Page 275 - Academic Press Encyclopedia of Physical Science and Technology 3rd InOrganic Chemistry

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Encyclopedia of Physical Science and Technology EN009M-428 July 18, 2001 1:6

532 Metal Particles and Cluster Compounds

short distance is that for the Cr Cr quadruple bond in by a rotation about the metal–metal bond without a serious
Li 6 [Cr 2 (σ-C 6 H 4 O) 4 ]Br 2 . From the spectroscopic data loss in δ bonding.
of bare Cr Cr dimers, studied under matrix isolation One further tool for establishing the presence and order
˚
condition, an even shorter Cr Cr distance of 1.71 A has of metal–metal bonds is theoretical analysis. Such analysis
10 2
been inferred. This nonligated dimer has a d s electron has provided results which are consistent with the simple d
configuration, hence, a sextuple bond has been postulated. orbital-d-orbital mixing ideas which lead to the one σ two
The remarkably short Cr Cr distance seems congruent π, and one δ bond. The energy of the bonding orbitals is
with a sextuple bond. The Mo Mo single bond in inversely proportional to the degree of atomic orbital over-
(C 5 H 5 )(CO) 3 Mo Mo(CO) 3 (C 5 H 5 ) has a bond distance lap, the opposite being true for the antibonding orbitals.
˚
of 3.21 A and is the longest distance between two metal The energy ordering scheme up through the σ antibond-
atoms which are considered to have a formal metal–metal ing orbital is shown in Fig. 14. Since metal–metal bonding
bond. In general, if two metal atoms which are not bridged essentially occurs through the d-orbitals, the d orbital oc-
˚
byanyligandscomewithin3.2 Aofeachothertheymaybe cupancy, or the oxidation state of the metals, necessarily
considered to have a metal–metal bond. The restriction of dictates the upper limit of the bond order which may be
this generalization to nonbridged metal atoms is important obtained. Filling the orbitals of the energy diagram clearly
4
4
as bridges may have a considerable effect on the metal– shows that the eight electrons of a d –d system, such as
2−
metal distance. The specific effect that a bridging ligand [Re 2 Cl 8 ] can produce a metal–metal quadruple bond.
will have on a metal–metal bond depends on what ligand is The isoelectronic Mo(II)–Mo(II) compound [Mo 2 Cl 8 ] 4−
acting as the bridge. Bridging hydride ligands, for exam- has also been structurally characterized. The Mo Mo
˚
˚
ple, are generally associated with metal–metal bonds that quadruple bond distance of 2.138 A is about 0.1 A shorter
are longer than similar nonbridged bonds. Carbonyl lig- than that of the Re Re quadruple bond. With all the ligand
ands, on the other hand, have the opposite effect, they tend environment of the metal being identical in these two com-
to shorten the metal–metal bonds they bridge. When com- pounds the difference in the metal–metal bond length may
paring bond distances we must be careful to consider what be wholly attributed to the inherent differences of each
type of ligands are present in the molecules. So that ligand metal.
consideration might be diminished it would be advanta- Another type of quadruply bound metal dimer is found
geous to have a complete series of metal–metal bonded in compounds with the general formula M 2 (L 2 CR) 4 where
dimers with the same ligands all of which are coordinated M = Cr, Mo, W and L = O, S, N. One example is
˚
in the same fashion. This, however, is not available so bond Mo 2 (O 2 CH) 4 , with a Mo—Mo distance of 2.091(2) A. In
distances alone do not provide sufficient information for a these compounds the metal–metal bond is bridged by four
goodcorrelationtobemadebetweenbondlengthandbond uninegative, bidentate ligands giving a paddlewheel ge-
order. ometry to the compounds (Fig. 15). In such compounds
Another structural feature of a metal dimer which the configurational requirements of the ligands as well as
may imply the presence of multiple metal–metal bond- the presence of a δ bond confine these molecules to an
ing is the stereochemical configuration of the molecule. eclipsed configuration. As well as restricting the rotation
2−
In [Re 2 Cl 8 ] , the compound for which the first quadruple about the metal–metal bond these bidentate ligands can
bond was proposed, the effects of the multiple bond on the influence the metal–metal bond distance. Variations in the
stereochemical configuration are clearly seen; [Re 2 Cl 8 ] 2−
has no bridging chloride ligands. As such, we might ex-
pect this compound to adopt a staggered configuration;
however, this is not the case. An eclipsed configuration
is maintained despite the fact that the short Re Re bond
˚
of 2.24 A brings the chloride ligands closer together than
the sum of their van der Waals radii. This seemingly high
energy configuration is necessitated by the presence of a
δ bond as it is only in this configuration that a full δ bond
◦
will remain intact. A 45 rotation about the Re Re bond
giving the staggered conformation, would result in zero
overlap between the two d xy -orbitals and total destruction
of the δ bond. The δ bond, however, is quite tolerant to ro-
◦
tation about the metal–metal bond. Even at a 30 rotation,
50% of the δ bond overlap remains. Therefore, quadruply FIGURE 15 Structure of Mo 2 (O 2 CH) 4 . Note the paddle-wheel
bonded metal dimers may alleviate some steric congestion geometry typical of carboxylate-bridged metal dimers.