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Encyclopedia of Physical Science and Technology EN003D-147 June 13, 2001 22:58

754 Coordination Compounds

TABLE VIII Coordination Numbers and Associated 2. The more polarizable the ligand, the lower is the
Shapes coordination number: [AlCl 4 ] , [AlF 6 ] .
−
3−
Coordination Point
number Shape group Examples For any one metal, the ions in increasingly positive
3+
oxidation states become smaller [i.e., rFe (spin free)
2 a Linear C ∞h [CuCl 2 ] − 5 2+ 6 9 10
+
2+
(3d ) < rFe (spin free) (d ); rCu (d ) < rCu (d )].
[Ag(NH 3 ) 2 ] +
The coordination number with a given ligand tends to
4 Tetrahedral T d [BeF 4 ] 2−
increase with this shrinkage, perhaps because the more
[CoCl 4 ] 2−
highly charged cation has much increased electron at-
[Zn(NH 3 ) 4 ] 2+
tachment enthaply, requiring more of the same ligands
2−b
Plannar D 4h PtCl 4
AuCl −b to become electroneutral. Examples (which abound) are
3−
−
−
2−
2−
3−
3−
4 TlCl , TlCl ; ICl , ICl ; CuF , CuF ; PtCl ,
ICl − 6 4 4 2 6 4 6
2+
4 PtCl ; SnCl , SnCl ; Ag(C 5 H 5 N) , Ag(C 5 H 5 N) ;
+
2−
2−
−
5 Pyramidal C 4v [OV(OH 2 ) 4 ] 2+ 4 6 − 3 4 2
(AuCl 4 ) , (AuCl 2 ) . In the cases of the electronic config-
−
Bipyramidal D 3h [InCl 5 ] 2− 8 6
urations d and d , for the same metal the interconversion
6 Octahedral O h [AlF 6 ] 3−
of one state to the other is often called oxidative addition
[RhCl 6 ] 3−
(or reductive elimination in the opposite direction):
a
On the whole, two coordination is found for the lower oxidation
MCl 2− + Cl 2 → MCl 2− (e.g., M = Pd, Pt) (76)
state (I) of the coinage metals (copper, gold, silver) and among their 4 6
isoelectronic neighbors [e.g., Hg(OH 2 ) 2 ) 2+ , Tl(OH 2 ) 3+ ].
2
b Note that these are isoelectronic (so the same structure is For the same ligand with varying metal ions there is
expected). (in crystalline binary compounds) a general tendency for
the coordination numbers of the cations to increase on
in simple coordination compounds varies (as it does in bi- going down a eutropic family (as in SiO 2 , GeO 2 , SnO 2 ,
nary and ternary crystalline structures) from 2 up to ∼12. PbO 2 ). This is not as true of isolated complexes in coor-
The most common (all others up to 12 are known) are 2, 4, dination compounds. The sizes of corresponding ions do
5, 6, and (for some of the larger ions—barium and radium; increase down the three transition series, but this increase
thorium; zirconium, hafnium, and some lanthanides and is often swamped by the sharing of the effect among sev-
actinides) 8. Table VIII gives examples of coordination eral ligands. For example, the bond lengths M ← N for
3−
numbers and the associated shapes. Co(NH 3 ) , Rh(NH 3 ) 3+ and Ir(NH 3 ) 3+ are sufficiently
6 6 6
There are many rules of thumb for rationalizing changes alike that many triads of their analogous salts, for example,
in coordination number for a particular metal and among [M(NH 3 ) 6 ](NO 3 ) 3 ·HONO 2 or [M(NH 3 ) 5 ](OH 2 )](NO 3 ) 3 ·
metals in general. For a given metal ion with a partic- HONO 2 , are isostructural for M = Co, Rh, Ir.
ular ligand in a particular solvent—usually water—such The growth in size of the s-block ions is well known, as
2+ ˚ 2+ ˚
changes are manifested by sudden discontinuities in prop- in rMg 2+ < rCa (1.06 A) < rSr (1.33 A) < rBa 2+ <
2+
2+
erties. With Hg in water, the successive stepwise stabil- rRa , and the coordination numbers with like ligands do
ity constants with chloride are K 110 > K 120 >>> K 130 ·· ·. tend to increase down these series. However, the metal
Whereas the first two chloride ligands attach to mercury ions [other than the very small ones of the higher oxida-
+ 0
very well, giving successively (HgCl) and (HgCl 2 ) , tion states, Mn(VII) and the like] of atomic number greater
the third one has little affinity. Presumably, the stable than 19 are all large enough to accommodate the higher
linear two-coordinated structure is being altered to a coordination numbers (8). For the electropositive ele-
−
three-coordinated HgCl (triangular) or four-coordinated ments at the start of the transition series, the lanthanide
3
n
−
Hg(OH 2 )Cl (tetrahedral). contraction ensures that (5d) ions are about the same
3
n
In general, if a particular metal ion in a particular oxi- size as their 4d congeners but both are larger than 3d n
4+
3+
dation state manifests, under different circumstances (i.e., (e.g., Ti 4+ < Zr 4+ Hf ;V 3+ < Nb 3+ Ta ;Cr 3+ <
3+
with a variety of ligands), more than one coordination Mo 3+ W ). Typical complex compounds contain
2−
4−
4−
number, the changes (based on the Pauling electroneutral- TiF , ZrF , and HfF .
6
8
8
ity principle) are as follows:
1. Anions have lower coordination numbers than
B. Structures and Their Symmetries
cations:
The coordination number 4 is fairly common, and there are
[CoCl 4 ] 2− versus [Co(OH 2 ) 6 ] 2+
two limiting shapes, as listed in Table VIII: the planar and
[MnCl 4 ] 2− versus [Mn(OH 2 ) 6 ] 2+ the tetrahedral. For main group metals, tetrahedral ions