Short Notes on Meta: Metal Bonded Compounds

By Renuka Miglani|Updated : April 26th, 2023

Are you looking for some short and reliable notes during your CSIR-NET preparations? Then, you have come to a perfect place!

Candidates preparing for their CSIR NET exam might need to get some short study notes and strategies to apply while preparing for the key exam of their life. At this point, We at Byjus Exam Prep come up with short notes on Metal—Metal Bonded Compoundswhich come under the Inorganic Chemistry section of the Chemical Science syllabus

Our experienced Exam experts have meticulously designed this set of short notes on the Metal-Metal Bonded Compounds to give you the most standard set of study materials to be focused upon. In this cut-throat competitive world, students need to prepare themselves with the best study materials to help them learn for their future. So, here we are offering the best study notes that are reliable and can be used by students during their preparations for the upcoming CSIR-NET 2023 exam.


Study Notes on Metal—Metal Bonded Compounds 

With the development of improved techniques for the determination of structure, it has been recognized that there are many d-metal compounds that contain metal-metal (M — M) bond distances comparable to or shorter than those in the elemental metal. A rigorous definition of metal clusters restricts them to molecular complexes with metal-metal bonds that can form triangular or larger structures. This definition, however, would exclude linear M − M compounds and is normally relaxed. 

(a). Metal-metal Bonds:

 Metal-metal bonds with bond orders up to five are formed by many d metals in low oxidation states.  The first d-block metal-metal bonded species to be identified was the ion of mercury (I) compounds, as occurs in Hg2Cl2, and examples of metal-metal bonded compounds and clusters are now known for most of the d metals. Some of their common structural motifs are an ethane-like structure (1) an edge-shared octahedron (2) and so on. 

If we consider the possible overlap between d orbitals on adjacent metal atoms, then,

a σ bond between two metal atoms can form from the overlap of dz2 orbital from each atom.

two π bonds can arise from the overlap of dzx or dyx orbitals.

♦ two δ bonds can be formed from the overlap of two face-to-face dxy or dx2-y2 orbitals


Thus, a quintuple bond formation can take place if all the bonding orbitals are occupied to give the electron configuration

Many other species with multiple metal-metal bonds, where the dx2-y2 orbital is involved in bonding to ligand species, are known, A well-known example is the quadruple bonded compound molybdenum (II) acetate which is prepared by heating Mo (CO), with acetic acid:

2Mo (CO)6 + 4CH3COOH → Mo2(O2CCH3)4 + 2H2 +12CO

The dimolybdenum complex is an excellent starting material to prepare other Mo-Mo compounds. For example, the quadruple bonded chloride complex is obtained when the acetate complex is treated with concentrated hydrochloric acid at below room temperature:

Mo2(O2CCH3)4(aq) + 4H+(aq) + 8 Cl(aq) → [Mo2Cl8]4−(aq) + 4CH3COOH (aq)

An incomplete occupation of the bonding orbitals can result in a reduction of the formal bond order to 3.5 or to the triply bonded M=M systems. These complexes are more numerous than the quadruple-bonded complexes and, because δ bonds are weak, M = M bond lengths are often like those of quadruple-bonded systems. A decrease in bond order can also stem from the occupation of both the δ* orbitals and, once these are fully occupied, successive occupation of the two higher lyings π* orbitals lead to a further decrease in the bond order from 2.5 to 1.

As with carbon-carbon multiple bonds, metal-metal multiple bonds are centers of reaction. However, the variety of structures resulting from the reactions of metal-metal multiple bonded compounds is more diverse than for organic compounds. For example:


In this reaction, HI adds across a triple bond, but both the H and I bridge the metal atoms; the outcome is quite unlike the addition of HX to an alkyne, which results in the formation of a substituted alkene. The reaction product can be regarded as containing a 3c,2e MHM bridge and an iodide anion bonding by two conventional 2c,2e bonds, one to each Mo atom. Larger metal clusters can be synthesized by addition to metal-metal multiple bonds. For example, Pt (PPh3)4, loses two triphenylphosphine ligands when it adds to the Mo = Mo triple bond, resulting in a three-metal cluster:



The above diagram represents the approximate molecular orbital energy level scheme for M – M interactions.


This diagram represents the approximate molecular orbital energy level scheme for the M – M interactions in a quadruple bonded system, where only the will be utilized in bonding.

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