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The growing use of photovoltaic (PV) cleaning machines (“robots”) raises the risk of abrasion to the antireflective coating (ARC) on modules’ front glass. ARC abrasion is often studied via accelerated lab tests, however field tests are needed to achieve real-world abrasion conditions. In this study nine types of PV modules and five types of ARC coupons were subjected to 18 months of dry-brush robot cleaning in the desert climate of Doha, Qatar. Three cleaning schedules were tested: daily, weekly, and never (reference samples subject to weathering alone). Modules’ power (Pmax), current (Isc), and reflectivity changes were measured and compared between the various cleaning schedules. It was found that the abrasion resistance of PV modules varied greatly. Five kinds of module showed greater losses with more frequent cleaning, while the other four did not. Lab profilometry of the coupons similarly found large variability of the depth and quantity of scratches for different ARCs, because of the difference in ARC durability between modules, and the likelihood that different cleaning robots will vary in their harshness, it is recommended to test specific robot/module pairs in the field to be confident of their ARC degradation rate.

Condensation (dew) on PV modules greatly affects their soiling. In this study, we compared several techniques for measuring and characterization of condensation on soiled glass samples in the desert environment of Doha, Qatar. The techniques were: two types of moisture sensor; optical microscopy; scanning electron microscopy image analysis; and dew-point/surface temperatures. It was found that moisture sensors are practical and useful for measuring condensation on PV modules. As condensation increased, dust particles in the soiling layer became more tightly clustered together by the droplets, and more cementation crystal needles were generally observed. Importantly, it was found that condensation commenced when the surface temperature was several °C above the dew point, indicating it is necessary to directly measure condensation on soiled surfaces rather than estimate it from environmental conditions.

Soiling consists of the deposition of contaminants onto photovoltaic (PV) modules or mirrors and tubes of concentrated solar power systems (CSPs). It often results in a drastic reduction of power generation, which potentially renders an installation economically unviable and therefore must be mitigated. On the other hand, the corresponding costs for cleaning can significantly increase the price of energy generated. In this work, the importance of soiling is assessed for the global PV and CSP key markets. Even in optimized cleaning scenarios, soiling reduces the current global solar power production by at least 3%–4%, with at least 3–5 billion € annual revenue losses, which could rise to 4%–7%, and more than 4–7 billion € losses, in 2023. Therefore, taking into account the underlying physics of natural soiling processes and the regional cleaning costs, a techno-economic assessment of current and proposed soiling mitigation strategies such as innovative coating materials is presented. Accordingly, the research and development needs and challenges in addressing soiling are discussed.

Soiling of photovoltaic (PV) modules can significantly reduce their energy yield by reflecting or absorbing the incident light and is of great importance for operation and maintenance of PV systems in dusty environments. In this paper, we examine the influence of dew—which is formed on PV modules during night and early morning hours—on soiling processes. Outdoor experiments were performed in desert conditions in Qatar using unheated and heated glass samples supplied with different heating powers, as well as outdoor microscopy studies. In addition, laboratory soiling experiments were performed using different dust types, controlled dust removal by centrifugal forces at different relative humidity levels, and defined dew cycles. Results from both the field tests and laboratory soiling experiments showed a significant impact of dew on particle adhesion, removal, and corresponding soiling rates, which was independent of the dust type. Microstructural investigations using electron microscopy were performed to determine the cause of increased particle adhesion. Heating of glass surfaces during the night can significantly reduce the processes of cementation, particle caking, and capillary aging and, thus, reduce soiling. This study indicates that the prevention of condensation can be a method to mitigate soiling.

Three fully transient numerical thermal models have been developed for photovoltaic (PV) modules in MATLAB environment, using 2-D finite difference (FD) method. One of the thermal FD model is based on explicit time scheme, while the other two are based on implicit time schemes. Out of the two implicit FD models, one has been modeled using preexisting toolboxes of MATLAB, while the other has been modeled using a self-developed novel method. All the three FD models are based on energy balance of different control volumes, which as a whole constitute the complete solid domain of the PV panel. The models have been tested against a variety of experimental data, ranging from sunny clear days, sunny cloudy days, rainy overcast days and consecutive sunny clear days. All the three models are found to agree very well with experimental results, i.e. the errors between the modeled and experimental data ranges between 0.2–0.7 °C. The main difference is between their computational speeds. In terms of average execution time per iteration for transient analyses, the self-developed novel implicit method (referred to as implicit BB throughout the work) was about 1200 times slower than the explicit method. However, overall, the implicit BB method took less time for the entire transient analyses, as it requires less number of iterations due to its tolerance to adapt longer time steps for each iteration. In other words, the explicit method although slightly more accurate, took approximately 900 times more CPU time to simulate the same time span test compared to implicit BB method. Thus, the self-developed novel method (implicit BB) has been recommended for these types of thermal models. [ABSTRACT FROM AUTHOR]

