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Mn 2 (CO) 10 is a simple and clear case of a metal-metal bond because no other atoms tie the two Mn atoms together. When several metals are linked by metal-metal bonds, the compound or ion is called a metal cluster. Many metal clusters contain several unsupported M–M bonds. Some examples are M 3 (CO) 12 (M = Ru, Os) and Ir 4 (CO) 12.
The strong bonding of metals in liquid form demonstrates that the energy of a metallic bond is not highly dependent on the direction of the bond; this lack of bond directionality is a direct consequence of electron delocalization, and is best understood in contrast to the directional bonding of covalent bonds.
Among the five bonds present between the metal centers, one is a sigma bond, two are pi bonds, and two are delta bonds. The σ-bond is the result of mixing between the d z 2 orbital on each metal center. The first π-bond comes from mixing of the d yz orbitals from each metal while the other π-bond comes from the d xz orbitals
The halides of low-valent early metals often are clusters with extensive M-M bonding. The situation contrasts with the higher halides of these metals and virtually all halides of the late transition metals, where metal-halide bonding is replete. Transition metal halide clusters are prevalent for the heavier metals: Zr, Hf, Nb, Ta, Mo, W, and Re.
Covalent bonding corresponds to sharing of a pair of electrons between two atoms of essentially equal electronegativity (for example, C–C and C–H bonds in aliphatic hydrocarbons). As bonds become more polar, they become increasingly ionic in character. Metal oxides vary along the iono-covalent spectrum. [4]
The bond results because the metal atoms become somewhat positively charged due to loss of their electrons while the electrons remain attracted to many atoms, without being part of any given atom. Metallic bonding may be seen as an extreme example of delocalization of electrons over a large system of covalent bonds, in which every atom ...
Covalent bonding also includes many kinds of interactions, including σ-bonding, π-bonding, metal-to-metal bonding, agostic interactions, bent bonds, three-center two-electron bonds and three-center four-electron bonds. [2] [3] The term covalent bond dates from 1939. [4]
σ bonding from electrons in CO's HOMO to metal center d-orbital. π backbonding from electrons in metal center d-orbital to CO's LUMO. The electrons are partially transferred from a d-orbital of the metal to anti-bonding molecular orbitals of CO (and its analogs). This electron-transfer strengthens the metal–C bond and weakens the C–O bond.
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