Short Notes on Meta: Metal Bonded Compounds

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 d_z² orbital from each atom.

♦ two π bonds can arise from the overlap of d_zx or d_yx orbitals.

♦ two δ bonds can be formed from the overlap of two face-to-face d_xy or d_x²-_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 d_x²-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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