BACKGROUND: Acute coronary syndromes remain a leading cause of preventable early deaths. However, previous studies have indicated that paramedics' compliance with chest pain protocols is suboptimal and that many patients do not receive the benefits of appropriate prehospital treatment. AIMS: To evaluate paramedics' level of compliance with national clinical practice guidelines and to investigate why, in certain circumstances, they may deviate from the clinical guidelines. SETTING: The Health Service Executive Mid-Western Regional Ambulance Service which serves a mixed urban and rural population across three counties in the west of Ireland. METHOD: A retrospective review of completed ambulance Patient Care Report Forms was conducted for all adult patients with non-traumatic chest pain treated between 1 December 2007 and 31 March 2008. During the same study period, paramedics were asked to complete a prospective questionnaire survey investigating the rationale behind their treatment decisions, their estimation of patient risk and their attitudes towards the clinical practice guidelines and training. RESULTS: 382 completed Patient Care Report Forms were identified for patients with chest pain, of whom 84.8% received ECG monitoring, 75.9% were given oxygen, 44.8% were treated with sublingual glyceryl trinitrate (GTN) and 50.8% were treated with aspirin. Only 20.4% of patients had a prehospital 12-lead ECG recorded. 58 completed questionnaires were returned (response rate 15%); 64% of respondents said they had received insufficient training to identify ECG abnormalities. CONCLUSIONS: Prehospital treatment with oxygen, aspirin, sublingual GTN and ECG monitoring remains underused by paramedics, even though only a small number of patients had documented contraindications to their use. The small number of patients who received a prehospital 12-lead ECG is a cause of particular concern and suggests that incomplete patient assessment may contribute to undertreatment. Further provision of training and equipment is necessary to enable paramedics to more accurately assess and treat patients with acute coronary syndromes.

An extensive accurate X-ray data set for diaqua(glycylglycinato)copper(ii) hydrate, Cu(glygly)(OH₂)₂·H₂O, at 10 K is reported. The molecular structure is very accurately defined. The Cu(glygly)(OH₂)₂ unit approximates to a square pyramid with the apical water molecule only loosely bound. The data have been analysed by multipole and valence orbital population models and the topology of the electron density has been examined for chemical bonding features. There are two independent molecules of the complex in the unit cell, and the agreement between their bonding features is used as a measure of the validity of obtaining quantitative valence-related parameters from X-ray data. The agreement is quite reasonable. Atomic charges evaluated by the Stockholder and monopole population methods do not conform with chemical expectations. Those from the sum of valence orbital populations and the atom-in-molecules approach are more reasonable but contain anomalies. The ‘hole’ in the 3d10 shell corresponding to the 3d9 configuration, predicted on the grounds of ligand field theory, is seen as a much lowered (1.16 e) population of 3d_z2 relative to the other 3d orbitals (av. 1.65 e). Populations of valence hybrid orbitals conform with chemical expectations and, for the (sp2)_σ hybridised carbon atoms of C&z.dbd;O groups, relocation of electrons from a p_π orbital to those σ hybrids is seen. Bond critical points have been located between all the appropriate non-hydrogen atom pairs. The electron densities at these points show values typical for mixed ionic/covalent character in the Cu–O and Cu–N interactions and of covalent character in the C–C, O–C and N–C bonds. The Laplacian of the density at the bond critical points also behaves in a fashion which conforms with those bond types, although the scatter in the results is greater. There is little evidence of the copper atom having influenced the electron densities of the bonds between light atoms within the glycylglycinato moiety.

