Metal Carbonyls -The EAN Rule

Metal Complexes in which Cabon monoxide (CO) act as a ligand are called metal carbonyls Example: Cr(CO)6 chromium hexacarbonyl, Fe(CO)5. pentacarbonyliron

Any coordination or complex compound consisting of a heavy metal such as nickel, cobalt, or iron surrounded by carbonyl (CO) groups comes under this category. Some common metal carbonyls include: tetracarbonylnickel Ni(CO)₄ and octacarbonyldicobalt Co₂(CO)₈.

EAN Rule:

“The sum of the electrons on the central metal atom or ion and the electrons donated from
the ligands in a complex compound is called the effective atomic number (EAN) of the metal and for stable complexes it is generally equal to the atomic number of next incoming noble gas.”

EAN = No. of electrons in the central metal atom or ion + No. of electrons donated by ligands

Note: An alternative and more general statement can be given instead of EAN rule as follows: “When the central metal ion or atom of a complex compound acquires noble gas electronic configuration (n-1)d 10 ns 2 np 6 there will be 18 electrons in the valence orbitals (or valence shell) and the electronic configuration will be closed and stable.” It is known as the 18-electron rule.

The complexes obeying the 18-electron rule are quite stable.18-Electron rule applied to metal carbonyls. The 18 – electron rule is very useful in predicting stabilities and structures of organometallic compounds.

There are two conventions for counting electrons in complexes:

a) Neutral atom or covalent model

According to the covalent model there is covalent bond between Co and H atoms in HCo(CO)4 and H atom acts as 1e-donor ligand.

Accordingly, no. of electrons in valence shell of Co in HCo(CO)4 =9(Co) + 4×2(CO) + 1X1(H) = 18.

b) Oxidation state or ionic model

According to the ionic model there is ionic bond between (OC)4Co+ and H- ions and H- ion acts as 2e-donor ligand.

Accordingly, no. of electrons in valence shell of Co in HCo(CO)4

=8(Co+) + 4×2(CO) +1×2(H-) = 18

The steps are generally followed for counting the electrons present in the valence shell of central metal in a metal carbonyl.

1. The electrons present in the valence shell [i.e., (n-1)d, ns and np orbitals] of the central metal atom are counted and then electrons are added to it or subtracted from it depending upon the nature of charge (negative or positive ) present on it in the metal carbonyl.
2. Each terminal carbonyl group contributes 2e to the valence shell of the central metal.
3. The bridging carbonyl group (M – CO – M) contributes 1e to the valence shell of each metal atom attached by it.
4. The metal-metal (M –M) bond contributes 1e to the valence shell of each metal atom.

Examples of electron counts in the metal carbonyls

Limitation of EAN rule:

1. This rule not obeyed by Square planar organometallic complexes of the late transition metals.
2. This rule not obeyed by some organometallic complexes of the early transition metals.
3. Some high valent d⁰ complexes have a lower electron count.
4. Sterically demanding bulky ligands force complexes to have less electrons than EAN.
5. The EAN rule fails when bonding of organometallic clusters of moderate to big sizes (6 Metal).
6. The rule is not applicable to organometallic compounds of main group metals as well as to those of lanthanide and actinide metals.
7. Counting the electrons helps   to predict stability of metal carbonyls. But it will not tell you whether a CO is bridging or terminal.

Types of metal corbonyls

On the basis of number of metal atoms present per molecule:

On this basis metal carbonyls are of two types.

1. Mononuclear metal carbonyls: The metal carbonyls which contain only one metal atom per molecule are called mononuclear metal carbonyls. Example [Ni(CO)4], [Fe(CO)5 ], [Cr(CO) 6 ] , etc.
2. Polynuclear metal carbonyls: The metal carbonyls which contain more than one metal atom per molecule are called polynuclear metal carbonyls. Example Mn2(CO)10 ,Fe2(CO)9 , Fe3(CO)12 , Co4(CO)12etc.

Physical properties:

• State: Majority of the metallic carbonyls are liquids or volatile solids.
• Colour: Most of the mononuclear carbonyls are colourless to pale yellow. V(CO)6 is a bluish-black solid. Polynuclear carbonyls exhibit dark colour.
• Solubility: Metal carbonyls are soluble in organic solvents like glacial acetic acid, acetone, benzene, carbon tetrachloride and ether.
• Magnetic Property: All the metal carbonyls other than vanadium hexacarbonyl are diamagnetic. The metals with even atomic number form mononuclear carbonyls. Thus, all the electrons in the metal atoms are paired. In case of dinuclear metal carbonyls formed by metals with odd atomic number, the unpaired electrons are utilized for the formation of metal-metal bonds.
• Thermal Stability: Most of the metal carbonyls melt or decompose at low temperatures. Solid carbonyls sublime in vacuum but they undergo some degree of degradation.
• Thermodynamic Stability: Metal carbonyls are thermodynamically unstable. They undergo aerial oxidation with different rates. Co2(CO)8 and Fe2(CO)9 are oxidized by air at room temperature while chromium and molybdenum hexacarbonyls are oxidized in air when heated.

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