Coordination compounds monodentate complexes

The CK" ion can act either as a monodentate or bidentate ligand. Because of the similarity of electron density at C and N it is not usually possible to decide from X-ray data whether C or N is the donor atom in monodentate complexes, but in those cases where the matter has been established by neutron diffraction C is always found to be the donor atom (as with CO). Very frequently CK acts as a bridging ligand - CN- as in AgCN, and AuCN (both of which are infinite linear chain polymers), and in Prussian-blue type compounds (p. 1094). The same tendency for a coordinated M CN group to form a further donor-aceeptor bond using the lone-pair of electrons on the N atom is illustrated by the mononuclear BF3 complexes... 

Coordination compounds containing bidentate ligands are often thermally more stable than those comprised of related monodentate ligands, e.g. ethylenediamine (en) complexes dissociate at a higher temperature than those of ammonia or pyridine. Compounds containing a ring structure, such as coordinated salicylaldehyde (sal) and acetyl-acetonate (acac), are particularly stable, and may often be sublimed... 

Numerous other aromatic nitrogen compounds are known, and a significant number have been used to prepare complexes of platinum(II). Phthalazine (101) is one such compound, and the complex ds-PtCl(phthalazine)(PEt3)2 has been prepared in order to compare its fluxional properties with the phenanthroline and naphthyridine analogues. The monodentate complex is fluxional, but in this phthalazine case the orientation of the lone pairs is unsuitable for the five-coordinate intermediate required by an intramolecular process and the mechanism of exchange becomes dissociative.11 6... 

A slightly different situation is observed in coordination compounds formed by hydrazides of carbonic acids [112]. In this case, together with complexes 419 and 420 which are closely related to 417 and 418, the structure 421 is formed, which for the examined compounds has been found to behave as a monodentate N-donor ligand [112] ... 

Reactivity studies of jj -acyl complexes are limited. Compounds (115) react with CO to afford octahedral cis.trans- ) -acyl complexes (121), probably via prior conversion to a coordinatively unsaturated monodentate (see Monodentate Ligand) acyl (equation 27). The cis,trans-( 2 ) then isomer-izes to trans,trans-( l ). Phenylacetylene reacts with (115 R = R = Et) to give a mixture of acyl-alkyne coupling products,that is (122). Reaction with the R = i-Pr compound proceeds in a different way, by equilibration to the -alkyl, followed by aUcyl-alkyne coupling, and finally CO insertion to give rf--diCy complex (123). ... 

Eanthanide triflates reacts with PhsPO in ethanol forming [En(OTf)2(Ph3PO)4] OTf (Ln = La, Nd, Er, Lu). The erbium and lutetium compounds have octahedral coordination, with monodentate triflates, but the complexes of the larger La and Nd have seven-coordination with one triflate being bidentate. 

The azide group can function as a monodentate ligand in coordination compounds such as [Co(N3)2en2]N03 and [Co(N3)(NH3)s](N3)2, - and here again the differences between N-N bond lengths are not too certain. In these complexes the angle between Co—N and the linear N3 group is around 125°. The... 

The monodentate mode of coordination of the pz group has been now found in many structurally characterized transition metal compounds. Thus, complexes of the type M(pz ), m-M(pz )2, and fran.v-M(pz )2 have been described. The utility of some of these species as building blocks for the preparation of heteronuclear complexes is well documented, although further studies are necessary in order to gain more insight into the properties of these systems. 

A large number of covalent tetrahydroborate complexes with first-row transition metals has been described. The most common coordination mode of the tetrahydroborate ligand is q2, being known as at least one example of q2 - BH4 for all the transition metals of this row. Although less abundant, several q1 and q3 complexes have been characterised experimentally, though only for a limited number of metals. In particular, most of the monodentate complexes are Cu compounds and most of the tridentate are Ti compounds. 

---