2. 2
• Organometallic compounds: direct metal-carbon bonds.
• An organometallic compound is broadly defined as one that involves one or more
carbon atoms of an organic group or molecule and a transition, lanthanide, actinide, or
main group metal atom.
• In 1848, E. C. Frankland synthesized dimethylzinc, and then subsequently discovered
Zn(C2H5)2, Hg(CH3)2, Sn(C2H5)4 and B(CH3)3.
• Among the oldest and well-used organometallic compounds include the Grignard
reagents RMgX, alkyl lithiums, RLi. They have found extensive use in the laboratory-
scale synthetic chemistry.
• Since the 1960s, exploratory synthetic research in organometallic compounds has been
dominated by studies of d-block elements.
Main group: (AlMe3)2
What are organometallic compounds?
3. 3
• Hapticity “” was originally developed to indicate how many carbons of a -system were
coordinated to a metal center.
• It is now used to describe the bonding mode of a ligand to a metal center.
• An 5-cyclopentadienyl ligand, for example, has all five carbons of the ring bonding to the
transition metal center.
• x values for carbon ligands where the x value is odd usually indicate anionic carbon ligands
(e.g., 5-Cp, 1-CH3, 1-allyl or 3-allyl, 1-CH=CH2). The number of electrons donated by the
ligand is usually equal to x+1.
• When x value is even, x usually indicate neutral carbon -system ligands (e.g., 6-C6H6, 2-
CH2=CH2, 4-butadiene, 4-cyclooctadiene). The number of electrons donated by the ligand in
the even (neutral) case is usually just equal to x.
Hapticity
Cationic 2e- donor :
Neutral 2e- donors :
Anionic 2e- donors :
Anionic 4e- donors :
Anionic 6e- donor :
4. 4
• Hapticity is the coordination of a ligand to a metal center via an uninterrupted and
contiguous series of atoms.
• The hapticity of a ligand is described with the Greek letter η ('eta'). For example,
η2 describes a ligand that coordinates through 2 contiguous atoms.
• Denticity refers to the number of donor groups in a single ligand that bind to a central
atom in a coordination complex. In many cases, only one atom in the ligand binds to the
metal, so the denticity equals one, and the ligand is said to be monodentate or
unidentate). Ligands with more than one bonded atom are called polydentate or
multidentate.
• The denticity of a ligand is described with the Greek letter κ ('kappa').For example, κ6-
EDTA describes an EDTA ligand that coordinates through 6 non-contiguous atoms.
• Denticity is different from hapticity because hapticity refers exclusively to ligands where
the coordinating atoms are contiguous.
Hapticity vs Denticity
5. 5
18-Electron Rule and Stability of Organometallic Complexes (1920 by Sidgwick)
• 18-Electron Rule: In mononuclear, diamagnetic complexes, the total number of electrons never
exceeds 18 (noble gas configuration). The total number of electrons is equal to the sum of d-
electrons plus those contributed by the ligands.
• Stability of many organometallic complexes, particularly those involving the first row transition
metals can be predicted by using 18-electron rule.
• This rule can be stated as, when a metal achieves an outer electron configuration of ns2(n-
1)d10np6, there will be 18 electrons in the valence orbitals and a closed stable configuration will
result.
• For many second and third row transition metals 16 electron complexes are also found.
• The 18-electron rule is quite advantageously predict the stability and existence of organometallic
compounds.
Examples:
Electron counting to determine the number of valence electrons about a metal center in a given
transition metal complex:
• Determine the oxidation state of the transition metal center(s). To do this one must:
i. note any overall charge on the metal complex
ii. know the charges of the ligands bound to the metal center
iii. know the number of electrons being donated to the metal center from each ligand
• Add up the electron counts for the metal center and ligands.
6. 6
Rh
Ph3P Cl
Ph3P PPh3
Rh is s1
d8
= 9e
since Cl is -1, Rh is +1 (the complex is neutral)
4 ligands x 2e each = 8e
9e - 1e + 8e = 16e
therefore coordinately unsaturated
Calculation - 18-Electron Rule
8. 8
Other Examples for practice: [V(CO)6]-, [Fe(H2O)6]2+, [Co(CN)6]3-, [Cu(NH3)4]+ Mn2(CO)10, Co2(CO)8
Cr(CO)6 Cr
6CO
Total
Ni(CO)4 Ni
4CO
Total
Fe(CO)5 Fe
5CO
Total
Cr(NO)4 Fe(Cp)2 Cr(C6H6)2
Fe(CO)4PPh3 Fe(CO)2(NO)2 [Mn(CO)5]-
• There are multiple examples of transition metal compounds with less (group 3, 4 and 10) or more than
18 electrons in the metal shell
18-Electron Rule – For Practice
9. 9
Transition metal carbonyl complexes (metal carbonyls) are among the most well studied
organometallic compounds. Almost all transition metals form carbonyls.
Ten of the CO ligands are terminal and two span an
Fe---Fe edge, resulting in C2v point group symmetry.
Mn2(CO)10 has no bridging CO ligands: it
can be described (CO)5Mn-Mn(CO)5.
The major isomer contains two bridging CO ligands linking the Co atoms
and six terminal CO ligands, three on each Co atom. It can be described
by the formula (CO)3Co(μ-CO)2Co(CO)3 and has C2v symmetry.
Metal Carbonyl Compounds
Pentacarbonylhydridomanganese has
octahedral symmetry, point group C4v. Ferrocene is an organometallic
compound Fe(C5H5)2.
