Nitrosyl complexes mononuclear, bonding

A five-coordinated complex [CoCl2(NO)(PCH3Ph2)2], shown in Figure 6, is known to exist in two isomeric forms. The trigonal bipyramidal isomer exhibits N—O stretching frequency at 1750 cm while the square pyramidal isomer absorbs at 1650 cm . The bond angle and nitrosyl-stretching frequencies of some mononuclear nitrosyl complexes of the first transition series is shown in Table 1. 

A description involving two extreme bonding formulations is currently used to explain the nature of the M—N—O linkage in most mononuclear complexes. In the sense of formal oxidation state it is now customary to think of linear nitrosyls as derived from NO+ and bent (120°) nitrosyls as derived from NO-.23,24 Bond formation between free nitric oxide and a transition metal ion must then be considered to involve either prior donation of an electron from NO (giving NO+) or prior acceptance of an electron by NO (giving NO-) 25 in each case this would of course be followed by lone pair donation from the nitrogen. Using this somewhat simplistic view it is easy to explain the... 

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