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

Cluster compounds
(IJ 5 -C,B,H 6 )Mn(CO),. (IJ 5 -C 2 B 4 H 6 )Fe(CO), and (IJ 5 -CB 5 H 6 )Co(CO),. ali of which
have the closo-pentagonal bipyramidal structure with a p.e.c. of 40.
The frontier orbitals of Fe(CO), and of CoCp are isolobal with those of B-H
(p. 207). They consist of a radial hybrid directed to the centre of the polyhedron
and two tangential orbitals at right angles to each other. In the Fe(CO), fragment
(or CoCp fragment) these orbitals, as in B-H. contain two valence electrons.
Fe(CO), can therefore replace B-H in deltahedral clusters. Mn(CO), provides one
less valence electron than Fe(CO),. so that o ne extra electron is required from the
remaining cluster atoms. With Co(CO), one electron less is needed for an
isoelectronic arrangement.
Il. 5 Transition metal clusters
Many cluster molecules are formed by transition metal atoms which are
coordinated to n-acid ligands, in particular carbon monoxide, cyclopentadienyl
and phosphines. Under these circumstances the 18-electron rule should be
obeyed so that a transition metal fragment has ten more valence electrons than
the corresponding isolobal Main Group fragment. This relationship is exemplified
in Table 11.3.
The 17-electron fragments such as Co(C0) 4 , Mn(C0) 5 or CpNi(CO) form the
dimers Co 2 (C0) 8 , Mn 2 (C0) 10 or CpNi(C0) 2 in which the total p.e.c. is 34. A
carbonyl group normally contributes two electrons to the p.e.c .. irrespective ofits
mode of bonding-bridging or terminal. Rings arise from 16-electron fragments
e.g. Fe,(C0) 12 , Cp,Co,(CO), (p.e.c. 48) and Cp 2Mn 2 (C0) 4 (ţi-CH 2 ) (p.e.c. = 38;
16t + 6 m = 38). Electron-precise tetrahedral clusters (p.e.c. 60) are exemplified
by Co 4 (C0) 12 and [CpFe(C0)] 4 , as well as by mixed clusters such as Co,CH(C0) 9
(p.e.c. 50; 15t + 5 m, t = 3, m = 1). The 15t p.e.c. is also consistent with the
structure ofRh 6 C(CO)~; (p.e.c. 90) which has a carbon atom encapsulated in the
centre of a prisma tic cluster. The incorporation oftransition metal fragments into
borane and carborane deltahedra is discussed above. Closo, nido and arachno
deltahedral clusters made up entirely from transition metal atoms are also
common. The octahedral cobalt carbonyl Co 6 (C0) 16 belongs to this class. Some
carbido-iron carbonyls illustrate some ofthese structural relationships as well as
providing novel chemistry (Fig. 11.10). Reduction ofFe(C0) 5 by strong reducing
agents such as Fe(CO)~- or Mn(CO); affords the cluster anion [Fe 6 (C0) 13 (ţL­
CO), (ţL 6 -C) ] 2 -. in the middle of which a carbide carbon, derived from a carbonyl
ligand ofFe(C0) 5 , is encapsulated. Subsequent oxidation with C 7 H;BF; removes
a vertex Fe(CO), group to give a nido octahedral cluster (p.e.c. 74) which on
further reduction and protonation gives a butterfly arachno octahedral derivative
(p.e.c. 62) in which there is a bridging CH group with an 'agostic' hydrogen. This
reaction sequence shows the conversion, albeit by a very involved route, or
coordinated CO to methane.
The structures of large transition metal clusters can be derived by bringing
together smaller triangular, tetrahedral and octahedral fragments. These 'con-
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