Practice for 18-Electron Rule – All the above complexes
10. 10
Bonding of CO
• The MO configuration of CO molecule is:
KK 1nb
2 22 14 3nb
2
• Carbonyl ligand is considered as a weak 2-electron
-donor and very strong -acceptor.
•Two types of interactions are involved in the
complexation of carbonyl with transition metal ion.
• Electron donation of the lone pair on carbon -
orbital to vacant metal d orbital.
• This electron donation makes the metal more
electron rich - compensate for this increased electron
density, a filled metal d-orbital may interact with the
empty * orbital on the carbonyl ligand.
• What stabilizes CO complexes is M→C π–bonding
or synergistic bonding
• The lower the formal charge on the metal ion the
more willing it is to donate electrons to the π–orbitals
of the CO .
• Thus, metal ions with higher formal charges, e.g.
Fe(II) form CO complexes with much greater difficulty
than do zero-valent metal ions.
Bonding in CO
11. 11
• Zeise’s Salt, potassium trichloro(ethene)platinate(II), K[Pt(C2H4)Cl3].H2O
synthesized in 1827 reported as the first olefin complex, confirmed to have
H2C=CH2 as a ligand in 1868.
• Zeise isolated stable yellow crystals when he refluxed an alcoholic solution of
potassium tetrachloroplatinate.
• This compound is commercially available as a hydrate. The hydrate is
commonly prepared from K2[PtCl4] and ethylene in the presence of
a catalytic amount of SnCl2. The water of hydration can be removed in vacuo.
K2[PtCl4] + C2H5OH K[Pt(C2H4)Cl3] + Cl- + H2O
Structure and Bonding (Study in Details from Reference Books)
• The alkene C=C bond is approximately perpendicular to the PtCl3 plane and
occupies the fourth coordination site of the square planar complex with the
carbon-carbon axis perpendicular to the platinum-ligand plane. In Zeise's salt
and related compounds, the alkene rotates about the metal-alkene bond with a
modest activation energy.
• The bond between the ethylene molecule and the Pt metal ion may be
considered as a dative –bond to a suitable hybrid orbital on the Pt atom (dsp2)
and a synergic –bond.
Zeise’s Salt
Important - Structure and Bonding in Zeise’s salt (Study in Details from Reference Books)
12. 12
• Ferrocene is an organometallic compound with the formula Fe(C5H5)2. It is the metallocene, consisting of two
cyclopentadienyl rings bound on opposite sides of a central metal atom. Such organometallic compounds are
also known as sandwich compounds.
• Ferrocene was first prepared unintentionally. In 1951, Peter Pauson and T. J. Kealy reported the reaction of
cyclopentadienyl magnesium bromide and ferric chloride with the goal of oxidatively coupling the diene to
prepare fulvalene. Instead, they obtained a light orange powder of "remarkable stability".
• A second group was also unknowingly discovered ferrocene in 1952. Miller, Tebboth and Tremaine were trying
to synthesize amines from hydrocarbons such as cyclopentadiene and ammonia in a modification of the Haber
process. The stability of the new organoiron compound was accorded to the aromatic character of the negatively
charged cyclopentadienyls, but they were not the ones to recognize the η5 (penta-hapto) sandwich structure.
• Robert Burns Woodward and Geoffrey Wilkinson deduced the structure based on its reactivity.
• Independently Ernst Otto Fischer also came to the conclusion of the sandwich structure and started to
synthesize other metallocenes such as nickelocene and cobaltocene.
Ferrocene
13. 13
Pauson and Kealy synthesised ferrocene using iron(III) chloride and a Grignard reagent, cyclopentadienyl
magnesium bromide. Iron(III) chloride is suspended in anhydrous diethylether and added to the Grignard reagent,
which is prepared by reacting cyclopentadiene with magnesium and bromoethane in anhydrous benzene.
More efficient preparative method is using either commercially available sodium cyclopentadienideor freshly
cracked cyclopentadiene deprotonated with potassium hydroxide and reacted with anhydrous iron(II) chloride
in ethereal solvents/THF.
2 NaC5H5 + FeCl2 → Fe(C5H5)2 + 2 NaCl
FeCl2·4H2O + 2 C5H6 + 2 KOH → Fe(C5H5)2 + 2 KCl + 6 H2O
The other early synthesis of ferrocene was by Miller et al.,who reacted metallic iron directly with gas-phase
cyclopentadiene at elevated temperature.
Ferrocene - Synthesis
Important - Structure and Bonding in Ferrocene (Study in Details from Reference Books)
15. 15
IR spectra and metal-carbon bonds
The υCO stretching frequency of the coordinated CO is very informative
Recall that the stronger a bond gets, the higher its stretching frequency
M=C=O (C=O is a double bond) canonical structure
Lower the υCO stretching frequency as compared to the M-C≡O structure (triple bond)
Note: υCO for free CO is 2041 cm-1)
[Ti(CO)6]2- [V(CO)6]- [Cr(CO)6] [Mn(CO)6]+ [Fe(CO)6]2+
υCO 1748 1858 1984 2094 2204 cm-1
Bridging versus terminal carbonyls
Bridging CO groups can be regarded as having a double bond C=O group, as
compared to a terminal C≡O, which is more like a triple bond:
the C=O group in a bridging carbonyl is more like the C=O in a ketone, which typically
has υC=O = 1750 cm-1
terminal carbonyl bridging carbonyl
(~ 1850-2125 cm-1) (~1700-1860 cm-1)
Bridging CO between 1700 and 2200 cm-1