Comparison has been made between theoretically obtained structure factors and those obtained from very low temperature X-ray and polarised neutron diffraction experiments on single crystals of Ni(NH₃)₄(NO₂)₂. This work has emphasised an approach in which interactions between neighbour molecules have been ignored and the unit cell charge density has been determined from single molecule calculations by summing the densities of the molecules in the unit cell. Agreement between experiment and theory has been assessed using the χ² and goodness of fit (R_w) statistics. For this molecule the agreement with experiment is little altered by change of basis up to the level of a valence triple-zeta set with polarisation functions (VTZP). The importance of the crystalline environment was examined using the CRYSTAL95 package. At the level of theory available in this program the effect of intermolecular interactions on structure factors has been shown to be negligible. Including electron correlation also made little change to the structure factors. The best of the single molecule calculated structure factors, using five parameters, fitted the X-ray experiment almost as well as 83 or 126 parameter multipole expansions. A similar result was obtained for the polarised neutron diffraction structure factors.

The crystal structures of the α‐alums rubidium chromium bis(­sulfate) dodecahydrate, RbCr(SO4)2·12H2O, and caesium chromium bis[tetraoxoselenate(VI)] dodecahydrate, CsCr(SeO4)2·12H2O, have been determined by X‐ray diffraction at 293 and 12 K. The metal atoms lie on sites and the anions lie on threefold rotation axes. The accurate and extensive data sets lead to much more precise determinations than are available from earlier work, particularly at 12 K. The changes in the atomic displacement parameters between 293 and 12 K correspond to the respective predominances of intermolecular and intramolecular vibrational effects.

Powder neutron diffraction measurements have been made at 2 and 20 K for the complex cis-[ReCl4(NH&z.dbd6;CMeNHPh)2]. The changes in diffracted intensities on magnetic ordering at 9.7(1) K were fitted in the Shubnikov group Pc′c′n to an antiferromagnetically ordered magnetic structure in which the spins are aligned along c in this orthorhombic crystal. A moment of 2.58(6) µB resides on the rhenium site in each molecule, while each chlorine site has a spin population of 1.03(8) µB due to covalent spin delocalisation. The N–C–N region of the other two ligands each has a spin population of –1.86(10) µB, indicating a substantial spin polarisation resulting from electron correlation effects. The antiferromagnetic structure is consistent with a reinterpretation of earlier single crystal magnetisation data. The very large amounts of spin transfer observed within the molecule are not unexpected in this heavy metal complex, and qualitatively reasonable given a synergistic interaction between the two ligand types. The previous observation of a metamagnetic transition in this crystal is explained as being driven by a change in canting angles within the molecule, from alignment closely along c to an arrangement in which opposite halves of the molecule are differently canted, with small but significant +b and –b components. The direction of magnetisation within the molecule is thus not all parallel.

The crystal structures of KOs(NH3)3(ClO4)2Cl2and KCr(NH3)6(ClO4)2Cl2were determined at 10 K by X-ray diffraction, and for the osmium salt also at 293 K. At 293 K the osmium salt is trigonal, space group R3m, with the same simple structure as others of this class of double salt. At 10 K, in agreement with previous radius ratio predictions, both crystals are best described as remaining R3m. All previously studied members, with larger alkali metal cations, are twinned R3 at low temperatures, with small, symmetry-breaking rotations of the hexaamminemetal(III) and perchlorate ions about the threefold axis. Differential scanning calorimetry on CsRu(NH3)6(ClO4)2Cl2suggests that the R3mto R3 change is very extended in temperature with only a small discontinuity at the transition temperature.

The magnetic susceptibilities of tris(acetylacetonato)ruthenium(III) have been measured between 2·5 and 300 K along the a, b, c, and a* axis directions, together with the magnetizations along the same directions up to a magnetic field of 5 T. There is a small amount of magnetic exchange interaction apparent below 10 K, with Weiss constants up to –0·35 K and the magnetization is fitted with exchange integrals up to –0·38 K in magnitude. A pathway for magnetic exchange in terms of a pair of symmetry-related Ru(acac) rings lying parallel and close is obvious from the structure. The susceptibility results above 10 K have been interpreted in terms of a four-parameter ligand field model (CF1) operating on the ground 2T2gterm of the d5configuration. The t2gorbitals are found to be split by 475 cm-1by a dominant trigonal symmetry component and then by –50 cm-1by a subsidiary rhombic component. The single-electron spin-orbit coupling constant is 875 cm-1and the orbital angular momentum reduction parameter is 0·7. The g-values deduced for the ground Kramers doublet are not in good agreement with those from e.s.r. experiments, but rather better agreement is found for closely allied ligand field parameter sets (CF2) which can fit the susceptibilities at particular temperatures but do not reproduce their temperature dependence well. Consideration of the variation of structural details with temperature indicate that, in fact, the CF2 sets may be more realistic.

