Page 245 - Academic Press Encyclopedia of Physical Science and Technology 3rd Organic Chemistry

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Encyclopedia of Physical Science and Technology EN011A-544 July 25, 2001 18:30

532 Organometallic Chemistry

are shown from the top in these pictures not from the side metals include Pt(II), Mo(0), Fe(0), Ir(I), Hg(II), Cu(I),
as in 2. and Ag(I). Soft ligands include CO, ethylene, benzene,
allyl, cyclopentadienyl.
A remarkable feature of the zerovalent metal carbonyls
Cl NH 3 Cl NH 3
is that there is a regular change in their formulas and
Pt Pt
therefore in their constitution as we move from left to right
Cl NH 3 H 3 N Cl across the periodic table: Cr(CO) 6 , (CO) 5 Mn Mn(CO) 5 ,
Fe(CO) 5 , (CO) 3 Co(CO) 2 Co(CO) 3 , Ni(CO) 4 . Alternate
6 7
compounds are mononuclear (i.e., contain one metal)
but have one less CO for every two steps to the right.
The simplest carbon ligand is the methyl group, CH 3 . This regularity is embodied in the Eighteen Electron rule,
Almost all metals form methyl complexes MMe n , where which states that special stability often accompanies a
n is the valency of the metal, which varies in a regular way valence electron count of 18 electrons per metal (and in
across the periodic table (e.g., Na: n = 1. Mg: n = 2.). The some cases, such as Pd(II), 16 electrons). To take the
M CH 3 bond consists of a single shared pair of electrons case of Cr(CO) 6 , the valence electron count is obtained
and is not very different from the case of M NH 3 . Since as follows: Cr is in group 6 of the periodic table, so has
carbon is more electronegative than the early metals (i.e., 6e; CO donates the 2e of its C lone pair to the metal
the ones on the left-hand side of the periodic table), the and so:
methyl group bears a negative charge in the methyl com-
6 + (2 × 6) = 18 (8)
plexes of these elements (e.g., LiMe, mentioned earlier).
The next simplest carbon-based ligand is carbon mono- In the general case of a complex of formula [MX a L b ] c+
xide, CO. Although superficially similar to the M Me (where M is a metal of group N, X is an anionic ligand
system in having a metal-carbon bond, M CO is a very such as Cl or CH 3 , and L is a neutral ligand, such as NH 3
different type of bond. This molecule has a lone pair or CO), the count is given by Eq. (9)
of electrons on the carbon atom, which can be shared
with a metal to give a metal carbonyl complex, such as e count = N + a + 2b − c (9)
Fe(CO) 5 . A new feature of the bonding, which differs
Usefulconceptsaretheoxidationstate(O.S.),givenbyEq.
from the situation in NH 3 or CH 3 , is that the CO is an n
(10), and the d configuration, or number of d-electrons,
unsaturated compound (i.e., has double or triple bonds).
given by Eq. (11).
In such a case, there is always the possibility of an ad-
ditional interaction not present to any significant extent O.S. = c + a (10)
in complexes of NH 3 or the methyl group. The metal
n = N − c + a (11)
may donate some of its electron pairs to the ligand (an
empty orbital allowing this to happen is almost always The (II) (i.e., + 2) of Pd(II) is the oxidation state; Pd(II)
8
present in an unsaturated ligand); this is called back do- is said to have the d configuration.
nation and accounts in part for the large modification In metal carbonyls, the CO removes metal electron
both in metal and ligand properties on binding. For CO, density so efficiently that the lower oxidation states of
the back bonding has the effect of weakening the C O the metal, which have more electrons, are strongly stabi-
bond (this can be detected by infrared spectroscopy) and lized. Methyl complexes and carbonyls tend to have very
+
−
preventing the M L bond from having the M −L po- different oxidation states for this reason, such as WMe 6
larity of M NH 3 (because for CO the ligand-to-metal (O.S. = 6) and W(CO) 6 (O.S. = 0).
electron donation is balanced by metal-to-ligand back Early metal alkyls tend to be reactive and nucleophilic
bonding). (i.e., tend to transfer an alkyl anion to a reagent), but the
So important is this back donation in stabilizing the late metals, such as Pt, Ir, Hg, can form very stable alkyls;
M CO bond that metals which do not have any elec- the very high stability of the water-soluble [HgCH 3 ] con-
+
trons to back donate form either no CO complexes or stitutes one of the major hazards of mercury pollution,
form very unstable ones. A metal will only bind CO because this species, formed by microbial action, can en-
strongly if it has d-electrons and therefore comes from ter the food chain. Metal alkyls that have a β-H (i.e., a
groups 2–11 ( = N) of the periodic table and has a va- hydrogen at the second carbon from the metal) can un-
lency state ( = v), such that N − v ≥ 1. Only unsaturated dergo the β-elimination reaction provided they are coor-
ligands (said to be soft ligands) have the power to ac- dinatively unsaturated (i.e., have an electron count of less
cept back donation. Metals are able to engage in back than 18) or, if coordinatively saturated (18e count), can
bonding if they have a low valency state (e.g., v ≤ 2). Soft lose a ligand so as to generate the necessary unsaturation.