inorganic chemistry

potentially photoreactive. It could be shown( 128 ) that only two
of those, the singlet metal-to-ligand^1 MLCT (dMn!pDAB) state
and the triplet sigma-bond-to-ligand^3 SBLCT (sMnH!pDAB)
state, define the photochemistry of the compound. While the
lower-lying triplet SBLCT (or mixed MLCT/LLCT) state should
induce a homolytic cleavage of the metal-hydride bond, visible
light excitation and population of the singlet MLCT state allow
an ultrafast direct dissociative process of the axial CO ligand
(Fig. 14), which is completed within 400–500 fs.

C.2. Controlling catalytic activity

The metabolic functions of living organisms are maintained by a
complex interplay of regulatory networks. Enzymatic activity and
gene expression are permanently adapted for an optimum perfor-
mance and may be completely switched on and off in a reversible
manner. Typical mechanisms involved in biological systems
include the stimulation and inhibition by control proteins or
metabolite molecules, allosteric interactions, proteolytic activa-
tion, redox transformations, and reversible covalent bond
modifications such as phosphorylation and dephosphorylation( 5 ).

2

6

5

4

3

2

3

4

5

Mn—H (Å)

Mn—COaxial (Å)

6

Energy (a.u.)

FIG. 14. Calculated potential energy surface of the^1 MLCT state of
Mn(H)(CO) 3 (DAB) as a function of the MnH and MnCO bond lengths
(DAB, 1,4-diaza-1,3-butadiene). Adapted from Ref. ( 128 ).

258 GÜNTHER KNÖR AND UWE MONKOWIUS