The crystal structure of [As(C6H5)4] [TcNCl4] was determined at 120 K by X-ray diffraction and at 28 K by neutron diffraction. The crystals are tetragonal, space group P4/n, with a 1260.4(3) and c 773.2(2) pm at 120 K. The [TcNCl4]-anion possesses exact C4v symmetry, with Tc≡N distances of 160.3(2) and 162.5(4)pm at 120 and 28 K respectively. Magnetic susceptibilities were measured on single crystals from 300 to 4.5 K. The results indicate a well behaved S ½ system following the Curie-Weiss law with θ -0.13 K

The X-ray crystal structure determinations of five complexes of rhenium(IV) containing the ReCl4L2 unit, with L monodentate , are reported, one at 92 K. They are: (1) cis -( HNCMeNHPh )2ReCl4, Pccn , a 21.677(4), b 7.253(1), c 12.470(2) Ǻ, R1(2σ) 0.0217, Rw 0.0473; (2) the di ( acetonitrile ) solvate of (1), Pī , a 10.005(2), b 12.120(2), c 12.744(2) Ǻ, α 72.19(1), β 67.65(1), γ 90.95(1)°, R1(2σ) 0.0311, Rw 0.0819; (3) cis -(HN= CMeNH (p-C6H4Me))2ReCl4, Pna21, a 12.782(2), b 8.237(1), c 21.279(3) Ǻ, R1(2σ) 0.0497, Rw 0.1066; (4) cis-( thf )2ReCl4, P 212121, a 7.785(2), b 12.666(4), c 14.253(5) Ǻ, R1(2σ), 0.0326, Rw 0.0827; and (5) trans-(Me2PhP)2ReCl4, P 21/c, a 9.701(2), b 13.739(6), c 8.324(2) Ǻ, β 106.83(2), R1(2σ) 0.0241, Rw 0.0613. The Re-Cl bond lengths in these and six other ReIVCl4L2 octahedral complexes are compared, and no distinct trans-effect of the donor atom on the chlorine atoms is seen either where L is monodentate as in our compounds or where L2 is a chelate ligand, as in the examples from the literature, although the results are much less clear there.

The magnetic susceptibilities along and perpendicular to the c axis (hexagonal setting) between 2.0 and 300 K at a magnetic field of 1.00 T, and the magnetizations at field strengths up to 5.00 T, are presented for single crystals of [Co(NH3)5(OH2)] [Cr(CN)6]. The results are interpreted in terms of zero-field splitting (2D) of the ground 4A2g term by spin-orbit coupling and of magnetic exchange interaction between the chromium atoms. The magnetic exchange is modelled as one of Ising or mean-field in type. The exchange is found to be quite small: J = -0.18(6) cm-1 if the Ising model is employed, and -0.03(1) cm-1 for the mean-field model. The model adopted for the exchange has a strong influence on the value of the parameter D obtained. When the Ising model is used D is deduced to be -0.28(9) cm-l; when the mean-field model is used D is -0.14(4) cm-l. The g-values deduced are in agreement with those from e.s.r. measurements at higher temperatures and do not depend on the exchange model. In any case, D is found to be sufficiently large that it must be considered in a polarized neutron diffraction experiment on the compound.

The single-crystal magnetic susceptibilities from 4.5 to 300 K, and the magnetizations at 4.5 K from 0.25 to 5 T, of Cs2KFe(CN)6 along a, b and c* are reported. The data are fitted well by a crystal field model with splitting of the ground-state 2T2g term of the hexacyanoferrate (III) ion. The three variable parameters of the model were the two representing the low-symmetry splitting of the t2g orbitals, and the effective reduction in the orbital angular momentum. Four other parameters were fixed at values known or estimated from other experiments on hexacyanoferrate (III) ions, including polarized neutron diffraction which determines the orientation of lower-symmetry crystal field components. The crystal field has only inversion symmetry with respect to the ligand axes, with a non-cubic component which is almost uniaxial with respect to the a,b,c * axis system.

