Quadruple bonds between metal

During the succeeding decades, many thousands of cluster compounds of transition metals have been synthesized, including hundreds containing quadruple bonds. Therefore, we need to consider briefly how metal atoms can bond to each other and, in particular, how quadruple bonds between metals are possible. 

In the 1960s quadruple bonds between metallic atoms were discovered in species such as Re2Clg . There would be no obvious way to explain quadruple bonds with cubic atoms or tetrahedra. In 2005, evidence was presented for a quintuple bond between the two chromium atoms in a crowded dichromium complex (RCrCrR). ... 

Although less numerous than complexes containing heavier transition metals, some cases are known in which there is a quadruple bond between first-row metals. One very interesting complex of this type is [Cr2(C03)4(H20)2] in which the carbonate ions form bridges between the two metal atoms as shown in Figure 21.25. 

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. 

The reactions and nature of multiple bonds between two metal atoms form a lively area of research and debate (1,2). Many examples of triple bonds between metals are known, including the extensively studied molybdenum or tungsten alkoxide, M2(OR)6, and amido, M2(NR2)6, complexes (3). Classic examples of quadruple bonds are epitomized by [Re2Cl8]2 (4) and molybdenum acetate, Mo2(OCOMe)4 (5). 

Rhenium occupies a very important place in the development of the chemistry of complexes containing metal metal double, triple and quadruple bonds. This chemistry involves coordination complexes that contain pairs or trinuclear clusters of rhenium atoms and has been surveyed in a recent monograph entitled Multiple Bonds Between Metal Atoms .5 This source, which covers the literature up to early 1981, has also been invaluable in preparing the present review and can be consulted for a more complete and detailed coverage of these classes of complexes. 

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... 

Complexes with multiple, especially quadruple, bonds between two metal atoms have been the subject of much research for at least 25 years. There have been numerous reports of the synthesis of species that contain... 

It is dear that to increase the bond order, the number of electrons must be reduced, so that fewer antibonding MO are occupied. In the complex [Re2(Cl)8(H20)2], each monometallic fragment [Re(Cl)4(H20)] has a d electronic configuration (Re(III)). There are therefore eight electrons to be placed in the MO of this complex, which leads to the electronic configuration (7tY af 8f (4-48b). As only the bonding orbitals are occupied, there is a quadruple bond between the two metal centres. There are two n bonds, one a bond, and a new entity that does not exist organic chemistry a 8 bond. 

Quadruple Bonds between Transition Metal Atoms... 

Textbooks on organic chemistry depict molecules with single, double, and triple bonds between elements. For a certain class of dinuclear transition metal complexes this description is not sufficient and a quadruple bond between the two metal centers has to be introduced. The fourth bond is due to overlapping d orbitals with 8 symmetry. Especially interesting are Cr2L4 complexes (L = bidentate, anionic ligand), some of which show extremely short metal-metal bonds (<2.00 A). For [Cr2 (5-CH3—2-CH30C6H3)4] (21) a chromium-chromium distance of only 1.83 A was determined, the shortest metal-metal bond known. 

Bimetallic complexes frequently have a direct intermetallic bond linking the metal centers together. For transition metal dimers it is possible to prepare complexes that have single, double, triple, or quadruple bonds between the metal centers. If the intermetallic bond is oriented along the z-axis, overlap of the d orbitals on the two metals results in the formation of a and a molecular orbitals. Overlap of the dxz and dyz pairs of orbitals on the metals gives a pair of degenerate... 

The existence of quadruple bonds between two metal atoms has been one of the most appealing developments in bonding theory of the last century. Its discovery in the 1960s triggered a vast amount of research that led to the synthesis and characterization of a wide variety of quadmply bonded complexes [1]. A few decades elapsed, though, before the first quintuple Cr-Cr bond was reported by Power and coworkers [2], followed soon by a variety of systems with quintuple Cr-Cr [3] or Mo-Mo bonds [4]. Here we are concerned with the structural and electronic effects associated with multiple metal-metal bonding [5] and with the analogies and differences between quadruple and quintuple Cr-Cr bonds. 

The "classic" molecule is M02(C CCHa), which is an important representative member of di-metal molecules containing a quadruple bond. The occupation of the delta-bonding orbital, which completes formation of the quadruple bond, is a special feature of these molecules. The classic question is the following To what extent does an electron in the delta-bonding orbital contribute to the total bond strength and force constant between the two metal centers ... 

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