Page 113 - Academic Press Encyclopedia of Physical Science and Technology 3rd InOrganic Chemistry

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Encyclopedia of Physical Science and Technology EN005F-213 June 15, 2001 20:32

Electron Transfer Reactions 355

N 2 H 2 is isoelectronic with O 2 , with two half-filled may be effected in one step. The following reaction could
π ∗ orbitals that are isolobal with half-filled t 2g or- be an example:
bitals on the V(IV) atoms resulting from the elec-
tron transfer step. This complex, if it is formed, is Mn(IV) glucolate → 2Mn(II) gluconate + O 2 . (2)
2
too unstable to detect due to the reaction 2N 2 H 2 →
It occurs when Mn(IV) gluconate in 1.0M NaOH is
[HN NH NH NH] → N 2 + N 2 H 4 . Similar electron
brought to pH 12. A µ,µ-dioxo bridged dimer is proposed
donors to hydrazine (N 2 H 4 ) are oxalate ( O 2 C CO )
−
−
2 as the activated state:
and thio-oxalate [OSC CSO] , which, when oxidized,
2−
break the C–C bond, which forms the gases CO 2 and CSO.

Two electrons from each bridging O 2− may be transferred
to Mn(IV), which forms O 2 and two Mn(II) gluconate
complexes.
A multiple-electron acceptor of interest is dinitrogen
(N 2 ), which must accept six electrons to be converted to
ammonia. A dinuclear complex of Zr(II) containing three
ligated N 2 molecules reduces one N 2 to N 2 H 4 when added
to acid solution. The two-step mechanism with postulated
Copper ions have two t 2g orbitals with proper symmetry
electron transfers is illustrated below:
to interact with the π-orbitals in the planar oxalate ions,
which provides a conduction path from each oxalate to
two adjacent copper atoms. Mild heating causes smooth
evolution of CO 2 and deposit of extremely fine copper
crystals—a process used in the production of microelec-
tronic devices.

b. Single acceptor—Two donors.

A highly conducting π-bonding system including t 2g or-
bitals on the zirconium atoms provides easy pathways for
electrons impelled by the potential provided by proton
attack on a terminal nitrogen atom.
A Mo(0) complex containing two N 2 and four substi-
Two Cr 3+ become coordinated to octahedral IO 5− to
6 tuted phosphine ligands, when added to acid, will reduce
form an activated state for two one-electron transfers from
one N 2 to either N 2 H 4 or 2NH 3 , which suggests that at
Cr(III) to I(VII). Cr(IV) is unstable and disproportionates
least three orbital pathways exist for pairs of electrons on
to Cr(VI) and Cr(III).
Motofindtheirwaytonitrogenunderfavorablepotentials.
Of course, one could also readily imagine that three suc-
cessive two-electron transfers take place when the bound
2. Four-Electron and Six-Electron Transfers
nitrogen forms ammonia.
A multielectron process of great interest is the conversion Seemingly complicated activated states for electron
of water to oxygen in the natural process of photosynthesis transfer clearly arise when possibilities of direct produc-
in green plants. Study of the photosystem has revealed that tion of stable products with most favorable geometries
O 2 evolution maximizes on every fourth pulse of light, occur. A spectacular instance occurs in the oxidation of
which suggests that the half-reaction dimeric (MoOH) 4+ illustrated below. All oxidants give the
2
same stoichiometry and product distribution. Mo(VI) as
2H 2 O → O 2 + 4H + 4e − oxidant gives the same products by
+