A charge density refinement using effective anharmonic isotropic thermal parameters was performed on the X-ray diffraction data for pentaammineaquacobalt(III) hexacyanochromate(III) and for hexaamminecobalt(III) hexacyanochromate(III), constraining the unit cells of the crystals to be electically neutral. Our previous refinements, which did not admit of anharmonicity, could not simultaneously give a good fit and provide electroneutrality of the unit cell. The effective anharmonic parameters correct a systematic error observed in these, and, less extremely, in other transition-metal complex crystals. Values of the derived charge density parameters change mostly at less than the 2σ level of significance in the new refinements except for overall ionic charges. It seems the limitations in our earlier refinements did not lead to serious errors in the chemical conclusions reached, apart from the ionic charges. In the [Co(NH3)5(OH2)][Cr(CN)6] case we observe five significant charge flows; from CN to Cr, σ bonding 0.5(3), and π back-bonding 0.6(2) e; from NH3 and H2O to Co, σ bonding 0.7(3), and π back-bonding 0.5(2) e; and a transfer of 1.5(4) from Cr(CN)6 to Co(NH3)5(H2O) through 17 CN…H hydrogen bonds, with N…H(3) noticeably more important. Bond overlap densities are also significant. The relative sizes of these charge flows are similar for the isomorphous hexaamminecobalt(III) hexacyanochromate(III) salt.

The title compound crystallizes at room temperature from aqueous solution as two distinct hydrated phases, both monoclinic and characterized by single-crystal X-ray structure determinations. The dihydrate ('blocks') is C2/c, a 42.58(2), b 9.099(8), c 21.14(1)Ǻ, β 112.12(4)°, Z= 8; R was 0.048 for 3784 'observed' reflections. The c. 3.75 hydrate ('Tetrahydrate'?) ('needles') is P21/c, a 9.348(2), b 36.663(8), c 23.071(8)Ǻ, β 93.51(2)°, Z 8; R was 0.050 for 6042 'observed' reflections. There are one and two independent cations in the asymmetric unit of each structure; all cations have the metal atom 'mer' coordinated by a pair of tridentate ligands. In the C2/c phase, Co-N (central) are 2.057(4) and 2.063(4)Ǻ with N-Co-N 175.8(2)°; Co-N (distal) are 2.163(6)-2.187(6)Ǻ. In the P21/c phase, in the first cation, Co-N are 2.045(8) and 2.050(7)Ǻ (central) [with N-Co-N, 167.0(3)°] and 2.193(8)-2.216(8)Ǻ (distal), while, in the second, Co-N (central) are 2.030(8) and 2.042(8)Ǻ with N-Co-N, 168.3(3)°; Co-N (distal) are 2.164(8)-2.227(8)Ǻ. These Co-N distances are longer than those observed in the [Co( tpy )2]2+ cations hitherto studied ( tpy = 2,2′:6′,2′-terpyridine), in which the electronic ground state has an appreciable low-spin component.

We have performed ab initio discrete variational Xα calculations on a Cs18[Mo(NCS)6]15+ cluster, with the external electric field appropriate for a Cs3Mo(NCS)6 crystal. The calculations show both σ and π bonding, and also the participation of diffuse molybdenum- centred orbitals . Compared with the charge density experiment these effects are qualitatively correct but are underestimated. We performed further calculations successively removing the external electric field and the 18 Cs+ neighbours . The effects on the calculated metal- ligand bonding in the complex ion are negligible. This is not in agreement with experiment, so that these calculations which invoke only electrostatic effects and orthogonality with neighbouring ions and which also use only simple basis sets are not adequate. Further calculations on the Cr(NCS)63- and W(NCS)63- ions show increasing covalence from chromium to tungsten as expected. The spin transfer to the ligand is calculated to triple, but charge transfer is almost constant.

