Bond metal atomic orbitals, quadruple

The rapid advances in synthetic and structural chemistry of the metal-metal bond have inspired detailed theoretical investigation. Unlike main group elements such as carbon, transition metals can form a quadruple bond between two metal atoms. The quadruple bond consists of a single a bond (overlap of two dz2 orbitals), two degenerate n bonds (overlap... 

Overlap of d orbitals leading to the formation of a quadruple bond between two metal atoms. Note that the z axis of each metal atom is taken to point toward the other, such that if a right-handed coordinate system is used for the atom on the left, a left-handed coordinate system must be used for the atom on the right. 

Figure 16-7 Energy level diagrams showing schematically how rf-orbital overlaps between two metal atoms (M2) can be modified by bonding ligands to give triple bonds in M2L6, strong quadruple bonds in M2Lg, and weaker quadruple bonds in M2L8X2.

Transition metals may form single, double, triple, or quadruple bonds (or bonds of fractional order) with other metal atoms. How are quadruple bonds possible In main group chemistry, atomic orbitals in general can interact in a ct or tt fashion, with the highest... 

Fig. 21.15 (a) The formation of metal-metal quadruple bond by overlap of appropriate metal orbitals. Both the d z and dy atomic orbitals are used to form 7r-bonds, but either the d y or d i yi atomic orbital is used for fi-bond formation, the choice being arbitrary depending on which has been chosen for Cr—O bond formation, (b) Approximate energy levels of the metal-metal bonding and antibonding MOs. 

In 1965 F. A. Cotton and C. B. Harris [1] synthesized K2[Re2Clg]-2H20, which presents a surprisingly short Re-Re distance of 2.24A. On the basis of this discovery, they introduced the concept of a multiple bond between two metal atoms. Cotton analyzed the bonding using simple molecular orbital (MO) theory and stated that the two Re atoms are bonded through a quadruple bond [1, 2]. 

Another example Br4Re-ReBr4, structure III, has two unusual structural features. The bromines attached to the two different metal atoms are not staggered to minimize repulsive interactions, but are eclipsed. Moreover, the rhenium-rhenium bond is very short. These structural features and the magnetic properties have been interpreted as indicating that a quadruple bond links the metal atoms. The bonds include one o--bond, two r-bonds, and a S (delta)-bond. A S bond can be visualized by placing the bonded atoms on a z coordinate axis and having overlap between two orbitals (one from each metal atom), structure IV. 

All of these examples have been cases in which all of the valence d electrons on each metal atom have paired in either bonding or antibonding orbitals. For zero-valent metal carbonyl dimers only one of the many valence d electrons on each metal is shared hence, only single bonds are known for these systems. The reason why the earlier transition metals in nonzero valent oxidation states form up to quadruple bonds and the later zero-valent transition metal carbonyls form only single bonds can be understood by considering the electronic needs of these different metal centers. 

The coordination about a transition metal and the chemistry of transition metal complexes is often dictated by the electronic needs of the metal center. The stabilization of compounds when the elements present acquire a closed-shell configuration is well known. For a transition metal, with its nine valence orbitals (one s, five d, and three p orbitals), the stabilizing closed-shell configuration is obtained when the electron count about the metal center reaches 18. When a metal center has fewer than 18 electrons in its valence shell it is said to be electronically unsaturated. One step toward alleviating this electronic unsaturation is the formation of metal-metai bonds. Because of their covalent nature, metal-metal single bonds are considered to add one electron to the valence shell of each metal atom. The electron donation from one metal atom to the other mirrors the bond order so a quadruple bond increases the valence count of each metal by four electrons. 

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