Chemistry Reference
In-Depth Information
Table 4.1
Variation of ion size and average bond distances found for first-row d-block metal ions
in common oxidation states and with six-coordinate octahedral geometry. Entries for a second and
third row element, to exemplify differences down a column of the Periodic Table d block, appear in
italics.
d-Electron
configuration
Free ion
radius (pm)
Ty
pical
M
Ty
pical
M
Ty
pical
Metal ion
O (pm)
N (pm)
M
Cl (pm)
d
0
Sc(III)
74.5
210
—
245
d
0
Ti(IV)
60.5
195
210
230
Ti(III)
d
1
67
185
215
235
V(IV)
d
1
58
185
205
215
d
2
V(III)
64
215
225
235
d
3
Cr(III)
61.5
195
210
235
Mn(IV)
d
3
53
185
210
230
Cr(II)
d
4
80
200
215
245
d
4
Mn(III)
64.5
195
205
230
Mn(II)
d
5
83
215
240
250
d
5
Fe(III)
64.5
190
205
230
d
6
Fe(II)
78
205
215
240
Co(III)
d
6
61
185
195
225
Rh(III)
d
6
66.5
195
205
235
d
6
Ir(III)
68
210
215
240
d
7
Co(II)
74.5
205
220
240
Ni(II)
d
8
69
205
210
235
d
9
Cu(II)
73
200
200
225
d
10
Zn(II)
74
205
210
230
displays bond distances that vary slightly across a usually large number of ligand systems
examined, indicating that each assembly does have a preferred metal-donor distance.
4.3.2
Moulding a Relationship - Ligand Influences
If we focus next on the bound donor molecules or ions, there are also two of the four key
factors mentioned earlier that we can consider ligand-centric. These are:
ligand-ligand repulsion forces; and
ligand rigidity or geometry.
Obviously, each ligand is unique in its shape and size. The effect of repulsion between
ligands, which we can term
nonbonding interactions
, can usually be recognized readily in
the solid state from distortions in shape as defined by crystal structure analysis. Two PH
3
molecules may bind in a square planar shaped complex with little preference for
trans
over
cis
geometry, whereas two very bulky P(C
6
H
5
)
3
molecules may exhibit strong preference for
coordination in a
trans
geometry, where they are much further apart. Another way that two
cis
-disposed bulky ligands can relieve ligand-ligand repulsion is for the complex to undergo
distortion from square planar towards tetrahedral, which leads to the two ligands moving
further apart in space; this is also observable in the solid state and, from spectroscopic
behaviour, in solution. However, there are also weak structure-making effects arising from
favourable hydrogen bonding interactions that can assist in stabilizing a particular shape.
This might occur where a carboxylate group oxygen participates in a hydrogen bond with