The room-temperature (295 K) crystal structures of potassium ferricyanide, K3[Fe(CN)6], have been determined for the simplest monoclinic (a reinvestigation) and orthorhombic polytypes by single- crystal X-ray diffraction. The monoclinic form is P21/c, a 7.047(3), b 10.400(3), c 8.384(3) Ǻ, β 107.29(3), Z 2. The iron atoms lie on special positions with symmetry 1. In the orthorhombic form, Pnca, a 13.422(6), b 10.396(4), 8.381(4) Ǻ, Z4, the iron atoms now lie on special positions with symmetry 2 (parallel to c). Residuals are 0.036 and 0.048 for 1232 and 855 ?observed? reflections respectively.

The crystal structure of the title compound, [Co(C9H13N5)2] I2,H2O, has been determined by single-crystal X-ray diffraction at 295 K and refined by least squares to a residual of 0.058 for 3255 ?observed? reflections. Crystals are triclinic, Pī, a 14.934(5), b 11.315(4), c 8.090(3)Ǻ, α 85.19(3), β 87.76(3), γ 74.10(3), Z 2. The distances from the cobalt to the central trimethine nitrogen atoms are 1.893(8) and 1.903(8)Ǻ consistent with the almost low-spin behaviour of the complex; the distal Co-N distances range between 2.045(8) and 2.140(8)Ǻ. One of the iodide anions appears to be disordered over two sites populated in the ratio 0.86:0.14; occupancy of the second site together with the magnetic moment of 2.1 BM may indicate a small proportion of the high- spin isomer.

The crystal structure of the title compound (C10H9N2) [Fe(C10H8N2)3] (ClO4)4 has been determined at c. 295 K from diffractometer data and refined by least squares to R 0.056 (7018 reflections with I > 3σ(I)). Crystals are triclinic, Pī, a 16.607(3), b 13.496(3), c 10.254(2) Ǻ, α 101.13(2), β 92.08(2), γ 102.77(1), Z 2. The iron atom is surrounded by three bidentate ligands, the six Fe-N distances being equal ( 1.963 Ǻ). The monoprotonated bipyridylium cation adopts the unusual cis configuration, with the proton clearly located on one of the nitrogen atoms. The e.s.r. g-tensor is approximately axial (g|| 1.474, g, 2.690) but its axes do not coincide with obvious geometric features of the cation.

The magnetic susceptibility of the perchlorate and iodide salts of the hexaureatitanium(III) and hexaureavanadium(III) cations have been measured to temperatures as low as 4 K in the former and 1.5 K in the latter cases. The results were interpreted to deduce the three parameters of the simplest theory for the splitting of the ground T term for each ion. The parameter sets agree only fairly well with results from the magnetic susceptibility and anisotropy data at higher temperatures.

Power station lighting covers a wide field. Exterior lighting includes the lighting of roadway systems, railway sidings or wharves, coal handling plant and coal storage areas, and uses standard street lighting and area floodlighting techniques.Interior lighting is more difficult. Boiler houses, however efficient, are always dusty, and the space much obstructed by pipes, flues, trunking, etc., making installation and maintenance awkward.Two basic systems of lighting are discussed for the illumination of the turbine houses.Problems arising in the control room—the nerve centre—depend closely on the interrelation of switch panel and control desk layouts and ceiling structure. Such lighting is best planned in the initial drawing board stage by close co-operation of all concerned.Emergency lighting systems must also be provided.

The polarized neutron diffraction data for deuterated trans-Ni(NH3)4(NO2)2 have been reanalysed using an improved method for accounting for the oribital component in the scattering of the polarized neutrons. In this case, where the applied magnetic field was not along a principal crystal or molecular magnetic susceptibility direction, there seemed to be a possibility that the usual 'dipole approximation' correction might not have been fully adequate within the accuracy of the data. However, because of the smallness of the orbital contribution to the angular momentum associated with the Ni2+ ion in the compound, the effects of the 'full correction' do not lead to a significantly different valence orbital population analysis of the spin density than does the dipole approximation.

In a polarized neutron diffraction experiment on the magnetically highly anisotropic ammonium cobalt Tutton salt (ND4)2Co(SO4)2·6D2O, 70 and 22 flipping ratios were measured in the hk0 and h01 zones respectively. These, and the underlying magnetization distribution around Co, more closely resemble those predicted by a model with a cubic crystal field with CoO6 axes, than those predicted from a generally orientated ellipsoidal crystal field, even though the latter provides a good fit to ESR and magnetic susceptibility data. However, all the real space magnetization distributions are substantially isotropic. For an examination of covalence as the experimental objective, these results indicate that a suitably scaled spin-only model for the magnetization distribution on the metal may suffice, even in highly anisotropic situations where a large orbital moment is present, such as the present case.

