Metal carbonyl

 *Metal carbonyls :-All transition metals form metal carbonyls that is a complex in viz carbon

monoxoide acts as a ligand.Almost all metal carbonyls follow 18 electron rule .

eg.Fe(CO)5=18 e-

     Mn(CO)5=17 e- =unstable ,so dimerizes
     Mn2(CO)10=(CO)5-Mn-(CO)5..........follow 18 e- rule

other by electron capturing mechanism;-
eg.V(CO)6+Na-------> [V(CO)6]-

-Classification of metal carbonyl on the basis of ligands...

a.Homoleptic carbonyl complex ;- Only CO is ligand
 eg.Fe(CO)5,Fe2(CO)9,Fe3(CO)12...etc

b.Heteroleptic carbonyl complex;-Ligands  other than CO are also present.
eg.Mo(CO)3(PF3)3,Mo(CO)3(PPh3)3,Cr(CO)3(NO)2 etc.

*Molecular orbital diagram for CO ;-
  C=1s2 2s2 2p2              valance electron=4
  O=1s2 2s2 2p4              valance electron=6
                                           total valance electron =10

CO donates sigma electrons from σ* s( HOMO)  to vacant d  orbitals of transition metal.But if metal can't accomodate the extra electron donated by carbonyl ligand .so metal donates electron  back to the LUMO of carbonyl group.  (  π *p ) and these electron comes back to homo of  CO.So when CO donates electron to metal.It is formed a sigma bond and this is called sigma donating ability.When metal donate electron back to the LUMO of carbonyl  it leads to formation of (  π  ) bond .This is called (  π) accepting  ability   of CO.so ligand is also called ( π  ) acceptor ligand.

[ The formation of sigma bond is supplimented by (π) bond formation & vice -versa]
 This process is called synergism & this effect is called synergestic effect.

* Carbonyl ;- Metal carbonyl bridging are of three types.
 1.μ2 Bridging;-    M-CO-M
2.Semibridging:- 
3. μ3Bridging

υCO   = semibridging > μ2- CO> μ3-Co

[ More negative charge on complex -> more charge on metal -> more  back π  bonding-> M-C bond strong ->  C=O bond long  ->  υ decreases.]

*Subtitution:- Subtitution reaction are of three types in the carbonyl mechanism.

1.Dissociative mechanism :-The comlex which follow  18 electron rule under go dissociative mechanism.Rate of dissociative mechanism will be slow because energy is required to break the bond.

General mechanism for dissociative mechanism:-
Step   1. MX -------->  LnM   +    X
             18e-                    16 e-
 Step2.LnM   +    y --------> LnM-y
             16e-                             18e-

eg.W(CO)6-------->    W(CO)5  +  CO
        18e-                          16e-
     W(CO)5  +   PPh3-------->    W(CO)5PPh3
       16e-                                              18e-

2.Associative mechanism:-The  comlex viz follow 16 e- ruleunder go  associative mechanism.  Rate will be fast.First bond is formed &  energy released in this process will be used in breaking bond.

General mechanism for associative mechanism:-
 Step1:- LnM-X   +    y-    ------->    LnM-Xy
                 16e-                                         18e-
Step2:-Ln-MXy ---------->       LnM-y     +      X-
                18e-                                   16e-

3.Interchange mechanism:- The leaving &  entering group exchange in a single step by forming a transition state .
LnM-X + y  ----------> [y-----LnM------X]+

eg. Ni(CO)4 + PR3 ---------> Ni (CO)3PR3
      Ni(CO)3PR3 + PR3 --------->  Ni(CO)2(PR3)2
     Ni(CO)2(PR3)2 + PR3 -------->  Ni(CO)(PR3)3
  Ni(CO)(PR3)3 +PR3 --------->Ni(PR3)4

*Effects of ion pairing :-
Polycationic complexes tend to form ion pairs with anions and these ion pairs often undergo reactions via the Ia pathway. The electrostatically held nucleophile can exchange positions with a ligand in the first coordination sphere, resulting in net substitution. An illustrative process comes from the "anation" (reaction with an anion) of chromium(III) hexaaquo complex:

[Cr(H2O)6]3+ + SCN− ⇌ {[Cr(H2O)6], NCS}2+
{[Cr(H2O)6], NCS}2+ ⇌ [Cr(H2O)5NCS]2+ + H2O                                                                     
 *Eigen-Wilkins mechanism:-
The Eigen-Wilkins mechanism, named after chemists Manfred Eigen and R. G. Wilkins,is a mechanism and rate law in coordination chemistry governing associative substitution reactions of octahedral complexes. It was discovered for substitution by ammonia of a chromium-(III) hexaaqua complex.The key feature of the mechanism is an initial rate-determining pre-equilibrium to form an encounter complex ML6-Y from reactant ML6 and incoming ligand Y. This equilibrium is represented by the constant KE:

ML6 + Y ⇌ ML6-Y
The subsequent dissociation to form product is governed by a rate constant k:

ML6-Y → ML5Y + L
A simple derivation of the Eigen-Wilkins rate law follows:

[ML6-Y] = KE[ML6][Y]
[ML6-Y] = [M]tot - [ML6]
rate = k[ML6-Y]
rate = kKE[Y][ML6]
Leading to the final form of the rate law, using the steady-state approximation (d[ML6-Y] / dt = 0),

rate = kKE[Y][M]tot / (1 + KE[Y])
*Eigen-Fuoss equation:-
A further insight into the pre-equilibrium step and its equilibrium constant KE comes from the Fuoss-Eigen equation proposed independently by Eigen and R. M. Fuoss:

KE = (4πa3/3000) x NAexp(-V/RT)
Where a represents the minimum distance of approach between complex and ligand in solution (in cm), NA is the Avogadro constant, R is the gas constant and T is the reaction temperature. V is the Coulombic potential energy of the ions at that distance:

V = z1z2e2/4πaε
Where z is the charge number of each species and ε is the vacuum permittivity.