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746 Coordination Compounds

2
TABLE II Equilibrium Constants K for Coordi- number 56, at the start of the third transition series) is 6s ,
nation of Hydroxide a 1
and the atom before it, cesium, has the 6s ground state.
Metal Ligand n x log K However, for cesium, the next electronic state lies not far
from the ground state, and indeed on compression, the
Pt 2,2-Bipyridyl 2 2 4.3
conductance of cesium changes sharply, as the d orbital

Pt 5,5 -DMB b 2 2 4.8 is squeezed below the s orbital. Barium shows a simi-
Pd 5,5 -DMB b 2 2 5.5 lar transition with pressure, corresponding to a change in

Pd HDMG c 2 0 5.5 2 1 1
configuration from 6s to 6s 5d . In view of the angled
a Equivalent to r =−1in β pqr : hybrids (s ∓ d) given by this configuration, the angular
(bent) structures as monomers in the vapor phase of MX 2
−
(ML n ) x+ + OH → [(ML n )OH] (x−1)+
for M = Ca, X = F; M = Sr, X = Cl, F; M = Ba, X = F,
b DMB, Dimethyl-2,2 -bipyridyl. Cl, Br, I are examples of transition metal chemistry. In

c HDMG, “Dimethylglyoxime,” 2,3-butanedionedi-
the same way, although the ground states of the atoms of
oxime. Equation (35) shows the structure of the analo-
2
3
8
2
vanadium and nickel are 4s 3d and 4s 3d , the shrinkage
gous [Ni(DMG) 2 ].
on ionization (which can be regarded as equivalent to the
effect of huge pressure) squeezes the 3d orbitals to lower
8-hydroxyquinoline (17)or H-oxinate, displaces some energies than the 4s, so that the ground states of the ions
8
3
protons, causing a fall in pH. Conversely, the addition of are 3d and 3d , respectively.
n
protonic acids to metal complexes such as 18 will reverse The concept that isoelectronic d configurations have
this formation and cause the dissociation of the coordinate related properties is most valuable. In particular, such
bond.Onecommonwayofmeasuringstabilityconstantsis properties as color, magnetism, and rates of chemical
to set up such competitive equilibria as shown in Eq. (41): reaction, which depend rather directly on numbers of
d-electrons, can be rationalized and predicted. For exam-
2HL + M 2+ + (41)
ML 2 + 2H
ple, the metal ions whose chiral coordination ions have
For example, enough kinetic inertness to be separated (resolved) into
long-lasting enantiomers most commonly have six d elec-
2H 3 NCH 2 COO + Cu 2+ M(H 2 NCH 2 COO) 2 + 2H + trons (n d configurations).

6
(42)
This is done in the presence of concentrations of metal ion
from zero to levels comparable with those of ligands. A. Splitting Diagrams for
−
For aqueous equilibria, where the species M ,L , and Octahedral Coordination
n−
OH are present, the utility of K pqr is clear. Coordination
−
When six ligands surround a metal ion to give octahedral
compounds may contain protons (particularly on poly-
coordination, the situation for two of the five d orbitals
dentate ligands). Values of r (0 in examples so far) rep-
of the metal is shown in Fig. 2. These two orbitals are
resent the involvement of protons in complex formation.
representative of others like themselves.
Of course, when r is negative, this may arise from loss of
a proton from somewhere in the coordination species or,
+ −1
because [H ][OH ] = K w , that is, [OH ] = K w [H ] ,it 1. The d yz orbital is typical of those that “point be-
−
+
−
may signify the gain of a hydroxide, as in the examples in tween” the axes (defined by the six ligands): These three
Table II.
III. ELECTRONIC CONFIGURATIONS
There are some 70 metallic elements. All metal ions
(Lewis acids) form coordination compounds. At present,
the coordination compounds of the 27 transition metals
are the most widely studied and applied, and this sec-
tion refers to them. In the periodic table, at the onset of
each of the transition series, the energies of the n s, n
p, and (n − 1) d [or (n − 2) f if appropriate] orbitals are
so close that they are made to interchange fairly readily.
FIGURE 2 Differing spatial distribution of the orbitals (a) d yz
For example, the ground state of the barium atom (atomic (between axes) and (b) d 2 −y 2 (along axis).
x