The structure of the elpasolite Cs2KCr(CN)6 has been determined at 7.0 K by neutron diffraction. 877 unique reflections refined to Rw = 0.020, Χ = 1.4. The Cr(CN)6 unit is close to regular octahedral coordination geometry, with bond lengths of 207.4(4) for Cr-C and 116.6(6) pm for C-N. The C-Cr-C angles average to 90.0(5) and the Cr-C-N angles to 178.3(1). The magnetic susceptibility as a function of temperature and the magnetization as a function of magnetic field strength correspond to the 4A2g ground term perturbed by a small magnetic exchange effect.

The structure of (ND4)2Ni(SO4)2.6D2O has been determined at 2 K by means of neutron diffraction using two crystals, one mounted about the b axis and one about the c axis, in normal-beam Weissenberg geometry. Totals of 642 and 689 averaged unique reflections were obtained and refined together, allowing differing scale and extinction parameters, to give R(F2) = 0.053. The weighted average cell dimensions and atomic positional parameters are reported. The crystals are monoclinic, P21/a; a 9.073, b 12.305, c 6.255 Ǻ, β 106.9°. The structure is compared to that obtained by X-ray diffraction at c. 295 K. The similarity of the structure, including the hydrogen bonding, to that of the manganese compound, is very great, apart from the metal-oxygen bond lengths. The structure of (NH4)2Ni(SO4)2.6D2O was obtained by the study of one crystal mounted down the c axis, collecting only the hk0 and hk1 layers, also at 2 K. A total of 441 averaged unique reflections were obtained, and the structure refined to Rw(F2) = 0.043. Given the poor resolution in the c direction, the resultant structure is almost indistinguishable from that of the deuterated compound, except that the NiO6 octahedron seems more regular. The magnetization at 2.00 K and 4.60 T is shown to be in reasonable agreement with the value expected from the 90 K E.S.R . z.f.s . parameters, and the experimentally observed 8% difference between the deuterated and hydrogenous salts could well arise from a difference in z.f.s .parameters consequent upon the structural alterations.

A quantitative description of transition-metal bonding has been obtained through combined analysis of 9(1) K X-ray and 13(1) K time-of-flight neutron diffraction data. It is shown that a simple valence-orbital model is too crude an approximation adequately to describe the electron-density distribution of Ni(ND3)4(NO2)2. To exhaust more fully the information present in the very-low-temperature diffraction data, a more flexible electron-density model was used. Quantitative measures describing the bonding in the complex have been achieved through topological analysis of the derived static model density. To study the effects of co-ordination and intermolecular interactions, comparisons were made with good-quality wavefunctions calculated for free nitrite and ammonium ions. Both ligands appear co-ordinated through predominantly electrostatic interactions. Contrary to previous studies of Ni(ND3)4(NO2)2, the topological analysis revealed that the metal–ligand interactions, besides cylindrical σ contributions, also have non-cylindrical π contributions to the covalent part of the bonding. Plots of the Laplacian of the electron density were used to locate regions of charge concentration and charge depletion in the valence regions of the atoms in the molecule. For all atoms, maxima in the valence-shell charge concentration are found in accord with the simple Lewis electron-pair concept of bonded and non-bonded charge concentrations. The study demonstrates that X-ray diffraction data measured carefully at very low temperatures have sufficient precision to allow for a reliable and detailed topological analysis of transition-metal electron-density distributions.

The X-ray crystal structure determinations of three complexes of hexaammineruthenium(III) are reported. They are: (1) NH4+.Ru(NH3)63+.2ZnCl42¯, a8·566(2), c12·829(4) Å, V941·3(4) Å3,I4/m, Z= 2, R1(2σ) = 0·021, wR2= 0·054; (2) Ru(NH3)63+.ZnCl4 2¯.Cl¯.H2O, a8·174(2), b8·979(2), c10·123(2) Å, α 80·20(2), β 86·41(2), γ 87·23(2)°, V730·2(3) Å3, P-1, Z= 2, R1(2σ) = 0·027, wR2= 0·074; (3) Ru(NH3)63+.HgCl42¯.HgCl3¯.HgCl2.H2O, a8·055(3), b21·493(11), c12·625(4) Å, β 90·52(3)°, V2186(2) Å3, P21/c, Z= 4, R1(2σ) = 0·058, wR2= 0·142. The hexaammineruthenium(III) cations in these structures adopt differing conformations in respect of hydrogen atom disposition. Compounds (1) and (2) have disordered NH3groups in the Ru(NH3)63+cations, and in compound (1) the NH4+cation is disordered as well.

