Reaction of the bis(phosphinoalkyl)silanes RSiH[(CH 2 ) n PR 1 2 ] 2 (R ) Me or Ph, n ) 2 or 3, R 1) Ph or Cy, i.e., cyclohexyl; biPSiH for R ) Me, n ) 2, R 1 ) Ph) with Pt(COD)Cl 2 (COD ) cycloocta-1,5-diene), [Ir(COD)Cl] 2 , trans-M(PPh 3 ) 2 (CO)Cl (M ) Rh or Ir), or Ru 3 (CO) 12 affords products that are complexes of tridentate biPSi silyl analogues, in which the Si center is anchored through attachment of trans P atoms at the transition-metal site. The platinum(II) compounds Pt[SiMe(CH 2 CH 2 PPh 2 ) 2 ]Cl (1), Pt[SiMe(CH 2 CH 2 PCy 2 ) 2 ]Cl (2), Pt[SiPh(CH 2 CH 2 PCy 2 ) 2 ]Cl (3), Pt[SiPh(CH 2 CH 2 PPh 2 )(CH 2 CH 2 PCy 2 )]Cl (4), Pt[SiMe(CH 2 CH 2 -CH 2 PPh 2 ) 2 ]Cl (5), Pt[SiMe(CH 2 CH 2 CH 2 PCy 2 ) 2 ]Cl (6), and Pt[SiPh(CH 2 CH 2 CH 2 PPh 2 ) 2 ]Cl ( 7), which have been characterized by 1 H, 13 C, and 31 P NMR spectroscopy, possess distinguishable square faces with Me on Si projecting in one direction and the buckled (CH 2 ) n backbones in the other. Cleavage of the Pt-Si bond in Pt(biPSi)Cl ( 5) by HCl affords a pair of diastereomers HPt[(PPh 2 CH 2 CH 2 CH 2 ) 2 Si(Cl)Me]Cl (8,8′) in which the two P atoms are also trans. The five-coordinate, 16e, d 6 Ir(III) complex IrH(biPSi)Cl ( 9) is isolated as a single diastereomer, but octahedral IrH(biPSi)(CO)Cl is formed as a 3:1 mixture of stereoisomers (10 and 11, CO or Cl trans to Si, respectively, i.e., H trans to Cl or CO) that react with SnCl 2 to afford (also 3:1) the analogues IrH(biPSi)(CO)SnCl 3 (12 and 13). Treatment of 10 and 11 (3:1) with LiAlH 4 yields a diastereomeric pair of cis-dihydrido complexes IrH 2 (biPSi)-(CO) (14,14′) (1:3), accompanied by a trans-dihydrido stereoisomer 15 and a fac-biPSi stereoisomer 16 as minor products; in 14, 14′ and 15 the hydride ligands are nonequivalent and show 2 J HHcis ) 5.1, 4.4 Hz, 2 J HHtrans ≈ 0, respectively. Similar treatment with LiAl 2 H 4 affords monodeuterioisotopomers 14a,14′a, while reaction of compound 9 with LiAlH 4 under CO gas initially affords 16 as a single product that subsequently isomerizes to 14,14′. The octahedral, 18e, d 6 Ru(II) homologues RuH[SiMe(CH 2 CH 2 PPh 2 ) 2 ](CO) 2 (17) and RuH(biPSi)-(CO) 2 (18) are formed in sealed-tube reactions at elevated temperature but in very poor yields. NMR spectroscopy suggests that in solution the biPSi backbones of 2-7 and 9-13 are equivalent, establishing planar symmetry (point group C s ) perpendicular to the ligand template, although this is not maintained in the solid state as shown crystallographically for compounds 5, 9, 13, and 18. Bond distances trans to Si are long in 5 and 13 (2.44 and 2.66 Å, to Cl or SnCl 3 , respectively), but in 9 Ir-Cl at 2.30 Å is short, occupying the equatorial plane with Si and H in a distorted TBP structure (Si-Ir-Cl 129°). The M-Si distances are 2.31 (5), 2.29 (9), 2.42 (13), and 2.46 Å (18). In 9 and 13, the H at Ir is anti vs Me on Si (of biPSi), whereas in 18, the Ir-H and Si-Me bonds are syn.

Slow uptake of molecular dihydrogen by the diiridium(I) prototype [Ir(mu-pz)(PPh(3))(CO)](2) (1: pzH = pyrazole) is accompanied by formation of a 1,2-dihydrido-diiridium(II) adduct [IrH(mu-pz)(PPh(3))(CO)](2) (2), for which an X-ray crystal structure determination reveals that (unlike in 1) the PPh(3) ligands are axial, with the hydrides occupying trans coequatorial positions across the Ir-Ir bond (2.672 A). Reaction with CCl(4) effects hydride replacement in 2, affording the monohydride Ir(2)H(Cl)(mu-pz)(2)(PPh(3))(2)(CO)(2) (3) in which Ir-Ir = 2.683 A. At one metal center, H is equatorial and PPh(3) is axial, while at the other, Cl is axial as is found in the symmetrically substituted product [Ir(mu-pz)(PPh(3))(CO)Cl](2) (4) (Ir-Ir = 2.754 A) that is formed by action of CCl(4) on 1. Treatment of 1 with I(2) yields the diiodo analogue 5 of 4, which reacts with LiAlH(4) to afford the isomorph Ir(2)H(I)(mu-pz)(2)(PPh(3))(2)(CO)(2) (6) of 3 (Ir-Ir = 2.684 A). Protonation (using HBF(4)) of 1 results in formation of the binuclear cation Ir(2)H(mu-pz)(2)(PPh(3))(2)(CO)(2)(+) (7: BF(4)(-) salt), which shows definitive evidence (from NMR) for a terminally bound hydride in solution (CH(2)Cl(2) or THF), but 7 crystallizes as an axially symmetric unit in which Ir-Ir = 2.834 A. Reaction of 7 with water or wet methanol leads to isolation of the cationic diiridium(III) products [Ir(2)H(2)(mu-OX)(mu-pz)(2)(PPh(3))(2)(CO)(2)]BF(4) (8, X = H; 9, X = Me).

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Addition of Me1 occurs irreversibly to [Ir(CO)2(p-pz)12 [(I), pzH = pyrazole] in two kinetically distinguishable steps (AM, ASS respectively 32 k 2, 40 k 5 kJ mol-l, -152 k 8, -157 k 16 J mol-l K-l); by contrast Me1 and [Ir(C0D)(p-pz)l2 [ ( Z ) , COD = cyclo-octa-1,5-diene]) enter into equilibrium, AG0298 = -5 k 2 kJ mol-1, with AM = 89 k 4 kJ mol-1, A S = 40 k 15 J mol-1 K-1 for the forward reaction, in striking contrast with activation parameters for related reactions including that of (1) and interpretable together with lack of any kinetic salt or solvent dependence as evidence for a concerted mechanism.

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