Page 114 - Academic Press Encyclopedia of Physical Science and Technology 3rd InOrganic Chemistry

P1: GPB/GJP P2: FYK Final Pages
Encyclopedia of Physical Science and Technology EN005F-213 June 15, 2001 20:32

356 Electron Transfer Reactions

+
6MoO 2− + 3(MoOH) 4+ + 14H →
4 2
3Mo 2 O 2+ + 2Mo 3 O 4+ + 10H 2 O
4 4
with Mo(VI) atoms equally distributed in the prod-
ucts. Evidently, (MoOH) 4+ on giving up electrons forms
2
oligomers. The first that is able to break up into only sta-
ble products has 12 molybdenum atoms with an average
oxidation state of 4.5.

FIGURE 3 Oxidation state–potential diagrams for vanadium and
chromium [1.0M H (aq)].
+

tance of kinetic control over the course of multistep pro-
cesses. One-electron reducing agents produce NO, which
escapes as a gas from an open system, before nitrogen is
VII. PATHWAYS AMONG reduced to oxidation number zero. Two-electron reduc-
+
+
OXIDATION STATES ing agents (in excess) produce N 2 O, NH 3 OH , and NH ,
4
bypassing all states that require an odd number of elec-
−
Oxidation state–potential diagrams for nonmetallic and trons to be transferred to NO . HNO, which is isoelec-
3
transition metal elements provide an interesting frame- tronic with O 2 , is apparently an active intermediate in all
work for analyzing the highly varied results obtained for two-electron reductions of nitric acid. Most product pat-
redox reactions involving as many as nine oxidation states. terns can be rationalized by postulating traps for HNO as
So many different products and stoichiometries are ob- follows:
tained from the reduction of nitric acid that early work
seeking patterns of reaction was abandoned after many HNO + HNO → HONNOH → N 2 O + H 2 O
years of frustrating effort.
(dimerization, no electron transfer).
Figures 3 and 4 show diagrams for the transition metals
vanadium and chromium and for the nonmetal nitrogen, Reducing agents:
all in 1.0M aqueous acid solution. The slopes of lines 2−
¨
¨
−
joining redox couples represent the potentials for the half- HNO+ :SnCl → Sn(IV) + H N O:
3
¨
¨
reactionsinquestion,apositivesloperepresentingareduc- 2−
¨
¨
+
H N O: + 3H → H 3 NOH +
tion potential and a negative slope an oxidation potential. ¨ ¨
An intermediate state above tie lines joining higher and −
(direct 2e transfer)
lower states will be unstable to disproportionation [e.g.,
¨
Cr(V) and NO 2 ] while species below such tie lines are sta- HNO + Mo(IV) → [Mo N O H]‡ →
¨
ble [e.g., Cr(III) and N 2 ]. The diagrams are a convenient 2−
¨
¨
−
way to postulate the feasibility of various possible one- Mo(VI) + :N O H (direct 2e transfer)
¨ ¨
and two-electron pathways in multistep redox processes
involving a particular element. The last case makes possible transfer of two unpaired t 2g
electrons from Mo(IV) by forming a triple bond in the
−
activated state using the π orbitals on NO , which are
∗
A. Reduction of Nitric Acid
isolobal with two t 2g orbitals on Mo(IV). [Under sim-
It is interesting to note that N 2 is seldom observed dur- −
ilar circumstances Mo(IV) reduces two ClO to ·ClO 2
3
ing reductions of nitric acid, which shows the impor- by separate one-electron transfers from the orthogonal