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752 Coordination Compounds

ions chromium(III) and rhodium(III) with ligands capa- Certainly, the presence of an imine-like carbon atom
ble of forming conjugate bases do not seem to exhibit this adjacent to the coordinated nitrogen seems necessary for
behavior. values of k 1 in rate equations (67) or (68) to be large.
3. Base hydrolysis (and in a very few cases attack 4. The unusually rapid coordination to initially inert
y−
by cyanide or other nucleophiles) on a number of co- aquo metal ions of a few oxo anions (XO ny ), for exam-
−
ordination compounds with the ligand C N. Often, the ple, X = N, n = 2, y = 1 for ONO , may involve a rate-
ligand is part of an aromatic ring. For instance, for all controlling reaction at the oxygen of water (path 29) rather
the base hydrolyses such as Eq. (66) (substitution of an than the metal (path 30).

N-heterocyclic ligand LL, usually 2,2 -bipyridyl or 1,10-
phenanthroline, by hydroxide), although there can obvi-
ously be no conjugate base formed by protonic dissoci-
ation (there are no acidic protons) the rate equations are
nevertheless as in Eq. (67):
M(LL) n+ + OH → [M(LL) 2 (OH)(OH 2 )] (n−1)+ + LL
−
3
(66)
− 2
−
Rate = [M(LL) 3 ] k 0 + k 1 [OH ] + k 2 [OH ] (67)
In a similar way, the rate equations for substitution by
cyanide ion are as in Eq. (68): H. Catalysis
− 2
Rate = [M(LL) 3 ] k 0 + k 1 [CN ] + k 2 [CN ] (68) If one defines a catalyst as “a species whose activity ap-
−
In the latter case, a typical example is the ready reaction of pears to a higher power in the rate equation than in the
ferroin with cyanide in water to give the Schilt–Barbieri stoichiometric equation,” many kinds of transformation
compound: of coordination compounds may be subject to catalysis.
For example, the replacement of fluoride in Eq. (70) has
−
[Fe(phen) 3 ] 2+ + 2CN → [Fe(phen) 2 (CN) 2 ] + phen
the stoichiometric and rate equations given in Eqs. (71)
(69)
and (72), respectively. The proton is said to be a catalyst,
The most reasonable interpretation (there have been probably through the intermediate compound (31) with
many) is to consider the hydroxide or cyanide as form- hydrogen fluoride as ligands:
3
ing first an sp -hybridized carbon atom (a pseudobase or
[Co(NH 3 ) 5 F] 2+ + H 2 O → [Co(NH 3 ) 5 (OH 2 )] 3+ + F −
Reissert-type adduct, respectively) and then being trans-
mittedfromcarbontometalion.Inotherwords,thechange (70)
inreactivityofanN-heterocycleoncoordinationtoametal
[Co(NH 3 ) 5 (OH 2 )][F]
ion is akin to that of the same N-heterocycle on classical K = (71)
quaternization by an organic agent such as methyl iodide. [Co(NH 3 ) 5 F]
The unusual rate equation [Eq. (67) or (68)] involving Rate = k[Co(NH 3 ) 5 F][H ] (72)
+
the nucleophile’s concentration in first- and second-order
Other ligands that are the conjugate bases of weak
terms arises because the rates of these reactions (appar-
−
Br¨onsted acids (e.g., NO ) show similar catalysis by a
entlyhydrolysisorsubstitutionbycyanideatthemetalion) 2
proton. In the case of coordinated nitrite, the proposed
are actually controlled by rates of reaction at the ligand
protonated intermediates (32) may actually be isolated in
(27 → 28).
solid salts [here the nitrate [Co(NH 3 ) 5 (HONO)](NO 3 ) 3 ].
In both 31 and 32. A represents the ligand NH 3 .

Genesis of the famous Zeise’s salt. K[Pt(C 2 H 4 )Cl 3 ],
made by the slow reaction of potassium tetrachloroplati-
nate(II) with ethene in water (for ∼5 days) is dramatically