placed on one of the triangular faces - not common in transition metals but seen in lanthanides

capped trigonal prismatic

as before, but an extra ligand on one of the square faces of the trigonal prism

cubic

each ligand sits at the corner of a cube with the metal in the centre

square antiprism

the form with the highest separation of ligands and is more favourable than cubic

dodecahedral

Can be seen as pentagonal bipyramidal with one planar ligand split above and below the plane