Polarized neutron and X-ray diffraction experimental results on simple Cr(III), Ni(II) and Co(ll) complexes are compared with theoretical calculations. A simple ionic (crystal-field) model is a useful first approximation for the description of the spin and charge densities. For the more covalently bound cases the effects of electron-electron correlation in the metal-ligand bonding are as important as the spin and charge transferred via simple covalence. Thus simple M.O. models do not offer much improvement, nor do ab initio calculations which do not include configurational interaction (viz. restricted Hartree-Fock). Unconstrained calculations which partly incorporate electron-electron correlation are in qualitative, but not quantitative, agreement with the experiments. The polarized neutron diffraction experiments provide such sensitive tests of theory because they spatially separate the small effects in the bonding region from the metal atom 3d region, whereas these are inextricably mixed in the energetic properties measured by spectroscopic techniques.

Flipping ratios for 923 reflections were measured in a polarized neutron diffraction experiment at 1·5 K with a magnetic field of 4·6 T applied along the [100] and [011] directions of a crystal of Cs2KFe(CN)6, containing the low-spin [2T2g] hexacyanoferrate(III) ion. Various multipolar models of the noncollinear magnetization density were fitted to this and to the earlier [010] data. The expected transfer of spin by π covalence and σ spin polarization to the cyanide groups is observed. The field applied along [011] breaks the magnetic monoclinic symmetry of the crystal to give two independent anions. The direction of the net magnetization on each of the four independent ions in the three orientations can be accounted for by a crystal field of uniaxial symmetry with polar angles θ = 95·4(16)°, φ = 92·5(16)° relative to ac*b. The magnetization is almost collinear with the applied field for the [100] and [010], but distinctly noncollinear for the [011] data. In addition, there are significant anisotropic noncollinear terms which make the magnetization direction within the Fe(CN)6 unit nonuniform. The distribution of magnetization derived from all three data sets shows almost cylindrical symmetry about the ligand direction closest to the applied field for all four ions, and for both collinear and noncollinear components. It does not follow the applied field, nor is it fixed with respect to ligand axes. A cubic crystal field model, in which the magnetization is dominated by the orbital moment, reproduces these results.

Spin-orbit coupling introduces orbital angular momentum into the 4A2 (CoCl2-4) or 2B2g (Co(Pc)) ground term of the Co2+ ion. Using a ligand field model real space magnetization densities and magnetic structure factors for the crystals are calculated. The shape of the magnetization and its non-collinearity with the magnetic field make the magnetic structure factors more complicated than is usually assumed in treating polarized neutron diffraction data for transition metal ions. However, it is found that for Cs3CoCl5 a systematic cancellation of correcting terms leads to results not distinguishable from those of the usual 'dipole' correction within the experimental errors. For Co(Pc) the 'anomalous' orbital scattering effects are larger and would be important if the data set were as accurate as that for Cs3CoCl5. An improved correction mechanism for PND data is developed but does not lead to significant changes in the valence orbital populations previously derived for the two compounds. Such corrections are, however, likely to be useful in other systems where orbital scattering effects are present.

The crystal structure of the title compound [CO(C15H11N3)2]Cl3 has been determined by single crystal X-ray diffraction methods at 295 K and refined by least-squares methods to a residual of 0.069 for 2953 'observed' reflections. Crystals are triclinic, PI, a 9.018(4), b 11.751(4), c 18.085(6), α 9O.82(6), β 9O.66(7), γ 90.79(7), Z 2. The determination establishes for the first time metal-nitrogen distances in the bis(terpyridyl)cobalt(III) species; (Co-N)(central, distal) are 1.86, 1.93 . The precision of the determination is adversely affected by widespread solvent/anion disorder.