The bonding of one-electron ligands to transition metals

Symmetry Metal orbitab Ligand orbitals Bond type 

In metal-alkyl (R, ) bonds there is probably little contribution by ir-bonding, as the and Py orbitals of the arbon atom, which are of the correct symmetry for ir-overlap with metal orbitals are already fully involved in bonding with carbon or hydrogen atoms which are also attached to this atom. 

As with all covalent bonds, the metal-carbon bond strength would be expected to increase with increasing electronegativity of the alkyl group  

Alkynyl complexes, which contain the grouping M—C=CR, are often more stable than the corresponding alkyls and aryls. An example is the nickel complex (Et3P)2Ni(C=CPh)2, wtiich melts at 149-15rc without decomposition. Alkynyl complexes are formally similar to metal cyanides and metal carbonyls. The species C=CR, CN and CO are all isoelectronic and presumably bond to a transition metal in a similar manner. All three ligands would be expected to possess both a-donor and ir-acceptor 

Evidence for donation from metal orbitals into the antibonding molecular orbitals of the alkynyl group has b en obtained from studies of the infrared spectra of such complexes. Foi the platinum compounds (Et3P)2Pt(C=CR)2 the C=C stretching frequencies occur ca. 2100 cm which is about ISO cm lower than those found in disubstituted acetylenic hydrocarbons (2260 — 2190 cm ), suggesting a lowering of the C=C bond order. The C=C distance in the compound (Et3P)2Ni(C=CPh)2 however, is very close (1-18 0 02 A) to that found in the free acetylene PhC=CPh (1-19 0-03 A). 

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