You may be a bit surprised to read that the resonating valence bond theory (VBT) says that metallic bonds are essentially covalent in nature. The statement sounds contradictory, but Linus Pauling proposed otherwise by introducing VBT in 1949, also known as Pauling’s theory of metals. What is all the mystery about, and how can a metallic bond behave as a covalent bond with resonating electron pairs? Let’s wait no more and quickly start reading!
What is resonating VBT of metals – Definition
According to Linus Pauling’s VBT, or the resonance model of metallic bonding, a solid metallic structure is believed to involve electron pair resonance between each atom and its neighbouring atoms. Many different resonance forms contribute to the overall stability of the metal structure.
Example: The resonating VBT model of lithium metal
The electronic configuration of lithium (Li) is 1s2 2s1.
It has one valence electron, which it loses to form a positively charged Li+ ion.
X-ray diffraction studies reveal that each Li atom is bonded to eight neighbouring atoms in the metallic crystal.
Considering a single valence electron of Li, it is impossible for it to bind with eight other atoms. However, the VBT of metallic bonding suggests that it is possible through the resonance model. A covalent bond can be formed between a Li atom and its neighboring Li atoms (as shown below).
The central metal atom gains a negative charge by accommodating an extra valence electron. Contrarily, the metal atom in front of it gains a positive charge on account of losing a valence electron as per the electron sea theory. Consequently, the covalent bonds are dynamic, i.e., these oscillate between alternate interatomic positions in metals.
The charges are not permanent; rather, they keep rotating from one position to another with a simultaneous movement of the covalent bonds. This charge delocalization is known as resonance. The resonance present in lithium metal leads to the following resonance forms. The actual lithium structure is a hybrid of all the contributing forms.
VBT of metallic bonding – History
Linus Pauling initially proposed the synchronized resonance of electron pairs in a metallic structure. It refers to a state in which all covalent bonds resonate in the same manner, and the valence electronic configuration of each atom stays undisturbed.
However, later on, Pauling realized that in metal atoms, instead of all bonds resonating at the same time, a single bond can resonate on its own. This gave birth to the concept of unsynchronized resonance. Unsynchronized resonance further endorsed VBT by conferring unusual stability to metal structures.
Fundamentals of the VBT of metallic bonding
While discussing the above example, if a question came to your mind that how can Li form two covalent bonds at a time if it has a single valence electron only? Then this is a very valid question. However, it is possible as per the VBT of metallic bonding. Similar to what we studied in the valence bond theory of simple covalently bonded molecules, hybridization occurs in the atoms bonded via metallic bonding. Two Li covalent bonds are formed by the sp hybridization of the central atom. Thus, empty orbitals must be available for hybridization in order to draw metallic resonance structures as per VBT.
The Li atom has three empty p orbitals in its outermost shell (n=2). One s and one p atomic orbital of lithium hybridize to form two sp hybrid orbitals. Each sp hybrid orbital overlaps with the s-orbital of an adjacent Li atom to form Li-Li-Li covalent bonds in the resonance model.
As another example, let’s see how we can draw the resonance model for sodium metal as per the valence bond theory of metallic bonding.
VBT in metals solved problem
Problem: How can we draw the VBT resonance model for potassium metal ?
Tip: Try it by yourself and then see the explanation given below.
Explanation:
The electronic configuration of potassium (K) is 1s2 2s2 2p6 3s1.
It has one valence electron while the 3p orbitals are empty, which can facilitate hybridization. Thus, we can draw the resonance structures for potassium metal, as shown below.
All K-atoms are unhybridized in resonance structure 1. Resonance structures 2 and 3 use sp hybrid orbitals of potassium to form covalent bonds with adjacent atoms.
Do you know metal atomic orbitals can also combine to form new molecular orbitals after metallic bonding? Learn more about it in our next article: band theory of metals.
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References
1. Mohallem, J. R., R. O. Vianna, A. D. Quintão, A. C. Pavão, and R. McWeeny. 1997. ‘Pauling’s resonating valence bond theory of metals: some studies on lithium clusters’, Zeitschrift für Physik D Atoms, Molecules and Clusters, 42: 135-43.
2. Pauling, Linus. 1949. ‘The resonating-valence-bond theory of metals’, Physica, 15: 23-28.
3. Sanaullah. 2016. Inorganic Chemistry.
