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Most metal carbonyl complexes contain a mixture of ligands. Examples include the historically important IrCl(CO)(P(C 6 H 5) 3) 2 and the antiknock agent (CH 3 C 5 H 4)Mn(CO) 3. The parent compounds for many of these mixed ligand complexes are the binary carbonyls, those species of the formula [M x (CO) n] z, many of which are
In the covalent bond classification method, κ 1-carbonate is anX ligand and κ 2-carbonate is an X 2 ligand. With two metals, the number of bonding modes increases because carbonate often serves as a bridging ligand. It can span metal-metal bonds as in [Ru 2 (CO 3) 4 Cl 2] 5-, where again it functions as an (X) 2 ligand.
The metal–ligand bond can be further stabilised by a formal donation of electron density back to the ligand in a process known as back-bonding. In this case a filled, central-atom-based orbital donates density into the LUMO of the (coordinated) ligand. Carbon monoxide is the preeminent example a ligand that engages metals via back-donation.
For example, calculations suggest that Fe 2 (CO) 9 lacks an iron–iron bond by virtue of a 3-center 2-electron bond involving one of three bridging CO ligands. [5] Representations of two kinds of μ-bridging ligand interactions, 3-center, 4-electron bond (left) and 3-center, 2-electron bonding. [5]
In this example, the cyanide ligands are "innocent", i.e., they have a charge of −1 each, −5 total. To balance the fragment's overall charge, the charge on {CrNO} is thus +2 (−3 = −5 + 2). Using the neutral electron counting scheme, Cr has 6 d electrons and NO· has one electron for a total of 7. Two electrons are subtracted to take ...
Compounds that obey the 18-electron rule are typically "exchange inert". Examples include [Co(NH 3) 6]Cl 3, Mo(CO) 6, and [Fe(CN) 6] 4−. In such cases, in general ligand exchange occurs via dissociative substitution mechanisms, wherein the rate of reaction is determined by the rate of dissociation of a ligand. On the other hand, 18-electron ...
The results of quantum chemical calculations also suggest that the bonding in alkaline earth octacarbonyl complexes can be described in terms of ionic bonding between a metal center with a formal +2 oxidation state and a ligand cage with a formal charge of -2 giving the general formula: Ca 2+ [(CO) 8] −2.
Examples of this type of ligand include CO, PR 3, NH 3, H 2 O, carbenes (=CRR'), and alkenes. Z-type ligands are those that accept two electrons from the metal center, as opposed to the donation occurring with the other two types of ligands. However, these ligands also form dative covalent bonds like the L-type. [2]