Phosphine ligands electron count

RhCl(PPh3) 3 The chlorine radical (Cl ) accepts an electron from rhodium metal (electronic configuration Ad1,5s2) to give Cl and Rh+. The chloride ion then donates two electrons to the rhodium ion to form a dative or a coordinate bond. Each PPh3 donates a lone pair of electrons on the phosphorus atom to the rhodium ion. The total number of electrons around rhodium is therefore 8 + 2 + 3X2=16, and the oxidation state of rhodium is obviously 1 +. The other way of counting is to take the nine electrons of rhodium and add one electron for the chlorine radical and six for the three neutral phosphine ligands. This also gives the same electron count of 16. 

Group 10 metal clusters with carbonyl and phosphine ligands sometimes fail to follow the counting paradigm, but the reason for this failure differs from those discussed up to now. The situation is illustrated by the two Pt carbonyl clusters shown in Figure 3.21 where the eve counts are four less than those obtained for group 8/9 metal clusters for the same cluster shapes. What is the origin of these lower electron counts ... 

Numerous examples of mono(tetrahydroborate) complexes illustrate this simple relationship between the BH4" coordination mode and the 18-electron rule. A nice case can be found in the series of Cu-borohydride complexes. The tetrahydroborate ligand is bound in an r]1 fashion in the Cu(BH4)(PMePh2)3 complex, in which the total number of electrons is 10(Cu) + 3 x 2(phosphines) + 2(BH4 ). The loss of a phosphine group in Cu(BH4)(PPh3)2 entails a change in the coordination mode of BFU" 1 2) in order to keep the 18-electron count. 

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