Page 276 - Academic Press Encyclopedia of Physical Science and Technology 3rd InOrganic Chemistry

P1: GNH Final Pages
Encyclopedia of Physical Science and Technology EN009M-428 July 18, 2001 1:6

Metal Particles and Cluster Compounds 533

electronic properties of these ligands can be achieved by π orbitals leaves only the σ bond intact. Rh 2 (O 2 CCH 3 ) 4
∗
7
changing the heteroatom donor from O to N or S or by with its two d Rh(II) atoms has the single bond electron
2
∗4
4 2 ∗2
changing the R group in the (L 2 CR) ligands. The R group configuration σ π δ δ π . The Rh—Rh single bond
˚
can also play an important role sterically. Certainly the hy- distance is 2.386 A.
drogen atom of the (O 2 CH) ligand is stereochemically in- All of these examples have been cases in which all of
active but R groups as large as 2-phenylphenyl have been the valence d electrons on each metal atom have paired
utilized. In such compounds the steric demands of the in either bonding or antibonding orbitals. For zero-valent
ligand should be considered. In [M 2 L 8 ] n− or [M 2 (LL) 4 )] metal carbonyl dimers only one of the many valence
systems the metal atoms are one ligand shy of being oc- d electrons on each metal is shared; hence, only single
tahedrally coordinated. The vacant coordination sites lie bonds are known for these systems. The reason why the
along the z axis opposing the metal–metal bond. Coor- earlier transition metals in nonzero valent oxidation states
dination does occur at these sites. The sensitivity of the form up to quadruple bonds and the later zero-valent tran-
metal–metalbonddistancetothiscoordinationvariesfrom sition metal carbonyls form only single bonds can be un-
metal to metal. Chromium dimers, which show the great- derstood by considering the electronic needs of these dif-
est tendency to accept axial ligands, are the most sensitive. ferent metal centers.
Some Cr dimers have been observed to increase the Cr Cr The coordination about a transition metal and the chem-
distance by ∼42% upon the axial coordination of pyridine. istry of transition metal complexes is often dictated by the
There are two ways of forming a M M triple bond. The electronic needs of the metal center. The stabilization of
4
2
triple bond electron configuration of σ π is provided by compounds when the elements present acquire a closed-
3
3
a d –d system. This situation exists in M 2 L 6 compounds shell configuration is well known. For a transition metal,
when M = Mo, W and L = NR 2 , OR, X. One example with its nine valence orbitals (one s, five d, and three
is Mo 2 (NMe 2 ) 6 , with a Mo Mo triple bond distance of p orbitals), the stabilizing closed-shell configuration is
˚
5
5
2.21 A. A triple bond can be obtained from a d –d system obtained when the electron count about the metal center
as well. This involves metal–metal bond order reduction reaches 18. When a metal center has fewer than 18 elec-
via the population of antibonding orbitals. Triple bonds trons in its valence shell it is said to be electronically unsat-
4 2 ∗2
2
of this sort have an electron configuration of σ π δ δ . urated. One step toward alleviating this electronic unsatu-
Completely filling the δ orbital annihilates the δ bond. A ration is the formation of metal–metal bonds. Because of
∗
∗
half-filled δ orbital would result in a metal–metal bond their covalent nature, metal–metal single bonds are con-
with an order of 3.5. Thus, [Re 2 Cl 8 ] 2− undergoes one- sidered to add one electron to the valence shell of each
3−
electron reduction to [Re 2 Cl 8 ] . The trianion will have metal atom. The electron donation from one metal atom
2
4 2 ∗1
the 3.5 bond order electron configuration σ π δ δ . Re- to the other mirrors the bond order so a quadruple bond in-
ducing the δ bond contribution to the overall bond order creases the valence count of each metal by four electrons.
to one-half should loosen the grip the δ bond has on the Consider the carbonyl complexes of Fe, Co, and Ni.
8
eclipsed configuration. The trianion is electrochemically Iron, a d metal, requires ten more electrons to reach the
generated in solution and has a very short lifetime. There- eighteen required for stabilization. The coordination of
fore, structural studies have not verified this expectation. five CO ligands, each of which donates two electrons,
The metal–metal bond length would also be expected to would satisfy this electronic requirement. Nickel, a d 10
increase. metal, requires four CO ligands to become electronically
A two-electron reduction of [Re 2 Cl 8 ] 2− may be carried saturated. In fact, Fe(CO) 5 and Ni(CO) 4 are stable forms
5
out chemically by treatment with P(C 2 H 5 ) 3 . The d –d 5 of these metal carbonyls. A problem arises, however, with
9
product, Re 2 Cl 4 (P(C 2 H 5 ) 3 ) 4 , has a metal bond electron the intermediate d cobalt. There is no way in which a
2
4 2 ∗2
configuration of α π δ δ . δ Bond destruction results in mononuclear, 18-electron binary metal carbonyl can be
9
a net bond order of three. However, the expected concomi- formed from a d metal; Co(CO) 4 and Co(CO) 5 would be
tant isomerization to a staggered conformation does not 17- and 19-electron complexes, respectively. Given that a
occur. Staggering the phosphorus atoms places the bulky metal–metal single bond supplies one additional electron
ethyl groups in closer contact with each other than when to each metal center an 18-electron cobalt carbonyl can be
the phosphorus atoms are eclipsed, therefore, the complex formed via the dimerzation of two 17-electron Co(CO) 4
is less sterically crowded in an eclipsed configuration. units.Thus,Co 2 (CO) 8 isastable18-electroncomplexwith
9
Further bond order reduction can be achieved by con- a single Co Co bond. This is a d 9 d system in which
tinuing to populate the metal–metal antibonding orbitals. only one electron on each cobalt atom was used for metal–
4 2 ∗2
2
For example, Ru 2 (O 2 C-C 3 H 7 ) 4 Cl with its σ π δ δ π ∗1 metal bonding unlike the earlier transition-metal dimers
electron configuration has a bond order of 2.5; the corre- in higher oxidation states in which all valence d-electrons
˚
sponding bond length is 2.281 A. Total population of the are used for metal–metal bonding. The earlier transition