_P.-Powell-auth.-Principles-of-Organometallic-Chemistry-Springer-Netherlands-1988

Cluster compounds
densations' do not normally occur in practice but are useful on paper as an aid to
correlating and sometimes to predicting structures. The principle is supported by
molecular orbita! calculations which take into account interactions between the
frontier orbi tais of the two combining fragments. The following rule results from
these considerations:
'The total electron count in a condensed polyhedron is equal to the sum of the
electron counts for the parent polyhedra minus the electron count characteristic
of the atom, pair of atoms, of face of atoms common to both polyhedra'. For
transition metal clusters which adhere to the 18-electron rule the characteristic
electron counts to be substracted are as follows: atom M (vertex sharing) 18; M­
M (edge sharing) 34; and M, (triangular face) 48. This rule is now applied in the
course of a brief account of some chemistry of osmium carbonyls. Osmium forms
an especially large number of neutra! and anionic cluster carbonyl complexes.
The triangular cluster Os,(C0) 12 is cleaved by halogens to give linear dihalides
p.e.c. 48
CO CO X
1/col/co l/co
OC-Os-Os-Os-CO
oc~ loc/loc/ 1
X OC CO
p.e.c. of Os 3 (COJ12 X2 =
(3x8) + (t2x2) + (2x1)= 50 or
(34+ 34-18)=50
The latter can be considered as arising from two binary groups (p.e.c. 34) linked
through one atom (p.e.c. 18).
When Os,(C0) 12 is heated in a sealed tube to 190°C, Os 6 (C0) 18 is formed,
together with Os 5 , Os 7 and Os 8 species (Fig. 11.11 ). The pentaosmium cluster
Os 5 (C0) 16 and the anion Os 5 (CO)~; which is derived from it by reduction possess
trigonal bipyramidal structures (p.e.c. 72). This figure can be viewed either as a
closo-deltahedron (14t + 2) oras a capped tetrahedron (2 tetrahedra sharing one
face; p.e.c. 60 + 60- 48). Os 5 (C0) 16 reacts with carbon monoxide to form
Os 5 (C0) 19 which has the fascinating 'bow-tie' arrangement offive metal atoms ali
lying in a plane.
In Os 6 (C0) 18 (p.e.c. 84), the main product from heating Os,(C0) 12 • two faces of
a central Os 4 tetrahedron are capped (3 x 60- 2 x 48). Reduction gives
octahedral Os 6 (CO)~; (p.e.c. 86, 14t + 2) and action of CO followed by tertiary
phosphite, Os 7 (C0) 17 L 4 , in which the planar bow-tie arrangement of osmium
atoms is extended stil! further.
11.6 Clusters and catalysis
Cluster complexes are being studied in the hope that they might provide novel
catalytic systems. In a mononuclear complex potential coordination sites must
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